To begin to explore the basis for the tissue-specific expression of mitochondrial carnitine palmitoyltransferase I (CPT I), we focused on three rat tissues (liver, heart, and skeletal muscle) in which the enzyme was known to display very different properties. In Northern blot analysis, a cDNA probe corresponding to liver CPT I readily hybridized to a 4.5-kilobase species of mRNA in liver and heart, but not in skeletal muscle. Using the same probe to screen a neonatal rat heart cDNA library, a full-length clone, surprisingly having 100% sequence identity to the liver CPT I cDNA, was isolated. The paradox was resolved by two additional experiments. First, in Western blots of mitochondrial membranes, an antibody raised against liver CPT I recognized the 88-kDa protein in heart, as well as in liver, but not in skeletal muscle. Second, high specific activity [3H]deschloroetomoxir (a covalent ligand for CPT I) reacted with a single form of CPT I in liver (~88 kDa) and skeletal muscle (~82 kDa), while proteins of both sizes were labeled in the cardiac myocyte. Tritiated ligand binding to the two heart proteins was blocked by excess unlabeled malonyl-CoA. It is concluded that liver and skeletal muscle each contains a single and distinct isoform of CPT I with monomeric size of ~88 and 82 kDa, respectively. The heart contains a CPT I protein of ~82 kDa in size (probably identical to the skeletal muscle protein) but, importantly, also expresses the liver-type enzyme. The results likely explain why previous studies of heart CPT I yielded an apparent K(m) for carnitine and I60 value for malonyl-CoA inhibition that were intermediate between those of the liver and skeletal muscle enzymes.
|Original language||English (US)|
|Number of pages||4|
|Journal||Journal of Biological Chemistry|
|State||Published - Jan 1 1994|
ASJC Scopus subject areas
- Molecular Biology
- Cell Biology