Normal protein content but abnormally inhibited enzyme activity in muscle carnitine palmitoyltransferase II deficiency.

The biochemical consequences of the disease causing mutations of muscle carnitine palmitoyltransferase II (CPT II) deficiency are still enigmatic. Therefore, CPT II was characterized in muscle biopsies of nine patients with genetically proven muscle CPT II deficiency. Total CPT activity (CPT I+CPT II) of patients was not significantly different from that of controls. Remaining activities upon inhibition by malonyl-CoA and Triton X-100 were significantly reduced in patients. Immunohistochemically CPT II protein was predominantly expressed in type-I-fibers with the same intensity in patients as in controls. Western blot showed the same CPT II staining intensity ratio in patients and controls. CPT I and CPT II protein concentrations estimated by ELISA were not significantly different in patients and in controls. Citrate synthase activity in patients was significantly increased. Total CPT activity significantly correlated with both CPT I and CPT II protein concentrations in patients and controls. This implies (i) that normal total CPT activity in patients with muscle CPT II deficiency is not due to compensatory increase of CPT I activity and that (ii) the mutant CPT II is enzymatically active. The data further support the notion that in muscle CPT II deficiency enzyme activity and protein content are not reduced, but rather abnormally inhibited when fatty acid metabolism is stressed.