SENSITIVITY OF INHIBITION OF RAT-LIVER MITOCHONDRIAL OUTER-MEMBRANE CARNITINE PALMITOYLTRANSFERASE BY MALONYL-COA TO CHEMICAL-INDUCED AND TEMPERATURE-INDUCED CHANGES IN MEMBRANE FLUIDITY

We have tested the possibility that alterations in the fluidity of the outer membrane of rat liver mitochondria could result in changes in the sensitivity of overt carnitine palmitoyltransferase (CPT I) to malonyl-CoA [Zammit (1986) Biochem. Soc. Trans. 14, 676-679]. The sensitivity of CPT I to malonyl-CoA inhibition was measured by using highly purified mitochondrial outer membranes prepared from fed or 48 h-starved rats in the presence and absence of agents that increase membrane fluidity by perturbing membrane lipid order [benzyl alcohol, isoamyl alcohol (3-methylbutan-1-ol) and 2-(2-methoxyethoxy)ethyl-8-(cis-2-n-octylpropyl)octanoate (A2C)]. All these agents resulted in marked decreases in the ability of malonyl-CoA to inhibit CPT I. This effect was accompanied by a modest increase in the absolute activity of CPT I in the absence of malonyl-CoA when the short-chain alcohols were used, but not when A2C was used, suggesting that the effect of increased membrane fluidity to decrease the malonyl-CoA sensitivity of CPT I may occur independently from other actions that may affect more directly the active site of the enzyme. In confirmation of the potential importance of fluidity changes, we showed that a marked increase in sensitivity of CPT I to malonyl-CoA could be produced when assays were performed at lower temperatures than those normally employed. These observations are discussed in the context of the slowness of the changes in CPT I sensitivity to malonyl-CoA inhibition that are induced by physiological perturbations.