Effect of Chronic Myocardial Ischemia on the Activity of Carnitine Palmitylcoenzyme A Transferase of Isolated Canine Heart Mitochondria

The effect of chronic ischemia on the activity of carnitine palmitylcoenzyme A transferase (PCoAT) was examined. Control mitochondria were isolated from the left ventricle of normal hearts and from the uninvolved areas of ischemic hearts. Mitochondria from ischemic tissue were isolated from the left ventricle of chronically ischemic hearts. Formation of the enzyme product, palmityl-14 C-carnitine, was markedly depressed in control mitochondria isolated from ischemic hearts; however, the pH optimums of the enzymes from control and ischemic mitochondria were in the same range. After 1 day of chronic ischemia, the kinetics of the enzyme from ischemic mitochondria (compared with that of the enzyme from control mitochondria) showed marked changes in Vmax. Periods of ischemia longer than 1 day significantly decreased the Km of carnitine PCoAT for carnitine from 2.59 ± 0.476 mM to 0.713 ± 0.148 mM, and Vmax was still depressed (15 to 3 nmoles/min mg-1 ). The Km for palmitylCoA was not significantly affected. Arrhenius plots for the enzyme from ischemic (32 hours) mitochondria revealed a single phasic response to temperature change. Two nonintersecting slopes were characteristic of the normal 1-day ischemic and the normal sonicated activity. As a result of ischemia, changes in the lipid components in the membrane containing carnitine PCoAT were postulated: alterations in the hydrophobic environment of the enzyme produce interference in the binding of palmitylCoA to the second substrate site and may result in a decrease in the Km of the enzyme for carnitine. The single phasic response of the enzyme to temperature change may be additional evidence for membrane lipid alterations. A defect in the transport of long-chain fatty acids may be an early change associated with myocardial ischemia.