The hypoglycemic sulfonylureas glyburide and tolbutamide inhibit fatty acid oxidation by inhibiting carnitine palmitoyltransferase.
The hypoglycemic sulfonylureas glyburide and tolbutamide were found to be excellent inhibitors of the rat liver, heart, and skeletal muscle carnitine palmitoyltransferases, but glyburide was by far the most potent inhibitor. Carboxytolbutamide, a sulfonylurea that has no hypoglycemic effect, produced little or no inhibition of the enzyme from the three tissues examined. Fasting decreased the degree of inhibition of carnitine palmitoyltransferase by the sulfonylureas, and in genetically diabetic BB Wistar rats, a decrease in sensitivity was also clearly demonstrated. Initial rate kinetics of the inhibition of carnitine palmitoyltransferase indicated that glyburide inhibits noncompetitively with respect to palmitoyl-CoA while inhibition by malonyl-CoA was cooperatively competitive. Inhibition by malonyl-CoA was noncompetitive with respect to carnitine, but inhibition by glyburide was uncompetitive. These studies indicate that the hypoglycemic sulfonylureas inhibit carnitine palmitoyltransferase by a mechanism that is much different from inhibition by malonyl-CoA, but are, nevertheless, potent inhibitors of the enzyme. These results have important implications for energy metabolism in the liver and heart in relation to the use of sulfonylureas and for understanding the mechanism by which the sulfonylureas act to lower blood glucose, but there are also important implications of these results on the study of the metabolic regulation of fatty acid oxidation.