Distinct kinetics of carnitine palmitoyltransferase I in contact sites andouter membranes of rat liver mitochondria

Carnitine palmitoyltransferase I (CPT I) of rat liver mitochondria is an in tegral, polytopic protein of the outer membrane that is enriched at contact sites. As CPT I kinetics are highly dependent on its membrane environment, we have measured the kinetic parameters of CPT I present in rat liver subm itochondrial membrane fractions enriched in either outer membrane or contac t sites. The K-m for palmitoyl-CoA was 2.4-fold higher for GPT I in outer m embranes than that for the enzyme in contact sites. In addition, whereas in contact sites malonyl-CoA behaved as a competitive inhibitor of CPT I with respect to palmitoyl-CoA, in outer membranes malonyl-CoA inhibition was no n-competitive. As a result of the combination of these changes, the IC50 fo r malonyl-CoA was severalfold higher for CPT I in contact sites than for th e enzyme in bulk outer membrane, The K-i for malonyl-CoA, the K-m for carni tine, and the catalytic constant of the enzyme were all unaffected. It is c oncluded that the different membrane environments in outer membranes and co ntact sites result in an altered conformation of L-CPT I that specifically affects the long-chain acyl-CoA binding site. The accompanying changes in t he kinetics of the enzyme provide an additional potent mechanism for the re gulation of L-CPT I activity.