MITOCHONDRIAL CARNITINE PALMITOYLTRANSFERASE-I ISOFORM SWITCHING IN THE DEVELOPING RAT-HEART

The expression pattern of mitochondrial carnitine palmitoyltransferase (CPT) enzymes was examined in the developing rat heart. Whereas the s pecific activity of CPT II increased similar to 3 fold during the firs t month of life, the profile for CPT I, which is composed of both live r (L) and muscle (M) isoforms, was more complex. Exposure of mitochond ria to [H-3]etomoxir (a covalent ligand for CPT I), followed by fluoro graphic analysis of the membrane proteins, established that while in t he adult heart L-CPT I represents a very minor constituent, its contri bution is much greater in the newborn animal. Use of the related inhib itor, -[6-(2,4-dinitrophenoxy)hexyl]oxirane-2-carboxylic acid (specifi c for L-CPT I), allowed the activities of the two CPT I variants to be quantified separately. The results showed that in the neonatal heart, L-CPT I contributes similar to 25% to total CPT I activity (in V-max terms), the value falling during growth of the pups (with concomitant increasing expression of the Id isoform) to its adult level of 2-3%. B ecause the myocardial carnitine content is very low at birth and rises dramatically over the next several weeks, it can be estimated that L- CPT I (K-m for carnitine of only 30 mu M compared with a value of 500 CCM for M-CPT I) is responsible for some 60% of total cardiac fatty ac id oxidation in the newborn rat; the value falls to similar to 4% in a dult animals. Should these findings have a parallel in humans, they co uld have important implications for understanding the pathophysiologic al consequences of inherited L CPT I deficiency syndromes.