ABSTRACT
Synthesis of glucose from lactate and generation of urea from ammonia were inhibited when sodium benzoate was added to suspensions of rat hepatocytes. Assays with isolated mitochondria suggested pyruvate carboxylase and the N-acetyl-L-glutamate (NAG)-dependent carbamoylphosphate synthetase (CPS-I) as potential sites of inhibition for both pathways, owing to a shared dependency on aspartate efflux from the mitochondria and its subsequent conversion to oxaloacetate in the cytosol. Assays with isolated hepatocytes indicated inhibition to be initiated by accumulation of benzoyl CoA with a resultant depletion of free CoA and acetyl CoA. Measurements of adenine nucleotides showed that benzoate metabolism did not sufficiently alter energy status to account for the observed inhibition. Consistent with these interpretations, acceleration of the conversion of benzoyl CoA to hippurate by the addition of glycine restored the levels of free CoA and acetyl CoA and the rates of gluconeogenesis and ureagenesis. Reduction of the levels of aspartate and glutamate, presumably by interference with the anapleurotic function of pyruvate carboxylase, most likely accounted for inhibition of gluconeogenesis by benzoate. Whether reduced flux through the urea cycle also contributed to inhibition of gluconeogenesis (by diminishing cytosolic conversion of aspartate to oxaloacetate) requires further study. Depression of glutamate and acetyl CoA to levels at or below the Km for NAG synthetase probably accounted for the observed inhibition of ureagenesis. Rates of urea production were observed to vary with changes in the levels of NAG, suggesting NAG-dependent CPS-I to be the primary site of inhibition of ureagenesis by benzoate.
Subject(s)
Benzoates/pharmacology , Gluconeogenesis/drug effects , Liver/drug effects , Urea/metabolism , Acetyl Coenzyme A/metabolism , Adenine Nucleotides/metabolism , Animals , Aspartic Acid/metabolism , Benzoic Acid , Bicarbonates/metabolism , Carbamoyl-Phosphate Synthase (Ammonia)/metabolism , Cells, Cultured/drug effects , Enzyme Activation , Glutamates/metabolism , Glutamic Acid , Kinetics , Liver/metabolism , Male , Mitochondria, Liver/enzymology , Orotic Acid/metabolism , Pyruvate Carboxylase/metabolism , Rats , Sodium/metabolism , Sodium BicarbonateABSTRACT
Pyruvate-dependent CO2 fixation by isolated mitochondria was strongly inhibited by sodium benzoate. Pyruvate carboxylase was identified as a site of inhibition by limiting flux measurements to assays of pyruvate carboxylase coupled with malate dehydrogenase. Benzoate reduced pyruvate-dependent incorporation of [14C]KHCO3 into malate and pyruvate-dependent malate accumulation by 74 and 72%, respectively. Aspartate-dependent malate accumulation was insensitive to benzoate, ruling out malate dehydrogenase as a site of action. Inhibition by benzoate was antagonized by glycine, which sharply accelerated conversion of benzoate to hippurate. Assays of coenzyme A and its acyl derivatives revealed inhibition to correlate with depletion of acetyl CoA and accumulation of benzoyl CoA. Depletion of acetyl CoA was sufficient to account for greater than 50% reduction in pyruvate carboxylase activity. Competition between acetyl CoA and benzoyl CoA for the activator site on pyruvate carboxylase was insignificant. Results support the interpretation that the observed inhibition of pyruvate carboxylase occurred primarily by depletion of the activator, acetyl CoA, through sequestration of coenzyme A during benzoate metabolism.
