COENZYME-A METABOLISM IN VITAMIN-B-12-DEFICIENT RATS

Vitamin B-12 (cobalamin) defiency results in decreased L-methylmalonyl-coenzyme A (CoA) mutase activity. The consequence of this defect on the cellular CoA pool was studied in rats with functional vitamin B-12 deficiency induced by administration of the cobalamin analogue hydroxy-cobalamin [c-lactam] or by dietary vitamin B-12 deficiency. Both types of vitamin B-12 deficiency were associated with methylmalonic acidemia (100-300-fold increases in plasma methylmalonic acid concentration compared with controls), but overall fuel homeostasis was intact. Liver from rats treated with hydroxy-cobalamin [c-lactam] contained a threefold greater concentration of total CoA (free CoA plus all acyl-CoA) compared with saline-treated rats. Fractionation of the CoA pool revealed higher levels of CoA, propionyl-CoA, methylmalonyl-CoA, acid insoluble CoA, as well as total CoA in the rats treated with hydroxy-cobalamin [c-lactam] compared with controls. Similar increases in liver CoA content were seen in dietary vitamin B-12 deficiency in both the fed and fasted states. To examine the hypothesis that sequestration of hepatic CoA as propionyl-CoA and methylmalonyl-CoA could increase CoA biosynthesis, the effect of propionate on CoA biosynthesis was studied in hepatocytes isolated from control rats. Propionate (1 mM) increased the formation of 14C-CoA from [14C]pantothenate (10 μM) by 27% in the hepatocyte system. When butyrate (1 mM) was provided as substrate, propionate (10 mM) increased [14C]CoA formation by 63%. α-Ketobutyrate, which is decarboxylated to propionyl-CoA, also increased hepatocyte [14C]pantothenate metabolism. Thus, vitamin B-12 deficiency is associated with hepatic accumulation of propionyl-CoA and methylmalonyl-CoA, and an increase in hepatic total CoA content. This increase in total CoA content may result from enhanced biosynthesis of CoA because cellular CoA is sequestered as poorly metabolized acyl-CoA. The increase in CoA biosynthesis may be an important compensatory mechanism to maintain cellular metabolism under conditions of acyl-CoA accretion.