Coenzyme function of cobalamin analogues from calf kidney.

Calf kidney has been used as a tissue source for the isolation of cobalamin analogues, which are defined here as cobalt-containing compounds of distinctive chromatographic behavior that are extractable from tissues by methods conventionally used to extract cobalamin and which, in radioisotope dilution assays, are more active with R-protein as binder than intrinsic factor and are relatively less active in microbiologic assays. Preparatory methods employed reverse affinity chromatography or a series of chemical extractions for the isolation of corrin followed by Dowex-50 chromatography. An analogue-containing fraction (peak 2) was eluted by acetate buffers between pH 4 and 5. This material was shown to contain cobalt, to migrate differently than the four cobalamins in Dowex-50 and paper chromatography, and to display a pattern of properties that is compatible with the above definition of cobalamin analogues. These analogues stimulated crude preparations of N5-methyltetrahydrofolate-homocysteine methyltransferase (EC 2.1.1.13) from Escherichia coli and rat liver at far lower concentrations (1-40 nmol/L) than the major cobalamins. No evidence of enzymatic conversion of cobalamin to analogue could be demonstrated.