Use of a Selective Inhibitor of Liver Carnitine Palmitoyltransferase I (CPT I) Allows Quantification of Its Contribution to Total CPT I Activity in Rat Heart: Evidence That The Dominant Cardiac CPT I Isoform Is Identical To The Skeletal Muscle Enzyme

It has recently been established that rat heart mitochondria contain two isoforms of carnitine palmitoyl-transferase I (CPT I), the minor 88-kDa variant being identical to liver CPT I (L-CPT I) and the dominant 82-kDa form resembling the skeletal muscle enzyme (M-CPT I) (Weis, B. C., Esser, V., Foster, D. W., and McGarry, J. D. (1994) J. Biol. Chem. 269, 18712-18715). To quantify the functional contribution of L-CPT I to over-all CPT I activity in heart mitochondria a selective inhibitor of the former was needed. The dinitrophenol analog of 2[6-(4-chlorophenoxy)hexyl]oxirane-2-carboxylic acid (etomoxir) (DNP-Et) was found to have this property. When liver and skeletal muscle mitochondria were exposed to DNP-Et in the presence of ATP and CoASH, the DNP-Et-CoA formed completely inhibited liver CPT I while leaving the muscle enzyme unaffected. Similar treatment of heart mitochondria blocked only the L-CPT I component. This had the effect of shifting the apparent K_m for carnitine from ∼200 to ∼500 μM and the I₅₀ value for malonyl-CoA (the concentration needed to suppress enzyme activity by 50%) from ∼0.18 to ∼0.06 μM, i.e. the heart system now behaved exactly the same as that from skeletal muscle. Taking the K_m for carnitine of L-CPT I and M-CPT I to be 30 and 500 μM, respectively, it could be calculated that the former contributes ∼2% to the total CPT I in heart. When the 82-kDa CPT I isoforms of heart and skeletal muscle were labeled with [³H]etomoxir and then exposed to trypsin, the fragmentation patterns obtained were identical and quite distinct from that given by CPT I from liver. We conclude that (i) DNP-Et, unlike other agents of the oxirane carboxylic acid class, has remarkable inhibitory selectivity for L-CPT I over M-CPT I; (ii) the previously puzzling observation that rat heart CPT I displays kinetic characteristics intermediate between those of the enzymes from liver and skeletal muscle is entirely accounted for by the low level expression of L-CPT I in the cardiac myocyte; and (iii) the dominant 82-kDa CPTI isoform in heart is identical to the muscle enzyme. The data reaffirm that, in contrast to CPT II, CPT I exists in at least two isoforms and that both are present in rat heart.