1. Administration of propionate caused a twofold increase in the concentrations of lactate and pyruvate in the blood of vitamin B12-deficient rats, whereas there was a slight decrease in lactate and a 50% increase in pyruvate in normal rats. 2. Concentrations of total ketone bodies in the blood of normal rats were not significantly altered by propionate administration but the [3-hydroxybutyrate]/[acetoacetate] ratio decreased from 3.0 to 2.0. In the vitamin B12-deficient rats there was a 40% decrease in total ketone bodies and a change in the ratio from 3.4 to 1.2. 3. The changes in the concentration of ketone bodies in freeze-clamped liver preparations were similar in pattern to those observed in blood. 4. Propionate administration caused a decrease in the concentration of acetyl-CoA in the livers of both groups of animals, but the absolute decrease was greater in the vitamin B12-deficient group. The decrease in the concentration of CoA was similar in both groups. 5. As in blood, there were threefold increases in the concentrations of lactate and pyruvate in the livers of the vitamin B12-deficient rats after propionate administration, whereas there was no significant change in the concentrations of these metabolites in the normal rats. 6. There was a 50% inhibition of glucose synthesis in perfused livers from vitamin B12-deficient rats when lactate and propionate were substrates as compared with lactate alone. 7. It is concluded that the conversion of lactate into glucose is inhibited in vitamin B12-deficient rats after propionate administration, and that this effect is due to inhibition of the pyruvate carboxylase step resulting from a decrease in acetyl-CoA concentration and a postulated increase in methylmalonyl-CoA concentration.

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