Effects of fasting and malonyl CoA on the kinetics of carnitine palmitoyltransferase and carnitine octanoyltransferase in intact rat liver mitochondria.

There has been considerable interest in the observation that the overt form of carnitine palmitoyltransferase (CPT1) in liver mitochondria is potently inhibited by malonyl CoA [I ,2]. It has been suggested [2] that this is a competitive type of inhibition against long chain acyl CoA substrates. However, this is based upon measurements of ketogenesis [3-51 or indirect calculations of CPT, activity [ 11. No studies have been performed in which CPTl activity has been measured directly in intact mitochondria at various concentrations of the carnitine and acyl CoA substrates. Accordingly this is undertaken here, using liver mitochondria from both fed and fasted rats since the sensitivity of CPTl to malonyl CoA is decreased in the fasted state [6]. The use of intact mitochondria is important both to avoid measurement of CPT2 which is insensitive to malonyl CoA [ 1 ] and to avoid disruptive procedures which may remove CPTl from mitochondrial membranes and render the enzyme malonyl CoA-insensitive [ 11. Rat liver mitochondria also appear to contain an overt carnitine octanoyltransferase (COT) [7,8] of unknown physiological function which is also inhibited by malonyl CoA [6]. The effects of this inhibitor upon COT and CPTl differ in that the former is more sensitive to malonyl CoA and this sensitivity is not diminished by fasting [6]. It was therefore also of interest to investigate the effects of malonyl CoA upon the kinetics of COT.