Only 1–2% of total Hcy is free in the plasma, 70–80%is combined with circulating proteins (mainly albumin) and theremaining section is composed of disulfides, Hcy and a mix ofHcy-cysteine disulfides. Hcy may transform itself through are-methylation process. This methionine-sparing process iscatalyzed by the methionine synthase enzyme (MS), which requires5-methyltetrahydrofolate (5′-MTHF) as a substrate and cobalamin(vitamin B12) as a cofactor in order to transfer the methyl groupof 5-MTHF to Hcy, thereby forming methionine and tetrahydrofolate(THF).
Blood was collected in tubes without ananticoagulant and the tubes were immediately centrifuged in orderto separate the serum from the corpuscular volume. Serum sampleswere frozen at −20°C until they were required for the serum folateand vitamin B12 assays. To do this we used the tool Unicel DXI 800produced by Beckman Coulter (Miami, FL, USA). The normal ranges forserum folates and vitamin B12 were 3.1–20 ng/μl and 211–911 pg/ml,respectively.
Biochemistry of B12-Cofactors in Human Metabolism
N2 - We have examined the distribution of cobalamin (coenzyme B12) synthetic ability and cobalamin-dependent metabolism among enteric bacteria. Most species of enteric bacteria tested synthesize cobalamin under both aerobic and anaerobic conditions and ferment glycerol in a cobalamin-dependent fashion. The group of species including Escherichia coli and Salmonella typhimurium cannot ferment glycerol. E. coli strains cannot synthesize cobalamin de novo, and Salmonella spp. synthesize cobalamin only under anaerobic conditions. In addition, the cobalamin synthetic genes of Salmonella spp. (cob) show a regulatory pattern different from that of other enteric taxa tested. We propose that the cobalamin synthetic genes, as well as genes providing cobalamin-dependent diol dehydratase, were lost by a common ancestor of E. coli and Salmonella spp. and were reintroduced as a single fragment into the Salmonella lineage from an exogenous source. Consistent with this hypothesis, the S. typhimurium cob genes do not hybridize with the genomes of other enteric species. The Salmonella cob operon may represent a class of genes characterized by periodic loss and reacquisition by host genomes. This process may be an important aspect of bacterial population genetics and evolution.