Conversion of Cyano- and Hydroxo-cobalamin in vivo into Co-enzyme Form of Vitamin B₁₂ in the Rat

Abstract

SINCE 5,6-dimethylbenzimidazolyl cobamide co-enzyme (DBCC) was shown to be one of the active forms of vitamin B₁₂ by Barker et al. in 1959 (ref. 1), its biochemical co-enzymatic activity (conversion of glutamate to methyl aspartate, methyl malonyl–CoA to succinyl–CoA, and 1,2-diols to aldehydes) and its physiological metabolism (tissue distribution, excretion and absorption) have been investigated^2–8. It is now believed that vitamin B₁₂ exists as a co-enzyme form in the liver, and takes part in the transformation of methyl malonyl–CoA to succinyl–CoA (ref. 9). On the other hand, enzymatic synthesis of DBCC from B₁₂ derivatives has been confirmed by several workers at the bacterial enzymatic level. It has been reported that the liver and kidney homogenate of rat could convert cyanocobalamin (CN–B₁₂) to co-enzyme B₁₂ in vitro¹⁰. But the only report indicating that CN–B₁₂ or hydroxocobalamin (OH–B₁₂) can be converted to co-enzyme form in vivo, on the quantitative base, is Fenrych's short communication reporting the conversion of CN–B₁₂ into the co-enzyme form in rabbit¹¹. Our preliminary report concerning this conversion, which was obtained by ⁵⁷Co-labelling of the DBCC fraction in rat liver following the intravenous administration of ⁵⁷Co-labelled CN–B₁₂ and ⁵⁷Co-labelled OH–B₁₂ in rat, will be described here.