ACCUMULATION OF METHYLMALONIC ACID CAUSED BY VITAMIN B-12-DEFICIENCY DISRUPTS NORMAL CELLULAR-METABOLISM IN RAT-LIVER

To clarify the relationship between intracellular concentrations of me thylmalonic acid and metabolic and growth inhibition in vitamin B-12-d eficient rats, hepatic methylmalonic acid levels were assayed and inhi bition of glucose and glutamic acid metabolism by methylmalonic acid w as studied in isolated hepatocytes. Vitamin B-12-deficient rats (14 we eks old) excreted more urinary methylmalonic acid and had lower body w eights than the control rats, Hepatic methylmalonic acid levels (3.6(S D 1.30)-5.3(SD 0.51) mu mol/g tissue; 7.9 (SD 2.90)-11.8 (SD 1.14) mM) were increased and correlated with the extent of the growth retardati on during vitamin B-12-deficiency. Isolated hepatocytes and mitochondr ia from normally fed rats were labelled with [C-14(U)]glucose and [C-1 4(U)]glutamic acid respectively, in the presence or absence of 5 mM-me thylmalonic acid. Although methylmalonic acid did not affect the incor poration of C-14 into protein and organic acid fractions in the hepato cytes, it inhibited (CO2)-C-14 formation (an index of glucose oxidatio n by the Krebs cycle) by 25% and incorporation of C-14 into the amino acid fraction by 30%. In the mitochondria, methylmalonic acid inhibite d (CO2)-C-14 formation (indicating glutamic acid oxidation by the Kreb s cycle) by 70%, but not the incorporation of C-14 into the protein fr action, The incorporation of C-14 into the organic acid fraction was s ignificantly stimulated by the addition of methylmalonic acid. These r esults indicate that the unusual accumulation of methylmalonic acid ca used by vitamin B-12-deficiency disrupts normal glucose and glutamic a cid metabolism in rat Liver, probably by inhibiting the Krebs cycle.