Coenzyme B12 and methylcobalamin in water are less active in promoting growth of Euglena gracilis Z strain than the same concentrations of cyanocobalamin and hydroxocobalamin which are equally active. When bound to human serum or human liver homogenate, however, the activities of these four cobalamins do not differ significantly with one exception. The results suggest that the Euglena assay using cyanocobalamin standards is not satisfactory for quantitation of coenzyme B12 and methylcobalamin in water but acceptable when coenzyme B12 and methylcobalamin are bound to serum or liver. Sulphitocobalamin in water is as active as cyanocobalamin and hydroxocobalamin but nitritocobalamin is less active. Factor B, the monocarboxylic acids of cyanocobalamin and hydroxocobalamin, and the dicarboxylic acid of cyanocobalamin in water were inactive.
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