MALONYL-COA-SENSITIVE AND MALONYL-COA-INSENSITIVE CARNITINE PALMITOYLTRANSFERASE ACTIVITIES OF MICROSOMES ARE DUE TO DIFFERENT PROTEINS

A carnitine palmitoyltransferase (CPT), extracted from microsomes with octyl glucoside, was purified and characterized as a 54-kDa protein a nd was found to show no malonyl-CoA inhibition (Murthy, M. S. R., and Bieber, L. L. (1992) Protein Exp. Purif: 3, 75-79). We show here that the malonyl-CoA-sensitive CPT of microsomes associates with their memb rane, whereas the above 54-kDa CPT is a soluble luminal protein. Weste rn blot probing with antibody to the 54-kDa CPT was found to show a po sitive response with the soluble microsomal fraction but not with thei r membranes. 2-Tetradecylglycidyl-CoA inhibited the membrane-associate d CPT activity irreversibly, whereas the inhibition of the soluble CPT was largely reversible, Exposure of microsomes to [H-3]etomoxir, ATP, and CoA led to the labeling of a similar to 47-kDa peptide that assoc iated with membranes, whereas no such peptide labeling was seen with t he soluble microsomal fraction. These and other results show (a) that microsomes have malonyl-CoA-sensitive, as well as malonyl-CoA-insensit ive, CPT activities, (b) that these two activities are due to distinct proteins, (c) that the malenyl-CoA-sensitive CPT of microsomes is a p reviously uncharacterized CPT isoform, and (d) that the [H-3]etomoxir- labeled similar to 47-kDa peptide is a likely candidate for the micros omal malonyl-CoA-sensitive CPT or its regulatory subunit.