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

SINCE 5,6-dimethylbenzimidazolyl cobamide co-enzyme (DBCC) was shown to be one of the active forms of vitamin B 12 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 B 12 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 B 12 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–B 12 ) to co-enzyme B 12 in vitro 10 . But the only report indicating that CN–B 12 or hydroxocobalamin (OH–B 12 ) can be converted to co-enzyme form in vivo , on the quantitative base, is Fenrych's short communication reporting the conversion of CN–B 12 into the co-enzyme form in rabbit 11 . Our preliminary report concerning this conversion, which was obtained by 57 Co-labelling of the DBCC fraction in rat liver following the intravenous administration of 57 Co-labelled CN–B 12 and 57 Co-labelled OH–B 12 in rat, will be described here.