VITAMIN B 12. Cobalamin. 2007/11/7 Cobalamine /11/7 Cobalamine 2

Transcription

1 VITAMIN B 12 Cobalamin 2007/11/7 Cobalamine /11/7 Cobalamine 2 1

2 (Bio)chemistry: Corrinoid: Corrin nucleus with 4 reduced pyrrol rings linked together Cobalt: linking the corrin ring through the 4 N on each pyrrol ring, 5 th bond attached to a nucleotide, the group on the 6 th bond is variable. 2007/11/7 Cobalamine 3 pyrrol ring 2007/11/7 Cobalamine 4 2

3 (Bio)chemistry coenzyme form: 1. methylcobalamin 2. adenosylcobalamin Variety of cobalamin methylcobalamin: Co-CH 3 medicine: cyanocobalamin: Co-CN hydroxycobalamin: Co-OH (commercial) Human body can convert other form to active form 2007/11/7 Cobalamine 5 Food source animal product bacteria, fungi, algae, not in plant product or in yeast 2007/11/7 Cobalamine 6 3

4 Digestion and Absorption 1-3% passive diffusion may occur; 80% absorption in low intake (~ 0.1μg/day) Less than 3% in high amount intake ( > mg) *Enterohepatic circulation: reabsorb cobalamin in the bile and intestinal secretion. 2007/11/7 Cobalamine 7 Digestion and Absorption In stomach, protein hydrolysis (pepsin) B 12 released from food intrinsic factor (IF): glycoprotein by gastric parietal cells 1. complexed with R protein (cobalophilin, baptocorrins in body fluid) small intestine 2. R protein hydrolyzed by pancreatic protease (insufficient cause malabsorption) B 12 released 3. complexed with IF (proximal small intestine) bind to ileum receptor 4. delayed 3-4 hrs absorbed, peak level in blood for 8-12 hr 5. it is stored mainly in the liver in amounts (3-5mg) sufficient to last a couple of years. 2007/11/7 Cobalamine 8 4

5 2007/11/7 Cobalamine 9 Cobalamin digestion Dietary cobalamin, obtained through animal foods, enters the stomach bound to animal proteins. Pepsin and hydrochloric acid in the stomach sever the animal protein, releasing free cobalamin. Most of the free cobalamin is then bound to R- protein, which is released from the parietal and salivary cells. Intrinsic factor (IF) is also secreted in the stomach, but its binding to cobalamin is weak in the presence of gastric and salivary R-protein. 2007/11/7 Cobalamine 10 5

6 Cobalamin absorption In the duodenum Dietary cobalamin bound to R-protein is joined by cobalamin-r-protein complexes that have been secreted in the bile. Pancreatic enzymes degrade both biliary and dietary cobalamin R-protein complexes, releasing free cobalamin. The cobalamin then binds with intrinsic factor. The cobalamin-intrinsic factor complex remains undisturbed until the distal 80 cm of the ileum. 2007/11/7 Cobalamine 11 Cobalamin transport In the ileum The cobalamin intrinsic factor complex attaches to mucosal cell receptors (cubilin) and the cobalamin is bound to transport proteins known as transcobalamin I, II and III (TC I, TC II and TC III ). Transcobalamin II, although it represents only a small fraction (about 10%) of the transcobalamins, is the most important because it is able to deliver cobalamin to all cells in the body. The cobalamin is subsequently transported systemically via the portal system. 2007/11/7 Cobalamine 12 6

7 Cobalamin metabolism Within each cell the transcobalamin II cobalamin complex is taken up by means of endocytosis and the cobalamin is liberated and then converted enzymatically into its 2 coenzyme forms, methylcobalamin and adenosylcobalamin. *Nitrous oxide a general anesthetic, causes multiple defects in cobalamin use, most of which are intracellular and clinically relevant only in people who have low or borderline-low serum cobalamin levels. 2007/11/7 Cobalamine 13 Transcobalamins TC I : circulating storage form (90% of cobalamin bound) TC II : transfer cobalamin to tissues (synthesized in liver) TC III : deliver cobalamin from peripheral tissue to liver Blood: 60~80% methylcobalamin, 20% adenosylcobalamin, cyano-, hydroxycobalamin TC II -cobalamin complex endocytosed by cell TC II degraded by lysosome hydroxycobalamin released into cytoplast methylcobalamin (mitochondria) react with ATP 5'-deoxyadenosylcobalamin 2007/11/7 Cobalamine 14 7

8 2007/11/7 Cobalamine 15 Storage Adenosyl- > hydroxo-, methyl- Liver (receptor for haptocorrin and TC II ) > muscle, bone, kidney, heart, brain, spleen Haptocorrin : circulating storage form of B /11/7 Cobalamine 16 8

9 Excretion 0.1%/day whole-body turnover (loss through bile to feces, not catabolism, little in urine) 65~75% of cobalamin (~5 g/day) in bile is reabsorbed 2007/11/7 Cobalamine 17 Functions -- Coenzymes reactions: Homocysteine Methionine: (in cytosol) methionine synthetase (methylcobalamin dependent transferase) cobalamin-apotransferase methylcobalamintransferase (CH 3 - from 5-CH 3 -THF) "Folate methyl trap hypothesis": 5-methyl THF accumulate, THF can not be regenerated 2007/11/7 Cobalamine 18 9

10 H 4 folate is converted to N 5 -methyl-h 4 folate in a number of different reactions as it accepts methyl groups. The methyl group can only be removed and the H 4 folate regenerated by the above reaction. (See folic acid) 2007/11/7 Cobalamine 19 Metabolism of homocysteine (CBS, cystathionine beta-synthase; MS, methionine synthase; MTHFR, methylenetetrahydrofolate reductase; SAH, S-adenosylhomocysteine; SAM, S-adenosylmethionine.) 2007/11/7 Cobalamine 20 10

11 2007/11/7 Cobalamine 21 Functions -- Coenzymes reactions methylmalonyl CoA succinyl CoA (in mitochondria) methylmalonyl CoA mutase: metabolism of oddchain fatty acid, some amino acids (methionine, isoleucine, threonine) defect in mutase, adenosylcobalamin synthesis or B 12 deficiency MMCoA & MMA 2007/11/7 Cobalamine 22 11

12 Functions -- Coenzymes reactions -leucine (intestinal bacteria) L-leucine leucine aminomutase (5'-deoxyadenosyl cobalamin required) 2007/11/7 Cobalamine 23 Deficiency 1. Dietary deficiency is rare (except for vegans). 2. Deficiency more commonly due to an absorption problem. 3. Auto immune disease can destroys the parietal cells that secrete the I.F. (75%) required for absorption. 4. Treatment is life long injections of vitamin B 12 (oral administration wouldn't be much use!) 5. Risk: elderly, alcoholics, gastrectomy patient, taperworm infection infant, very young child 6. RDA : 2 g/day; 1 g/day injection for pernicious anemia patient, no recorded for toxicity 2007/11/7 Cobalamine 24 12

13 Deficiency Hematologic: pernicious anemia (megaloblastic anemia); elevated bilirubin and LDH with low or normal retic count; low normal platelet count and WBC count; hypersegmented neutrophils; Bone marrow is hypercellular with megaloblastic erythroid hyperplasia and giant metamyelocytes. Pernicious anemia: 95% caused by inadequate absorption (USA); vegetarian : > 20~30 years. 2007/11/7 Cobalamine 25 Megaloblastic anemia 2007/11/7 Cobalamine 26 13

14 Deficiency reasons Impaired methionine synthase traps H 4 folate as N 5 - methyl-h 4 folate ("folate trap"). This can lead to a secondary or artificial deficiency of folic acid and develop further anemia. Impaired methylmalonyl CoA mutase causes accumulation of unusual odd number carbon fatty acids. These accumulate in nerve cell membranes (demyelination of nerves) causing irreversible neurological disorders. SAM (S-adenosylmethionine) required for methylation, myelin maintenance 2007/11/7 Cobalamine 27 Deficiency between B 12 and Folic acid Anaemia due to a true folic acid deficiency it is megaloblastic anemia. Anaemia due to a true folic secondary folic acid deficiency caused by primary B 12 deficiency is pernicious anemia. 2007/11/7 Cobalamine 28 14

15 Deficiency Neurologic: Subacute combined degeneration (SCD) of dorsal/lateral spinal columns usually presents as paresthesias, numbness, and ataxia associated with loss of vibratory sense and proprioception, symmetrical in nature and affecting LEs more than UEs. Result to severe weakness, spasticity, clonus, paraplegia, incontinence, and rarely optic neuritis. Can include neuropsychiatric symptoms including personality changes, memory/cognitive impairment, and dementia. 2007/11/7 Cobalamine 29 Deficiency prevention Anemia could be due to folic acid deficiency or vitamin B 12 deficiency. In either case folic acid would cure the anemia but if the true underlying deficiency involved vitamin B 12 the patient would still go on to develop the irreversible neurological disorders. For this reason such patients are always given folic acid and vitamin B 12 supplements until the true cause of the anemia is identified. 2007/11/7 Cobalamine 30 15

16 2007/11/7 Cobalamine 31 Megaloblastic anemia 2007/11/7 Cobalamine 32 16

17 B 12 Assessment & Diagnosis Serum B 12 RBC MCV Serum methylmalonic acid (MMA) Serum homocysteine level Antiparietal cell and intrinsic factor antibodies Schilling test 2007/11/7 Cobalamine 33 B 12 Assessment & Diagnosis Serum B 12 levels helpful but do not necessary rule out diagnosis of B 12 deficiency; likewise, specificity of a low B 12 in ruling in disease is variable. B 12 levels <200 pg/ml reportedly had almost 60% specificity, while levels <100 pg/ml had 90% specificity according to one study. RBC MCV Poor sensitivity and specificity to rule in or rule out disease, particularly with early or mild disease. 2007/11/7 Cobalamine 34 17

18 B 12 Assessment & Diagnosis Serum methylmalonic acid (MMA): B 12 catalyzes conversion of methylmalonyl-coa to succinyl-coa; absence of B 12 causes accumulation of methylmalonyl-coa and subsequent conversion to methylmalonic acid (MMA), resulting in elevated serum MMA levels in B 12 deficiency states. MMA elevation may precede development of clinical symptoms of B 12 deficiency and decrease in cobalamin level. Pathologic elevation is considered when levels are >3 SD above the mean. Levels normalize within 7-14 days of initiating therapy. 2007/11/7 Cobalamine 35 B 12 Assessment & Diagnosis Serum homocysteine level: Methionine is synthesized from homocysteine using B 12 as a cofactor; elevated levels are often found in both folate and B 12 deficiency. Antiparietal cell and intrinsic factor antibodies: Antiparietal cell antibodies are sensitive (~90%) but have poor specificity for the diagnosis of pernicious anemia; Anti-IF antibodies are confirmatory for the diagnosis if positive; however, sensitivity is only ~50%. 2007/11/7 Cobalamine 36 18

19 B 12 Assessment & Diagnosis Schilling test (1): Nuclear medicine test which documents decreases in GI absorption of radiolabelled B mg of radiolabelled cyanocobalamin is given orally followed by 1000 g IM B 12 1 hour later; 24 hour urine collection is then performed for excreted radiolabelled cyanocobalamin. Normal excretion = 10-35% of radiolabelled dose. <9% excretion indicates malabsorption. Renal insufficiency can cause false-negative results. 2007/11/7 Cobalamine 37 B 12 Assessment & Diagnosis Schilling test (2): Second stage of Schilling test involves addition of oral IF, which can help to differentiate between intestinal malabsorption and pernicious anemia (should normalize B 12 absorption in those with pernicious anemia but not intestinal malabsorption. Should be performed after several weeks of B 12 replacement. 2007/11/7 Cobalamine 38 19

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