Carnitine interacts with the metabolically-active coenzyme A pool through formation of acylcarnitines from acyl-coAs. Carnitine has been reported to have efficacy in the treatment of hereditary organic acidemias and other pathologic conditions. The mechanism for these effects is presumed to be generation of acylcarnitines and consequent alterations in cellular intermediary metabolism. However, proof of this mechanism is lacking, and fundamental information on the dynamics of acylcarnitine generation and its relationship to oxidative metabolism are needed. This proposal tests the hypothesis that acylcarnitine formation provides a mechanism for maintaining normal hepatic metabolism under conditions which acylCoAs accumulate and may impair liver metabolism. Vitamin B12 deficiency in rats will be used as a model. During vitamin B12 deficiency, propionyl-CoA and methylmalonyl-CoA accumulate secondary to impaired methylmalonyl- CoA utilization. We have demonstrated an accumulation of acylcarnitines, and specifically propionylcarnitine, in urine and tissues in the rat during the development of vitamin B12 deficiency. The characterization of the vitamin B12 deficient rat model will be extended in the current proposal to measurements of the hepatic CoA pool and overall fuel metabolism. The administration of cobalamin analogs to rats will be used to induce functional vitamin B12 deficiency of a more severe degree over a short time period. Using these models, exogenous carnitine will be administered, and the response of the carnitine and coenzyme A pools determined. The transition from the fed to fasted state will be used to assess in vivo the effect of vitamin B12 deficiency on hepatic gluconeogenesis and ketogenesis, and how this is altered by carnitine supplementation. Studies will be done in isolated hepatocytes from vitamin B12 deficient rats. Acylcarnitine formation and alterations in the coenzyme A pool will be determined in the hepatocyte and correlated with carnitine induced changes in the hepatocyte's ability to oxidize pyruvate and palmitate in the presence of propionate. Specific inhibitors of acylcarnitine formation will be used to further define the relationships between acylcarnitine production and alterations in metabolism. These studies will provide insight into how carnitine's interaction with the co-enzyme. A pool may perturb hepatic metabolism and how acylcarnitine formation may improve hepatocellular function. The work proposed will provide a rational basis for extended in vitro and in vivo studies of carnitine's effects on metabolism in disease states.
