STUDIES ON THE ROLE OF VITAMIN B12 FOR THE SYNTHESIS OF METHIONINE IN LIVER

Vitamin B(12) in photosynthetic bacteria and methionine synthesis by Rhodopseudomonas spheroides

1. Assay of some photosynthetic bacteria for vitamin B(12) showed them to be relatively rich in this factor. Rhodopseudomonas spheroides, grown photosynthetically in Co(2+)-supplemented medium, contained about 100mug./g. dry wt. 2. Extracts of wild-type Rps. spheroides methylated homocysteine by a mechanism similar to the cobalamin-dependent pathway present in Escherichia coli. However, no mechanism similar to the cobalamin-independent N(5)-methyltetrahydrofolate-homocysteine transmethylase of E. coli could be detected in Rps. spheroides. 3. N(5)N(10)-Methylenetetrahydrofolate-reductase activity was found in Rps. spheroides. 4. A methionine-requiring mutant strain of Rps. spheroides (strain 2/33), which does not respond to homocysteine, made the same amount of vitamin B(12) as the parent organism. Extracts did not form methionine from N(5)-methyltetrahydrofolate and homocysteine even in the presence of cofactors shown to be necessary with the parent strain, and it is concluded that the mutant is blocked in the formation of the apoenzyme of a homocysteine-methylating system similar to the vitamin B(12)-dependent one in E. coli.

Regulation of 5-methyltetrahydrofolate: homocysteine methyltransferase activity by methionine, vitamin B12, and folate in cultured baby hamster kidney cells

Rapid growth of BHK cells in methioninedeficient medium required supplementation with homocysteine, B(12), and over 40-fold greater levels of folic acid than growth in methionine-supplemented medium. The activity of the B(12)-dependent 5-methyltetrahydrofolate: homocysteine methyltransferase was studied in extracts of BHK cells grown in media containing various concentrations of the compounds of the enzyme reaction. The methyltransferase activity increased over 4-fold when B(12)-deficient deficient medium was supplemented with optimal levels of B(12); this increase was not prevented by puromycin. Addition of homocysteine to growth medium containing methionine, B(12), and folic acid was without effect. However, methyltransferase activity increased 2.5- to 4.0-fold further beyond the highest levels obtained in the presence of methionine, B(12), and folic acid when homocysteine was substituted for methionine in the growth medium. This increase was blocked by puromycin and was not due to removal of feedback inhibition of activity by the product methionine. These results suggest that methyltransferase activity may be regulated in part by derepression of the enzyme's synthesis on substitution of the substrate homocysteine for the product methionine.

Biochemical functions of corrinoid compounds. The sixth Hopkins memorial lecture

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Enzymatic decyanation of cyanocobalamin in rat tissues

Cyanocobalamin is decyanated in vitro to hydroxocobalamin by rat liver and kidney. This appears to be due to an enzyme system which we have called “cyanocobalamindecyanase”. The enzyme is in the soluble fraction of the cells; it requires reduced cozymases and flavins and has an optimum pH of 7.2 under anaerobic conditions. This is consistent with the fact that conversion of cyanocobalamin to hydroxocobalamin and/or its reduced forms (B12r, B12s) involves a reductive process in the earliest stage of the biosynthesis of coenzyme B12.