Intact mitochondria and inverted submitochondrial vesicles were prepared from the liver of fed, starved (48 h) and
streptozotocin-diabetic rats in order to characterize
carnitine palmitoyltransferase kinetics and
malonyl-CoA sensitivity in situ. In intact mitochondria, both starved and diabetic rats exhibited increased Vmax., increased Km for
palmitoyl-CoA, and decreased sensitivity to
malonyl-CoA inhibition. Inverted submitochondrial vesicles also showed increased Vmax. with
starvation and diabetes, with no change in Km for either
palmitoyl-CoA or
carnitine. Inverted vesicles were uniformly less sensitive to
malonyl-CoA regardless of treatment, and diabetes resulted in a further decrease in sensitivity. In part, differences in the response of
carnitine palmitoyltransferase to
starvation and diabetes may reside in differences in the membrane environment, as observed with Arrhenius plots, and the relation of
enzyme activity and membrane fluidity. In all cases, whether rats were fed, starved or diabetic, and whether intact or inverted vesicles were examined, increasing membrane fluidity was associated with increasing activity.
Malonyl-CoA was found to produce a decrease in intact mitochondrial membrane fluidity in the fed state, particularly at pH 7.0 or less. No effect was observed in intact mitochondria from starved or diabetic rats, or in inverted vesicles from any of the treatment groups. Through its effect on membrane fluidity,
malonyl-CoA could regulate
carnitine palmitoyltransferase activity on both surfaces of the inner membrane through an interaction with only the outer surface.