THE OXIDATION OF CATECHOL BY TYROSINASE

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1 THE OXIDATION OF CATECHOL BY TYROSINASE BY CHARLES I. WRIGHT AND HOWARD S. MASON (From the Division of Physiology, National Institute of Health, and the Dermatoses Section, Industrial Hygiene Division, Bureau of State Services, United States Public Health Service, Bethesda, Maryland) (Received for publication, May 14, 1946) Several investigators (l-5) have concluded that the complete oxidation of catechol in the presence of tyrosinase requires only 2 atoms of oxygen per molecule. M7e have reinvestigated this enzymic reaction and have found the consumption of oxygen to be dependent upon the conditions of the experiment. Under some circumstances, it may be as high as 3 atoms per molecule. EXPERIMENTAL The oxygen consumption measurements were made with Barcroft differential type manometers fitted with side arm flasks of approximately 18 ml. capacity. The total volume of fluid in each flask was always brought to 3.0 ml. by varying the quantity of buffer solution added to the main compartment of the flask. The tyrosinase was diluted with water until 0.5 ml. contained the desired concentration of the enzyme and thii quantity was then delivered into the side arm of the flask. After introduction of the solutions, the manometer flasks were immersed in a water bath at 25.6 and flushed out with water-saturated oxygen for 5 minutes while shaking at a rate of 110 oscillations per minute. After closing the manometers, 5 minutes were allowed to assure equilibrium. The enzyme wa8 then tipped from the side arm and the measurement of the rate of the reaction begun. The volumes of the flasks and manometer tubes were determined from the weight of mercury required to fill them and the conversion factors for O2 and COZ calculated according to Dixon (6). As a further check the COZ factors were determined by tipping an excess of HzS04 on a known amount of NaHC03. The greatest difference between any pair of the COt factors determined by the two methods was less than 2 per cent. The 1.0 ml. pipette used to measure the catechol solutions delivered with an error of less than 0.5 per cent. Two tyrosinase preparations were employed in thii study. The first (enzyme I) was made by extracting the natural substrate from frozen, ground mushrooms (7). The subsequent steps followed were those described by Ludwig and Nelson (4) to the point at which our preparation 4s

2 46 OXIDATION OF CATECHOL BY TYROSINASE corresponded to their Preparation ClOlA. It initially contained 311 catecholase units per ml., or 104 units per mg. of dry weight, as determined by the chronometric method of Miller et al. (7). At the time it was employed in this study some months later, it contained 273 catecholase units per ml., or 90 units per mg. of dry weight. Enzyme II was obtained by following the directions of Ludwig and Nelson (4) throughout and again corresponded to their Preparation ClOlA. It contained 96.5 catecholase units per ml., or 121 units per mg. of dry weight. This enzyme was used within a few days of its preparation. The catechol was obtained from the Eastman Kodak Company, and melted at (corrected). 200, I I I I I I I I I I I I I I I I I I I I I I 0 IO so 100 MINUTES FIG. 1. The oxygen consumed during the enzymic oxidation of 0.73 mg. of catechol with increasing quantities of tyrosinsse. The number of catecholase units is indicated for each curve. McIlvain s phosphate-citrate buffer was used (ph 5.1). Results The measurement of the oxygen consumption during the enzymic oxidation of catechol was carried out in a series of experiments in which enzyme concentration, catechol concentration, and ph were systematically varied. Fig. 1 shows the results obtained when the enzyme concentration was varied from 2.1 to 137 catecholase units per 3 ml., while the amount of catechol was fixed at 0.73 mg. and the ph at 5.1. The total oxygen consumed increased from 57 c.mm. with 2.1 enzyme units to 188 c.mm. with

3 C. I. WRIGHT AND H. S. MASON units. With 137 units, 180 cmm. were consumed. The volumes of oxygen utilized at each enzyme concentration, as shown in Fig. 1, were recalculated in terms of atoms of oxygen consumed per molecule of catechol. The data so obtained, up to 34 units of enzyme, are plotted in Fig. 2. The atoms of oxygen consumed increased rapidly to 2.5 wit h approximately 10 units of enzyme. Further increase in the concentration of enzyme resulted in no further increase in the oxygen taken up. The results obtained with fixed enzyme concentration and varying concentrations of catechol at different hydrogen ion concentrations are 3.0r-----l FIG.~. Atoms of oxygen consumed per molecule of catechol as a function of catecholase units in 3 ml. of reaction volume. shown in Figs. 3, 4, and 5. The enzyme concentration was fixed in these experiments at 41 units because at this value it was unlikely that the total oxygen consumed would be a critical function of the enzyme concentration (Fig. 2). The upper graph in Fig. 3 shows the results obtained at ph 3.1. Only 0.36 atom of oxygen was consumed per molecule of catechol oxidized, but the enzyme was found to be rapidly inactivated at this hydrogen ion 1 After this manuscript was completed, a sample of tyrosinase was kindly furnished us by Dr. Irwin Sizer. This sample was described as containing 2750 chronometric catecholase units per ml., or per 2.3 mg. of extractive solids. The experiments described in Figs. 1 and 2 were repeated with this preparation. The number of atoms of oxygen consumed per molecule of catechol reached 2.53 at 34 units of enzyme. Higher concentrations of enzyme gave the same value.

4 48 OXIDATION OF CAT%%02 BY TYROSINASE concentration. This inactivation was proved by the addition of more catechol from the second side arm of the manometer flask without further consumption of oxygen. At each of the other hydrogen ion concentrations the enzyme was shown to be active at the conclusion of the experiments. The lower graph in Fig. 3 shows the results obtained at ph 4.5. The PH 3.1 mz 300 B z i %./.-e-2.45 p-tlr-p2.40 z 200 : 0 o--o-o t ii :: IO t-t A-A--A-A MINUTES FIG. 3. The oxygen consumed in the oxidation of increasing concentrations of catechol at ph 3.1 and 4.5, with enzyme concentration fixed at 41 catecholase units. Reading from the lowest curve up, the amounts of catechol used per 3 ml. of reaction volume were 0.18 mg., 0.37 mg., 0.73 mg., 1.10 mg., and 1.46 mg. McIlvain s phosphate-citrate buffer was used. The atoms of oxygen consumed per molecule of catechol are indicated for each curve. number of atoms of oxygen consumed per molecule of catechol varied between 2.45 and The highest value was obtained with the lowest concentration of catechol. In order to prove that the number of atoms of oxygen consumed was not a characteristic peculiar to the enzyme initially employed, two preparations were used to obtain the data illustrated in Fig. 4. The data given

5 C. I. WRIGHT AND H. 6. MASON 49 in the left half of the figure were obtained with enzyme I. The atoms of oxygen consumed per molecule of catechol varied between 2.46 and In the right half of t.he same figure are the data obtained with enzyme II. The atoms of oxygen consumed per molecule of catechol varied in this case between 2.47 and The experimental conditions for these two sets of data were the same, except for the enzyme preparation, and the results were identical. 400 I I I PH 5.1 ENZYME I to.z L g 200 r 2 z p--p-o 2.6C 2 si 100 /A-A-A-A 2.64 z 0 0 2s IOU MINUTES FIG. 4. The oxygen consumed in the oxidation of increasing concentrations of catechol by two enzyme preparations at ph 5.1 (McIlvdn s buffer). The concentration of enzyme I was 41 catecholase units per 3 ml. and the concentration of enzyme II wee 39 catecholase units per 3 ml. Reading from the lowest curve up, the amounts of catechol per 3 ml. were 0.11, 0.18,0.37,0.73,1.10, and 1.46 mg. 2.6% Fig. 5 shows the results obtained at ph 7.0 and at ph 7.9. It was found necessary at these hydrogen ion concentrations to extend the duration of the measurements because a slow but appreciable oxidation proceeded for hours after the initial rapid reaction. At ph 7.0, the atoms of oxygen consumed per molecule of catechol at 46 minutes varied between 1.98 and 2.37; at 30 hours these values varied between 2.35 and At ph 7.9 the atoms of oxygen consumed at 55 minutes varied between 1.97 and 2.34; at 36 hours these values then varied between 2.34 and 3.01.

6 50 OXIDATION OF CATECHOL BY TYROSINASE Again, the highest consumption of oxygen per molecule of catechol was obtained with the lowest concentrations of catechol. The experiments Ii--lreported above were controlled by adding 41 units of enzyme to the buffer (ph 7.0) in the absence of catechol. No significant oxygen consumption was observed over a period of 7 hours. As a further control, to obviate the possibility of the oxidation of citrate, an experiment I I I I I I I I ph 7.0 ph r MINUTES 2.54 I I I I 1 I IO s 30 HOURS F b-a-+-d I I MINUTES /- d---a--a,-+- 1 I I I I I I I I 1 I I I-IOURS FIG. 5. The oxygen consumed in the oxidation of increasing concentrations of catechol at ph 7.0 and at 7.9 (McIlvain s buffer), with enzyme concentration fixed at 41 catecholase units per 3 ml. Reading from the lowest curve up, the amounts of catechol per 3 ml. were 0.18 mg., 0.37 mg., 0.73 mg., 1.10 mg., and 1.46 mg. The atoms of oxygen consumed per molecule of catechol are indicated at approximately 50 minutes and again at the termination of the experiment. was carried out with phosphate buffer at ph 5.1. The results of this experiment and a simultaneous experiment with citrate-phosphate buffer at the same ph are shown in Fig. 6. While the kinetics of the two reactions differed, the numbered atoms of oxygen consumed per molecule of catechol were approximately the same at 145 minutes. DISCUSSION Fig. 2 shows that with fixed catechol concentration (0.73 mg. per 3 ml.) the amount of oxygen consumed increases with the amount of enzyme used,

7 C. I. WRIGHT AND H. S. MASON 51 up to 2.5 atoms with 10 units. Beyond 10 units there is no further increase in oxygen utilization. The first part of this curve corroborates Graubard and Nelson (8,9) who showed that, within limits, by increasing the amount of enzyme a greater quantity of oxygen was consumed. The results they obtained did not, however, reach 2.0 atoms of oxygen per molecule of catechol. Robinson and McCance (1) in a single experiment showed that under the conditions employed 2 atoms of oxygen were consumed per molecule. The same value was later obtained by Wagreich and Nelson (2), Dawson and Nelson (3), Ludwig and Nelson (4), and Kubowita (5). However, none of these authors published a systematic study of the effect of varying enzyme and catechol concentration upon the total consumption of oxygen. The low consumptions of oxygen at ph 3.1 (Fig. 3) are the results of the inactivation of the enzyme and confirm the findings of Graubard and Nelson (8). By comparing the results in Figs. 3, 4, and 5 it may be noted that as the ph was increased the initial rate of oxidation was increased, but the time required for complete oxidation was increased. It seems therefore that, of the reactions involved in the complete oxidation of catechol by tyrosinase, at least one is accelerated by increasing the concentration of hydroxyl ions, while another is accelerated by increasing the concentration af hydrogen ions. Furthermore, these reactions are rate-determining steps in the over-all oxidation. From the results as a whole, it may be seen that two conditions lead to high consumption of oxygen per molecule of catechol. These are high enzyme concentration and low catechol concentration. Figs. 3, 4, and 5 show that the highest oxygen consumptions per molecule of catechol were found when the lowest concentrations of catechol were used. It may be pointed out that our ratios of enzyme to substrate concentration were, in every instance of complete oxidation, greater than in any previously reported. Some other possibilities may be advanced to explain our relatively high values for oxygen consumption. One is that some oxidizable substance has been added with the enzyme. This, however, seems to be precluded by the fact that the addition of 13 times as much enzyme as is required for the consumption of 2.5 atoms of oxygen per molecule of catechol does not further increase this value (Fig. 1). Another possibility is that the citrate of the buffer was oxidized by the tyrosinase-catechol system. This also seems unlikely because the enzymic oxidation of catechol in the absence of citrate (Fig. 6) resulted in the consumption of 2.5 atoms of oxygen. Furthermore, the addition of 41 units of enzyme to phosphate-citrate buffer in the absence of catechol resulted in no significant consumption of oxygen.

8 52 OXIDATION OF CATECHOL BY TYROSINASE Fig. 6 illustrates that the rates of the reaction in the two buffer systems were different. The rate of oxidation in phosphate buffer changed sharply at a consumption of approximately 1 atom per molecule of catechol. A similar but much more pronounced change has been observed by Dawson and Nelson (3) and interpreted by them (10) as due to a slow hydration 3- I I I i J >- I 4 D.- )- I I MINUTES Fro. 6. The effect of type of buffer on the rate and total oxygen consumption in the enzymic oxidation of catechol at ph 5.1. In the experiment indicated by + McIlvain e buffer and 0.72 mg. of catechol were used with 14 catecholase units; in the experiment indicated by 0 the buffer wa8 0.2 M NaHzPOd-NtiHP04, and 0.75 mg. of catechol w&8 used with 14 catecholase units of enzyme. The atoms of oxygen consumed per molecule of catechol are indicated for each curve. of o-benzoquinone at ph 5 in dilute solutions. Since only the nature of the buffer was changed in the experiments illustrated in Fig. 6, it is evident that a change of rate at 1 atom may also be produced by a change in the buffer system. The fact that more than 2 atoms of oxygen were consumed during the complete enzymic oxidation of catechol, together with the failure to detect hydroxy-p-quinone spectroscopically during this oxidation (1 l), is not

9 C. I. WRIGHT AND H. S. MASON 53 consistent with the mechanism of oxidation proposed by Wagreich and Nelson (2). We have also determined the oxygen consumed during the enzymic oxidation of hydroxyhydroquinone and have found that 2 atoms are utilized per molecule. These results, together with others bearing upon the mechanism of the enzymic oxidation of catechol and hydroxyhydroquinone, will be published in subsequent papers. SUMMARY 1. The oxygen consumed during the oxidation of 0.73 mg. of catechol in the presence of tyrosinase was dependent upon the concentration of enzyme, only up to 3.3 catecholase units per ml. Further increase in enzyme concentration produced no further increase in oxygen consumption. 2. The number of atoms of oxygen consumed during the complete oxidation of a molecule of catechol in the presence of tyrosinase was dependent upon the concentration of catechol, being highest at the lowest concentration. 3. The kinetics of the oxidation of catechol in the presence of tyrosinase is dependent upon the hydrogen ion concentration and upon the nature of the buffer. 4. Under the conditions of ph, catechol concentration, and enzyme concentration employed, the values for the number of atoms of oxygen consumed during the complete enzymic oxidation of a molecule of catechol varied between 2.34 and We wish to thank Anne H. Wright for technical assistance in the conduct of the manometric measurements. BIBLIOGRAPHY 1. Robinson, M. E., and McCance, R. A., Biochem. J., (1925). 2. Wagreich, H., and Nelson, J. M., J. B&d. Chem., 116,459 (1936). 3. Dawson, C. R., and Nelson, J. M., J. Am. Chem. SOL, 60,250 (1933). 4. Ludwig, B. J., and Nelson, J. M., J. Am. Chem. Sot., 61,260l (1939). 5. Kubowitz, F., Biochem. Z., 262, 221 (1937); 299, 32 (1938). 6. Dixon, M., Manometric methods, Cambridge, 2nd edition, 27 (1943). 7. Miller, W. H., Mallette, M. F., Roth, L. J., and Dawson, C. R., J. Am. Chem Sot., 66, 514 (1944). 8. Graubard, hf., and Nelson, J. M., J. Biol. Chem., 111, 757 (1935). 9. Graubard, M., and Nelson, J. M., J. Biol. Chem., 112, 135 ( ). 10. Nelson, J. M., and Dawson, C. R., in Nord, F. F., and Werkman, C. H., Advances in enzymology and related subjects, New York, 4, 112 (1944). 11. Mason, H. S., Schwartz, L., and Peterson, D. C., J. Am. Chem. Sot., 67, 1233 (1945).

10 THE OXIDATION OF CATECHOL BY TYROSINASE Charles I. Wright and Howard S. Mason J. Biol. Chem. 1946, 165: Access the most updated version of this article at Alerts: When this article is cited When a correction for this article is posted Click here to choose from all of JBC's alerts This article cites 0 references, 0 of which can be accessed free at ml#ref-list-1

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