BY FREDERICK CRESCITELLI* From the Department of Biology, University of California, Los Angeles, CA 90024, U.S.A. (Received 20 February 1981) SUMMARY

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1 J. Physiol. (1981), 321, pp With 8 text-figures Printed in Great Britain THE GECKO VISUAL PIGMENT: A ph INDICATOR WITH A SALT EFFECT BY FREDERICK CRESCITELLI* From the Department of Biology, University of California, Los Angeles, CA 924, U.S.A. (Received 2 February 1981) SUMMARY 1. Unlike rhodopsin, the extracted 521-pigment of the Tokay gecko (Gekko gekko) is ph-sensitive and changes its spectral absorbance in the ph range of 45--7'3. The colour change is reversible and ph can be employed to adjust the spectral maximum anywhere between 49 and its native location at The hypsochromic shift with increasing acidity is opposite to that expected for the protonation of the Schiff base nitrogen and suggests an action on the secondary system of interacting charges that have long been postulated to adjust vertebrate visual pigment colour within the visible spectrum. 3. Chloride ions modulate this ph effect in a systematic and significant manner. For the pigment extracted in the chloride-deficient state the colour change occurs in the ph range of 6(-7-, the midpoint being close to 6-5, suggesting the possible participation of the imidazole group of histidine as the functional moiety. With added NaCl the colour shifts to the region below ph The modulating action of chloride is postulated to be a conformational change of the opsin leading to a shift of the secondary interacting site from one functional group to another or else to a change in pk of a single group due to the conformational alteration of the electrostatics of the system. 5. At ph values between 7-5 and 9 a different mechanism becomes apparent. In this region a decrease occurs in the photopigment density as well as a shift in absorbance toward the blue. This alkaline effect is readily reversed either by adding NaCl or else by lowering the ph. Along with the other protective effects of chloride these ions serve to reduce or prevent this alkaline loss in density. 6. Associated with this reversible photopigment loss is a reversible appearance of a product with a maximum at about 366. The spectrum of this product is like that produced by the addition of 1 I-cis retinal to the extract. Acidification of the alkaline preparation leads to a restitution of the photopigment as well as to a reduction of the 366-product. 7. Addition of hydroxylamine to the alkaline extract in appropriate concentration inhibits the restitution of pigment-521 with acid or NaCl, but adding 1 1-cis retinal to the system leads to restoration of the photopigment after acidification. All the * Aided by grant no. 5 ROl EY from the National Eye Institute, National Institutes of Health, U.S. Public Health Service. 13 PHTY 321

2 386 F. CRESCITELLI evidence suggests that product-366 is either free 11 -cis retinal or else held to the opsin in a form that does not alter its spectral absorbance. The alkaline effect is therefore a disruption of the aldimine bond of the visual pigment. 8. In many respects the gecko 521-pigment behaves like the chicken cone pigment, iodopsin, suggesting that an investigation of the latter in terms ofph may be a worthy project for future study. 9. With its ability to change colour with ph, with chloride, with nitrate, etc. the extractable gecko pigment offers possibilities for the investigation of mechanisms responsible for adjusting visual pigment absorbance throughout the visible spectrum. The techniques of circular dichroism, Raman spectroscopy, infra-red spectroscopy, etc. may find here a suitable material for these studies. INTRODUCTION Though a Schiff base, the chromophore of rhodopsin has a spectral absorbance unaffected by changes in ph of the medium. Lythgoe (1937) noted that extracted frog rhodopsin is stable and has the same colour over the ph range of and Wald (1938) confirmed this ph stability with bullfrog rhodopsin. The Schiff base appears to be enclosed within an hydrophobic surround of amino acids resistant to attack by aqueous reagents that would ordinarily react with the aldimine bond (Bownds, 1967). In contrast, certain intermediates and products of photic bleaching, such as the meta-i and meta-il system and N-retinylidene opsin, change with ph (Matthews, Hubbard, Brown & Wald, 1963; Collins & Morton, 195). At these stages, apparently, the molecule unfolds and exposes specific groups that can exchange protons. Accordingly, it is of more than passing interest to report that there is one visual pigment whose spectral absorbance is alterable by ph without prior bleaching, an effect that is readily reversible. Moreover, this ph action differs according to the presence or absence of chloride. This is the behaviour of the extracted 521-pigment of the Tokay gecko (Gekio gelko). In some respects P521 is similar in properties to rhodopsin. It is a Schiff base with 11 -cis retinal; it has the same stereospecificity; its spectral absorbance conforms to the Dartnall nomogram and the final products of bleaching behave as do the rhodopsin indicator yellows (Crescitelli, Dartnall & Loew, 1976; Crescitelli, 1977 a). In certain other respects P521 is unlike rhodopsin. It does not reveal a meta-iji intermediate after photic bleaching, it is attacked and bleached by NH2OH and NaBH4 in the dark, and its spectral absorbance is reversibly altered by sulphydryl reagents, by chloride and by nitrate (Crescitelli, 1975, 1977a, 198a). To this list I can now add a ph effect which is the first point to be made in this report. Within the visual cell this photopigment absorbs maximally at 521 (Crescitelli, Dartnall & Loew, 1976) but when the retina is removed, washed with distilled water and with buffer at ph 7 (}7-5, and then extracted into 2 digitonin (w/v) at these ph values, the absorbance is shifted some 2 toward the blue. Addition of chloride (or bromide) leads to an immediate bathochromic recovery, the magnitude of which is a function of chloride concentration (Crescitelli, 1977 b). In addition there is an increase in density (hyperchromic effect) with the higher chloride concentrations. This dual 'ionochromic' response was initially observed with the pigment at ph around

3 GECKO VISUAL PIGMENT 387 neutrality. It now appears that this chloride effect varies with ph and this is the second point of this report. METHODS Tokay geckos were purchased from an animal dealer and after dark adapting them overnight they were quickly decapitated, the eyes removed and placed in 4 % potassium alum for an hour in order to harden them and facilitate handling during the dissection, the retinas were removed and washed once with distilled water and then placed in the alum solution for an hour. After centrifugation, the alum was then removed, the retinas were washed twice with distilled water and once with Tris-maleate buffer at the desired ph, and, finally, extraction of the pigment was made using 2 % digitonin made up in the Tris buffer at this ph. The spectral absorbance was measured using about -15 ml of extract in a microcell (1 cm path length). A Beckman DU spectrophotometer was employed, the temperature, during the measurements, being kept at 5 IC by means of cold water from a controlled refrigerated bath (Lauda) circulated through the blocks of the cell compartment. Addition of reagents to change the ph or to test other effects was made in the dark room illuminated by deep red light and the concentrations of the reagents were determined by weighings before and after each addition. Unless otherwise stated the readings were corrected for the dilution effect of the reagent by multiplying by the appropriate dilution factor. The ph after each experiment was determined by means of a cup electrode capable of handling as little as -1 ml solution. In a few experiments it was necessary to make a number of successive additions of either NaOH or maleic acid to the same extract or aliquot in order to obtain the effect of a series of ph changes on the same sample of pigment. In such cases it was not possible, without loss of valuable extract, to determine the ph after each addition. Consequently, a ph calibration curve was first obtained with digitonin solution employing a sequence of known additions of NaOH or maleic acid. From such a curve the expected ph resulting from each addition was read off. It is conceded that this indirect method is not as dependable as reading the ph after a single addition but the results obtained by the indirect procedure were always checked and found to be similar to those of the experiments with the ph read directly. Because of the known thermal sensitivity of P521 (Crescitelli, 1974) temperatures were always kept close to 5C in the preparation and analysis of the gecko photopigment. RESULTS A study of the gecko pigment over the range brought out the fact that there are two fairly distinct regions of ph over which two different types of responses are obtained. One is the range from 4-5 to 7 3, the other is roughly between 7-5 and 9. I shall refer to these as the first effect and the alkaline effect, respectively, and will describe them separately. Both these effects are easily revesible but below ph about 4' and above about 9 irreversibility occurs. The first effect A typical experiment for this region of ph (Fig. 1) pictures the spectral absorbances of two aliquots of an extract prepared with no added chloride, one at ph 5*43 (curve 1), the second at ph 7*33 (curve 2). In this case the spectral maxima were at 491 and 51, respectively. The addition of NaCl to both aliquots led to the characteristic ionochromic response (curves 3, 4), the maxima being shifted to 51 and 52, respectively. To convince myself that some experimental condition other than ph was not the source of this unexpected ph effect I repeated the procedure with rhodopsin of the rabbit and frog. Neither a ph, nor a chloride effect was observed. In some thirty experiments the detailed procedure was varied, starting, in different 13-2

4 388 F. CRESCITELLI trials, with the extract in the acid, the neutral or the alkaline segments of this ph range, and sometimes the chloride was added before changing the ph, at other times after. Regardless of the procedure, the results were the same and revealed the 521-pigment of Gekko gekko to be a ph indicator with a modulating chloride action. A complete description (Fig. 2A) of this ph behaviour for the pigment without I l 4 ~~~~~~~~~~~4-7 3 //1 *4 ;> /&\354 2~~~~~~~~~~~~~~~~~ Fig. 1. The ph effect. Curves 1,2: chloride-deficient pigment at ph 5-43 and Curves 3,4: after adding NaCi to a concentration giving maximal responses, 1-7 x 1-1 M & 9-72 x 1o-2 M. respectively. Optical density vs. wave-length. Temperature: 5 C. (curve 1) and with (curve 2) NaCl added to the preparations shows the spectral maxima as functions of ph in the two states. These maxima were read off curves similar to those of Fig. 1 but employing only those with a ratio (density at minimum to density at maximum) of 5 or less. This selection was made in order to minimize the error inherent in reading the maxima from spectra with significant quantities of spectral impurities at shorter wave-lengths. Three significant features emerge from the general summary of all the analyses of Fig. 2A. The first is that there are two separate functional regions over which the pigment changes colour, one for the pigment in the chloride-supplemented state, the other without this anion. For the latter state the change occurs between ph 6 and 7, the midpoint being close to 6*5. With chloride the functional region is below 6 2 and there is very little overlap between the two regions. The second feature, directly related to the first, is the dependence on ph of the chloride bathochromic shift. Absent at ph close to 4 5, this shift increases to ph 6 2, beyond which it decreases due to the ph bathochromic shift of the chloridedeficient system (curve 1). The third feature is that without chloride the pigment cannot achieve its native position at 521 by ph alone. Chloride is essential to accomplish this. The chloride ionochromic shift as a function of chloride concentration differs with phi (Fig. 2B) and these results agree with the three features just enumerated for the data of Fig. 2A. At ph 4 62 there is no response to chloride even at concentrations more than normally required to produce maximal 'red' shifts. This proves that the

5 GECKO VISUAL PIGMENT 389 failure of a chloride action at ph in the region of ph 4'5 in Fig. 2 A was not the result of an insufficiency of NaCl. This salt was added in quantity more than necessary to evoke maximal responses at all the ph values that were studied for Fig. 2A. One additional feature only hinted at in Fig. 2A is the difference between the two curves above ph 7 5, curve 1 lacking data points above this while curve 2 shows an extension 52 _ B I I X 1 I T Ir v I l l log NaCI x A 9* ~ * S *e * 2 E 5 49 x x I II ph Fig. 2. A, spectral maxima as functions of ph for pigment aliquots with no added NaCl (curve 1) and for other aliquots after adding NaCI to give maximum responses (curve 2). B, spectral maxima as functions of log NaCl (molarity x 16) at six different ph values. x xx x to greater alkalinity. This, of course, is part of the alkaline effect that will be described under the next heading. The data points of Fig. 2A were obtained by separate and laborious analyses of many extracts and aliquots adjusted to the various ph values which were read at the end of each analysis. A simpler and just as convincing demonstration of the chloride-modulated ph effect was obtained with a single extract by spectral readings taken after changing the ph in stages by successive additions of known quantities of NaOH. In this case the ph values at each stage were determined from the calibration curve as explained under Methods. The experiment was carried out with two aliquots of the same extract, one with, the other without NaCl, the two being analysed simultaneously in the same cuvette holder and using the same procedures. The results (Fig. 3) show the spectral curves, without correction for dilution, the

6 39 F. CRESCITELLI interrupted tracing being the profile of the maxima of the several curves. The aliquot with chloride (Fig. 3A) pictures this profile starting at 53 for ph 5-11 and curving bathochronically to 518 at ph 6-3 beyond which the maxima are little changed. The titration curve (inset) resembles the result (curve 2) of Fig. 2A for the collected data. The pigment without chloride and with its initial maximum at c c Fig. 3. A, spectral absorbances (uncorrected for dilution) for an aliquot with NaCi at varying values of ph. Initial ph was 5-11 and NaOH was added successively to give values shown. These were read off from previously calibrated ph curve. Dashed line traces successive spectral maxima. Inset: plot of spectral maxima as functions ofph. Data points (not shown) fall on curves as in Fig. 1. B, same as A for the second aliquot with no added NaCi. The two aliquots were analysed simultaneously in separate microcells under similar conditions and treatments.

7 - GECKO VISUAL PIGMENT 391 for ph 5-11 retained this position to about ph 6-3 beyond which there occurred the bathochromic shift shown by the profile (Fig. 3B). Beyond 7i25 there was a hint of the beginning of a 'blue' shift which again is part of the alkaline effect to be described shortly. Once more the inset shows the result to be like that of the chloride-deficient extracts of Fig. 2 A (curve 1). In this one experiment, therefore, one can see pictorially 2 { < X 4-82~~~~~~~ l l l l l l Fig. 4. The alkaline effect. Curve 1: chloride-deficient pigment at ph 7 5. Curve 2: aliquot of this made alkaline (ph 8 25) with NaOH. Curve 3: aliquot made acid (ph 5-97) with maleic acid. Curve 4 (continuous line) NaCl added to aliquot of curve 2 (ph 8 25). Curve 5 (@): NaCl added to aliquot of curve 3. the two functional regions discussed for the collected data of Fig. 2 A. A second similar experiment was made with another preparation but starting at a ph of 7-87, rather than 5-1 1, and adding maleic acid in successive stages to lower the ph. The two functional regions were again revealed but in this case the profiles of the change in maximum were reversed in order of appearance. The alkaline effect As already stated, the results in Figs. 2A and 3B carry the suggestion of a change in behaviour at a ph greater than 7.5. This prompted a study of the pigment's responses in this alkaline region, a study that led to the discovery of a reversible pigment change in properties that differs from that of the first effect. The nature of this alkaline effect is illustrated by an experiment (Fig. 4) in which an extract without chloride and at ph 7 5 (curve 1) was divided into two aliquots, one made alkaline to ph 8-25 (curve 2), the other acidified to ph 5*97 (curve 3). The alkaline aliquot lost considerable density while. the acidified control changed chiefly by a shift to shorter wave-lengths as expected. The loss at ph 8-25 was not irreversible for the addition of NaCl without change in ph produced a prompt restoration of pigment

8 392 F. CRESCITELLI density (curve 4). As expected, this restoration was accompanied by the typical chloride bathochromic shift with the maximum, originally at 54 (curve 1) passing to 52 (curve 4). Apparently, the addition of chloride completely restored the pigment to what it would have been without the alkaline effect since the addition of NaCl to the acidified aliquot of curve 3 led to curve 5, virtually identical with curve 4. Complete reversibility of the alkaline effect is indicated by this experiment. 23 os w_~~~~~~~~~-64.1 adetp-1 uv :N~ de oaiuto uv 3 at2h 6-81 uv 5 lq Fig. 5. Reversibility of the alkaline effect. Curve 1: extract prepared in chloride-deficient state at ph Curve 2: aliquot to ph 6-45 with maleic acid. Curve 3: aliquot with NaCi added at ph Curve 4: NaCi added to aliquot of curve 2 at ph Curve 5: aliquot of curve 3 made acid (ph 6-45) with maleic acid. Such reversibility was also obtained by acidification of an extract subjected to alkaline loss. In one such experiment (Fig. 5) the retinas were extracted with 2 digitonin made up in Tris buffer at ph 8-81 (curve 1). This extract was divided into two samples placed in separate cuvettes and analysed simultaneously. The first sample was made acid to ph 6-45 while the second sample was kept at ph 8.81 but NaCl was added to it. Both procedures led to pigment recovery, the first with a maximum at 485 (curve 2) the second with the typical bathochromic shift (curve 3). To complete the experiment NaCl was added to the aliquot of curve 2, keeping the ph at 6A45 (curve 4) and maleic acid was added to the aliquot of curve 3, changing the ph to 6-45 (curve 5). Both additions led to the expected results, the change from curve 3 to curve 5 probably representing further restoration of pigment from the alkaline effect which was present, though to a lessened degree, even in the presence of chloride. Accordingly, the alkaline loss of photopigment is not the result of irreversible damage to the opsin and assumes greater interest on this account. A further result of the alkaline treatment, suggested by the curves of Figs. 2A and 3B, is of tendency toward a 'blue' shift rather than a continuation of the bathochromic response of the first effect. Since this possibly implies a different site of action by this alkalinity I carried out several special experiments to establish the reality of this hypsochromic response. One such experiment (Fig. 6) involved an

9 GECKO VISUAL PIGMENT 393 extract without chloride prepared at ph 7-25 and divided into an aliquot with NaCl and an aliquot left in the chloride-deficient condition as prepared. Known amounts of NaOH were added in successive steps to both aliquots, the ph at each step being estimated from the previously established calibration curve. For the pigment with chloride (panel A) the spectral data are shown only for the initial (ph 7 25) and for 4 - A I I I l _ B o I,, I I Fig. 6. Protection by chloride of pigment against alkaline effect. Panel A: pigment with chloride at ph 7-25 and 8-5. Panel B: pigment without chloride at ph successively changed from 7-25 to 8-93 by the addition of NaOH. the final (ph 8 5) steps. The three intervening ones are not shown for they fell neatly in between the two. This half of the experiment makes the point that chloride stabilizes the pigment so that its spectral location changes little, if at all, and loss of pigment density is relatively minor, in confirmation of the change shown in going from curve 3 to 5 (Fig. 5). In contrast, the moiety without chloride (panel B) lost considerable density in the transition from ph 7'25 to 8-93 and suffered a shift of spectrum toward the blue. The shift in spectrum was confirmed in several other experiments including some in which the 521-pigment was light bleached and the difference spectra so obtained showed plain signs of the hypsochromic movement. The loss of photopigment in these experiments begs an answer to the question as to the nature of the product that appears with such loss. A priori, it is reasonable to conclude that this cannot be similar to the alkaline indicator yellow of light bleaching since the addition of NaCl to the latter does not result in the restoration of the gecko pigment. Is it then I1 -cis retinal freed from its bonding to the opsin so as to cause loss of pigment colour? An experiment suggesting that this in fact may be the action of this ph region is summarized in Fig. 7. An extract without chloride

10 394 F. CRESCITELLI and at ph 7 5 (curve 1) was made successively alkaline in stages to yield the spectra 2-6 of panel A at ph values from 7 5 to Concomitant with the pigment loss was the appearance and progressive growth of a product at 366. Once the pigment had been reduced to the level in curve 6, maleic acid was added to change the ph to 7-42 (curve 7, panel B). This resulted in the simultaneous restoration of photopigment 4 2 *4 -A 6-88: *42 5 ~ ~ Fig. 7. Alkaline effect and nature of product. Panel A: without NaCI, showing effect of increasing alkalinity to ph Curve 6 taken 1 hr after curve 5 at same ph. Product at 366 increases as pigment is lost. Panel B: Curve 6 is duplicate of curve 6 of panel A. Curve 7: ph to 7-42 by adding maleic acid. Curve 8: NaCl added next to give ionochromic effect. Curve 9: 11 -cis retinal in digitonin added. Explanation in text. and reduction of product-366. Subsequently, the addition of NaCl to the restored pigment led to the usual chloride shift (curve 8) without causing any significant change in the remaining product-366. In this case, apparently, restoration had occurred to its maximum and the chloride action was simply a response of the restored moiety. The final step of the experiment (curve 9) was the addition of 11 -cis retinal which led to the appearance of an absorbance resembling that of product-366 without any further increase in photopigment density. The product that appears with alkaline treatment and disappears upon acidification is spectroscopically similar to I I-cis retinal. The identity of product-366 with 11 -cis retinal was also confirmed by inactivating the former with NH2OH and showing that recovery of P521 by acidification or chloride is thereby prevented. This experiment (Fig. 8) was performed in the light

11 GECKO VISUAL PIGMENT 395 of knowledge (Crescitelli, 1979) that while NH2OH is able to destroy P521, at sufficiently low concentration this reagent reacts with retinal while still permitting regeneration of the photopigment, a reaction that takes place rapidly. Because of this it was possible to show that the density loss that followed upon treatment of an extract first with NaOH, then (NH2OH) H2SO4 (curves 1, 2; panel A) was not repaired by maleic acid (curve 3). The reality of this inhibition is made evident by the concomitant control experiment (panel B) in which no NH2OH was used and which shows the alkaline pigment loss (curve 2) and its restoration with maleic acid (curve 3). To show that the inhibitory action of NH2OH was not a damage to the opsin, o L Fig. 8. Further identification of product-366. Panel A: effect of NH2OH. Curve 1: chloride-deficient pigment at ph 7 3. Curve 2: ph shifted to 8-9 by NaOH and then (NH2OH)2H2SO4 added. Curve 3: ph shifted to 7-8 by maleic acid. Curve 4: 11-cis retinal then added at ph 7-8. Panel B: control with no added NH2OH. Curve 1: ph 7 3 comparable to curve 1 of panel A. Curve 2: ph shifted to 8 9 and recorded at same time as curve of panel A. Note that the absence of NH2OH led to less loss of pigment than in case of pigment of panel A with NH2OH. Curve 3: ph shifted to 7-8 by maleic acid and recorded at same time as curve 3 of panel A. Note greater restitution of P521 than in case of panel A. Curve 4: effect of adding 1 1-cis retinal. This shows no significant gain over that, of curve 3 showing that most of P521 had been restored by the acidification. This is in contrast with curve 4 of panel A which shows major restitution of the original pigment by 11 -cis retinal. Explanation in text.

12 396 F. CRESCITELLI thus preventing restoration, 1 1-cis retinal was added to both aliquots and the experimental (curve 4) gave a complete recovery of pigment density. The action of NH2H appears to have been a specific attack on the retinaldehyde. Altogether, these experiments support the idea of an action of alkalinity on the Schiff base, resulting in a release of the II -cis isomer in a form that is spectroscopically indistinguishable from the free isomer. An experiment similar to that of Fig. 8 was performed except that NaCl was employed as the restoring agent. The results were similar in demonstrating the inhibition of restoration by NH2H. The only difference between the NaCl and maleic acid treatments was the failure of added 1 -cis retinal to effect major recovery after NaCl. This is not unexpected since the procedure made at alkaline ph might be at a ph value removed from the optimum for regeneration. Apparently, NaCl is not able to eliminate this aspect of ph sensitivity. The central point of both types of experiments is in support of the idea that pigment restoration due to NaCl and maleic acid is due to recombination in Schiff base form of the released prosthetic group which is still in its native 11 -cis configuration. One additional, but not unimportant, detail may be added to this description of the alkaline effect. It is the anion-specific nature of the restoration. At concentrations of NaCl and NaBr that were more than required to produce maximal restoration, nitrate, fluoride, iodide and sulfate were ineffective. This is similar to the anion bathochromic shift (Crescitelli, 1977 a), to the NH2OH-protective action (Crescitelli, 198a) and to the inhibition of thermal destruction (Crescitelli, 198a). DISCUSSION This study, once again, emphasizes the apparent biological singularity of the gecko rod visual system. Added to the already known results (Crescitelli, 1977 a) the ph behaviour serves to set apart the gecko 521 -pigment from the rhodopsins in which no such ph effects have been observed. One photopigment, however, resembles the gecko system in a number of properties and that is the chicken cone pigment iodopsin (Fager & Fager, 1979; Knowles, 198). A ph effect has not yet been investigated in iodopsin although it does appear to have a more restricted range of ph stability than chicken rhodopsin (Wald, Brown & Smith, 1955). It would be no surprise to learn that idopsin, like the gecko pigment, has ph indicator properties and if this is ever discovered it would add one more item to those already adumbrated that suggest a phylogenetic relationship between fully evolved cones and the so-called transmuted or intermediate rods of geckos. From the chemical point of view the ph sensitivity of pigment colour is of special interest. Unlike rhodopsin, the gecko pigment has a more open structure that permits entrance to certain critical sites involved in adjusting colour and maintaining the integrity of the molecule. The Schiff base appears to be the site of attack by NH2H and NaBH4. This cannot be the case for the first ph effect because the spectral shift to acid is the reverse of that expected for an increased protonation of the Schiff base nitrogen. Accordingly, the site for this first ph effect is probably the structures postulated to modulate the colour via a seconday, non-covalent interaction between protein and prosthetic group (Dartnall, 1957; Hubbard, 1958; Dartnall & Lythgoe,

13 GECKO VISUAL PIGMENT ). A possible group for this interaction is the imidazole of histidine. Radding & Wald (1956) reported the appearance, after bleaching rhodopsin, of an acid-binding group with a pk of 6-6, and subsequently the possible role of proton binding by imidazole was considered for the meta-i to meta-il transition in rhodopsin bleaching (Matthews, Hubbard, Brown & Wald, 1963). The titration curve for the chloridedeficient unbleached gecko pigment (Fig. 2A) with its mid-point near ph 6-5 could be interpreted in the same manner, but this suggestion must be tempered by the knowledge that the dissociation constants of amino acids within the polypeptide enviroent are not necessarily comparable with the values of the free amino acids. Whatever may be the site of proton exchange in the chloride-deficient system, it is clear that a significant change occurs upon adding chloride (Fig. 2A). This could be the result of a chloride-induced conformational change exposing a functional group with a different pk or it could be a shift of the pk of the original site brought on by a change in the electrostatics of the system. Either of these ideas may be related to the double point charge model of Honig, Dinur, Nakanishi, Balogh-Nair, Gawinowicz, Arnaboldi & Motto (1979) which led to the proposal that photoisomerization induces a charge separation with subsequent changes in the pk values of specific groups (Honig, Ebrey, Callender, Dinur & Ottolenghi; 1979). For the gecko photopigment charge separation could be the result of a chloride conformational action. The hypsochromic shift of the alkaline effect (Fig. 6) implies a different site of action by ph. Along with the appearance of product-366, indistinguishable from 1 1 -cis retinal, the implication is that the aldimine bond itself is directly assaulted, resulting in deprotonation and possible rupture of the bond. Furthermore, the ease and rapidity of pigment recovery with lowering of ph or adding chloride (Figs. 4, 5) suggest that the 11 -cis retinal remains anchored within the stereospecific cleft of the opsin possibly held by fixation of the beta-ionone as proposed by Matsumoto & Yoshizawa (1975). Although simple Schiff bases are believed to have pk values between 6 and 8 (Favrot, Sandorfy & Vocelle, 1978) this may not be the case for the aldimine bond of the visual pigments. The influence of neighbouring groups may shift the pk to higher values and this, in fact, was proposed for bacteriorhodopsin (Ehrenberg, Lewis, Porta, Nagle & Stoeckenius, 198). In any case there is no reason to discount the possibility of a Schiff base attack in the alkaline effect. Moreover, the readiness with which chloride was able to restore pigment colour could be the result of a specific conformational change bringing a proton donating group closer to the Schiff base nitrogen. Such reprotonation has been postulated for bacteriorhodopsin in its light-driven proton exchange (Lewis, Marcus, Ehrenberg & Crespi, 1978). Finally, attention may be directed to the specificity of the chloride in protecting pigment colour and integrity. To the protective actions of this anion against attack by NH2OH, by temperature, by nitrate and in the dark exchange of its prosthetic moiety (Crescitelli, 1977a, 198a) may now be added the greater stability of the native pigment colour with chloride at lower ph values (Fig. 2A) and the prevention or reversal, by chloride, of the alkaline pigment loss (Figs. 5, 6). These are all anion-specific responses to chloride and bromide but not to fluoride, iodide or sulphate. Nitrate, with its ability to induce a 'blue' shift, (Crescitelli, 198b) is able to antagonize the chloride bathochromic shift, to produce a small protection against

14 398 F. CRESCITELLI temperature (Crescitelli, 198a) and in connection with the alkaline effect of this report is able to restore some pigment colour if the nitrate concentration is sufficiently high. In terms of sensitivity even the latter is not comparable to the chloride action. The nature of these highly specific, non-lyotropic actions, is suggestive of a conformational change associated with electrostatic effects at more than one site. If ever the number and nature of these pigment adjusting sites are discovered the knowledge so gained will be of interest beyond the scope of visual science. REFERENCES BOwNDS, D. (1967). Site of attachment of retinal in rhodopsin. Nature, Lond. 216, COLLINS, F. D. & MORTON, R. A. (195). Studies on rhodopsin. 2. Indicator yellow. Biochem. J. 47, CRESCITELLI, F. (1974). The gecko visual pigments. I. The thermosensitive property. Vision Res. 14, CRESCITELLI, F. (1975). The gecko visula pigments. II. Colour and the sulphydryl group. Vision Res. 15, CRESCITELLI, F. (1977a). The visual pigments of geckos and other vertebrates: an essay in comparative biology. In Handbook of Sensory Physiology, VII/5, ed. CRESCITELLI, F., pp Berlin-Heidelberg: Springer-Verlag. CRESCITELLI, F. (1977 b). Ionochromic behaviour of gecko visual pigments. Science, N. Y. 195, CRESCITELLI, F. (1979). The gecko visual pigments. The behavior of opsin. J. gen. Physiol. 73, CRESCITELLI, F. (198a). The two visual pigments of the gecko: the labile behviour. J. comp. Physiol. 138, CRESCITELLI, F. (198b). The gecko visual pigments: the nitrate effect. Vision Res. 2, CRESCITELLI, F., DARTNALL, H. J. A. & LoEw, E. R. (1976). The gecko visual pigments: a microspectrophotometric study. J. Physiol. 268, DARTNALL, H. J. A. (1957). The Visual Pigments. London: Methuen. DARTNALL, H. J. A. & LYTHGOE, J. N. (1965). The spectral clustering of visual pigments. Vision Res. 5, EHRENBERG, B., LEWIS, A., PORTA, T. K., NAGLE, J. F. & STOECKENIUS, W. (198). Exchange kinetics of the Schiffbase proton in bacteriorhodopsin. Proc. natn. Acad. Sci. U.S.A. 77, FAGER, L. Y. & FAGER, R. S. (1979). Halide control of colour of the chicken cone pigment iodopsin. Expl Eye Res. 29, FAVROT, J., SANDORFY, C. & VOCELLE, D. (1978). An infrared study of the basicity of non-aromatic imines: relation to visual pigments. Photchem. & Photobiol. 28, HONIG, B., DINUR, U., NAKANISHI, K., BALOGH-NAIR, V., GAWINOWICZ, M. A., ARNABOLDI, M. & MOTTO, M. G. (1979). An external point-charge model for wavelength regulation in visual pigments. J. Am. chem. Soc. 23, HONIG, B., EBREY, T., CALLENDER, R. H., DINUR, U. & OTTOLENGHI, M. (1979). Photoisomerization, energy storage, and charge separation: A model for light energy transduction in visual pigments and bacteriorhodopsin. Proc. natn. Acad. Sci. U.S.A. 76, HUBBARD, R. (1958). On the chromophores of the visual pigments. In Visual Problems of Colour, National Physical Lab. Symposium no. 8, pp , London: H.M.S.O. KNOWLES, A. (198). The chloride effect in chicken red cone receptors. Vision Res. 2, LEWIS, A., MARCUS, M., EHRENBERG, B. & CRESPI, H. (1978). Experimental evidence for secondary proton-chromophore interactions at the Schiff base linkage in bacteriorhodopsin: molecular mechanism for proton pumping. Proc. natn. Acad. Sci. U.S.A. 75, LYTHGOE, R. J. (1937). The absorption spectra of visual purple and of indicator yellow. J. Physiol. 89, MATSUMOTO, H. & YOSHIZAWA, T. (1975). Existence of a beta-ionone ring-binding site in the rhodopsin molecule. Nature, Lond. 258,

15 GECKO VISUAL PIGMENT 399 MATTHEWS, R. G., HUBBARD, R. BROWN, P. K. & WALD, G. (1963). Tautomeric forms of metarhodopsin. J. gen. Physiol. 47, RADDING, C. M. & WALD, G. (1956). Acid-base properties of rhodopsin and opsin. J. gen. Physiol. 39, WALD, G. (1938). On rhodopsin in solution. J. yen. Physiol. 21, WALD, G., BROWN, P. K. & SMITH, P. H. (1955). lodopsin. J. yen. Physiol. 38,