The release of carnitine palmitoyltransferase (CPT) activity from rat liver mitochondria by increasing concentrations of digitonin was studied for mitochondrial preparations from fed, 48 h-starved and diabetic animals. A bimodal release of activity was observed only for mitochondria isolated from starved and, to a lesser degree, from diabetic rats, and it appeared to result primarily from the enhanced release of approx. 40% and 60%, respectively, of the total CPT activity. This change in the pattern of release was specific to CPT among the marker enzymes studied. For all three types of mitochondria there was no substantial release of CPT concurrently with that of the marker enzyme for the soluble intermembrane space, adenylate kinase. These results illustrate that the bimodal pattern of release of CPT reported previously for mitochondria from starved rats [Bergstrom & Reitz (1980) Arch. Biochem. Biophys. 204, 71-79] is not an immutable consequence of the localization of CPT activity on either side of the mitochondrial inner membrane. Sequential loss of CPT I (i.e. the overt form) from the mitochondrial inner membrane did not affect the concentration of malonyl-CoA required to effect fractional inhibition of the CPT I that remained associated with the mitochondria. The results are discussed in relation to the possibility that altered enzyme-membrane interactions may account for some of the altered regulatory properties of CPT I in liver mitochondria of animals in different physiological states.

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