An Alternate Pathway of α-Ketoglutarate Catabolism in the Isolated, Perfused Rat Liver

Abstract

The pathway of ar-ketoglutarate catabolism in mammalian liver to glucose, fatty acids, and other products is generally considered to proceed in the mitochondria through the Krebs cycle to succinate.Thus, both ol-ketoglutarate-2-and -5-l4C would be expected to give carboxyl-labeled products.However, an examination of the enzymes of citric acid and isocitric acid metabolism, their subcellular localization, and their properties suggests that a-ketoglutarate can be catabolized by an alternate pathway in the cytoplasm.This pathway proceeds by the reductive carboxylation of a-ketoglutarate to isocitrate, the isomerization of isocitrate to citrate, and the cleavage of citrate to oxaloacetate and acetyl coenzyme A, and can be summarized as follows.a-Ketoglutarate + isocitrate -+ citrate + acetyl-CoA + oxaloacetateBecause citrate behaves asymmetrically during the cleavage reaction (2, 3), carbon atom 2 of ketoglutarate will be converted to the methylene carbon of oxaloacetate, whereas carbon atom 5 will appear in acetyl-CoA.Thus it would be anticipated that a-ketoglutarate-2-and -5-14C would produce distinctly different labeling patterns in products such as glucose and fatty acid derived from acetyl-CoA and oxaloacetate.This paper presents evidence for the existence of such a pathway in the isolated, perfused rat liver.EXPERIMENTAL PROCEDURE nL-Glutamic acid-2-and -5-14C, each with an approximate specific activity of 1.5 mc per mmole, were purchased from Nuclear Research Chemicals.In all experiments a tracer dose of 12.5 PC of the DL compound was used.Yields are referred to as percentages of the dose of L isomer, since it has been shown that n-glutamate is not utilized to any great extent by rat liver (4).The isolated, perfused rat liver preparation