Inhibition of carnitine palmitoyltransferase in normal human skeletal muscle and in muscle of patients with carnitine palmitoyltransferase deficiency by long- and short-chain acylcarnitine and acyl-coenzyme A

Muscle Carnitine Palmitoyltransferase II Deficiency: A Review of Enzymatic Controversy and Clinical Features

CPT (carnitine palmitoyltransferase) II muscle deficiency is the most common form of muscle fatty acid metabolism disorders. In contrast to carnitine deficiency, it is clinically characterized by attacks of myalgia and rhabdomyolysis without persistent muscle weakness and lipid accumulation in muscle fibers. The biochemical consequences of the disease-causing mutations are still discussed controversially. CPT activity in muscles of patients with CPT II deficiency ranged from not detectable to reduced to normal. Based on the observation that in patients, total CPT is completely inhibited by malony-CoA, a deficiency of malonyl-CoA-insensitive CPT II has been suggested. In contrast, it has also been shown that in muscle CPT II deficiency, CPT II protein is present in normal concentrations with normal enzymatic activity. However, CPT II in patients is abnormally sensitive to inhibition by malonyl-CoA, Triton X-100 and fatty acid metabolites. A recent study on human recombinant CPT II enzymes (His₆-N-hCPT2 and His₆-N-hCPT2/S113L) revealed that the wild-type and the S113L variants showed the same enzymatic activity. However, the mutated enzyme showed an abnormal thermal destabilization at 40 and 45 °C and an abnormal sensitivity to inhibition by malony-CoA. The thermolability of the mutant enzyme might explain why symptoms in muscle CPT II deficiency mainly occur during prolonged exercise, infections and exposure to cold. In addition, the abnormally regulated enzyme might be mostly inhibited when the fatty acid metabolism is stressed.

Limited trypsin proteolysis renders carnitine palmitoyltransferase insensitive to inhibition by malonyl-CoA in patients with muscle carnitine palmitoyltransferase deficiency

Carnitine palmitoyltransferase (CPT) was studied in muscle homogenates of two patients with muscle CPT deficiency heterozygous for the Ser-113 Leu mutation in the CPT II gene. Total CPT activity was normal in both patients but was almost completely inhibited by malonyl-CoA and Triton X-100 whereas in controls 38% and 58% of total activity remained in the presence of malonyl-CoA and Triton X-100, respectively. The addition of 1% Tween 20 abolished about half of the activity in patients but not in controls. Preincubation of muscle homogenate with trypsin slightly increased the total activity and rendered the activity greatly insensitive to inhibition by malonyl-CoA in both patients and controls. The data support the view that in patients with muscle CPT deficiency both CPT I and II are active, but that CPT II is abnormally accessible to inhibition by malonyl-CoA.