Are there two forms of carnitine palmitoyltransferase in muscle?

Mitochondria were isolated from rat skeletal muscle, heart, and liver and from human skeletal muscle. The distribution of CPT I and CPT II was studied by measuring CPT activity and malonyl-CoA sensitivity before and after disruption of the mitochondria. Neither sonication, freezing and thawing, nor detergent treatment increased CPT activity in heart or skeletal muscle mitochondria, but these procedures did increase CPT activity in liver mitochondria. These results cannot be attributed to different kinetics of CPT I and II to palmitoyl-CoA or carnitine, or to different effects of electrolytes on CPT I and II. Sensitivity to inhibition by malonyl-CoA also failed to distinguish convincingly between CPT I and II in skeletal muscle. Because the presence of CPT I and II in muscle cannot be ascertained, the notion of a selective CPT I or II deficiency in muscle cannot be entertained.