Factors Affecting the Rate of Oxidation of Fatty Acids in Animal Tissues EFFECT OF SUBSTRATE CONCENTRATION, pH, AND COENZYME A IN RAT LIVER PREPARATIONS* JOSEPH A. ONTKO AND DARNELL JACKSON

Since 1866, when the oxidation of fat by fasting subjects was reported (l), a considerable reservoir of information on the biological oxidation of fatty acids has accumulated. The present understanding of this process has largely unfolded since 1943 when Munoz and Leloir (2) described a cell-free preparation which oxidized short chain fatty acids. It was later shown that isolated mitochondria oxidize short and long chain fatty acids at comparable rates (3). Phosphate acceptors (4), carnitine (5), fasting (6), high fat diets (7), exercise (8), pantothenic acid deficiency (9), and the administration or deletion of a variety of hormones (10) affect the rate of fatty acid oxidation. Fritz (10) has reviewed fatty acid metabolism with emphasis on conditions which influence the rates of fatty acid oxidation and synthesis. This report presents studies on factors which alter the rates of fatty acid oxidation in rat liver preparations. This study was initiated to supply information on possible mechanisms by which the metabolic activity of this multireaction process is regulated. The dynamics of unesterified fatty acids in cells are complicated by protein binding, compartmentation, and by inter-relationships between several enzyme systems including those which catalyze lipolysis, esterification to form glycerides and other lipids, fatty acid synthesis, and fatty acid oxidation. Liver homogenates were used in many of these experiments in order that various intracellular enzyme systems, which may influence fatty acid oxidation and therefore have bearing on the mechanisms which control the rate of oxidation of fatty acids at the cellular level, would be present. The high endogenous respiration of such preparations makes manometric evaluation of fatty acid oxidation difficult. Ketone body formation, 14C02 production from palmitate-1-*4C, fatty acid disappearance and fatty acid incorporation into glycerides, phosphatides, and cholesterol esters were concurrently measured to evaluate fatty acid conversions in tissue homogenates. In addition to these measurements, oxygen consumption was also measured in the isolated mitochondrial system. The effects of substrate concentration and pH on palmitate conversions in liver homogenates were determined. The coenzyme A-induced decrease in the formation of ketone bodies from palmitate by liver homogenates (11) was further studied. Coenzyme A decreased i4CO2 formation from palmitate-l-% without increasing the incorporation of this long chain fatty acid into glycerides or phospholipids. The