Conversion of Cyano- and Hydroxo-cobalamin in vivointo Co-enzyme Form of Vitamin B12in the Rat

SINCE 5,6-dimethylbenzimidazolyl cobamide co-enzyme (DBCC) was shown to be one of the active forms of vitamin B12by 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 investigated2–8. It is now believed that vitamin B12exists 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 B12derivatives 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 B12in vitro10. But the only report indicating that CN–B12or 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–B12into 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–B12and 57Co-labelled OH–B12in rat, will be described here.