Regulatory properties of a mutant carnitine palmitoyltransferase in human skeletal muscle

Carnitine palmitoyltransferase (EC 2.3.1.21) was studied in sonicated muscle homogenates of seven patients who had recurrent attacks of myoglobinuria and marked deficiency of carnitine palmitoyltransferase in the isotope exchange assay, and in control subjects. When L‐palmitoylcarnitine was reduced from 0.5 mM to 0.05 mM in the isotope exchange assay, enzyme activity returned to normal in the patients but was not significantly altered in the controls. When the forward assay was performed in the presence of 80 μM palmitoyl‐CoA and 0.1% albumin, all patients showed normal carnitine palmitoyltransferase activity. The apparent K_m values for DL‐carnitine and palmitoyl‐CoA were also normal in the patients. When albumin was omitted from the forward assay, 72–105% of the initial activity was observed in the controls, but only 31–55% in the patients. When the palmitoyl‐CoA concentration in the forward assay exceeded 0.08 mM the enzyme activity was inhibited in both patients and controls, but the inhibition was significantly greater in the patients. The addition of either L‐palmitoylcarnitine or DL‐palmitoylcarnitine to the forward assay progressively inhibited enzyme activity in both patients and controls, but the inhibition was significantly greater in the patients. In the controls but not the patients D‐palmitoylcarnitine was less inhibitory than the L‐isomer or the DL‐racemate. When the forward assay was performed with muscle homogenates preincubated with 0.4% Triton X‐100 only 7–21% of the original enzyme activity remained in the patients, but 86–110% was found in the controls. Increasing concentrations of malonyl‐CoA inhibited both the forward and the isotope exchange assays. When the inhibition was maximal, only 14–18% of the CPT activity remained in homogenates of patients but 32–47% in homogenates of controls. The I₅₀ (median inhibitory concentration) and K_i values for malonyl‐CoA determined in the forward assay were not significantly different in the patients and controls. The data imply that CPT deficiency is caused by altered regulatory properties of a mutant enzyme and/or by altered interaction between the enzyme and its membranous environment rather than lack of catalytically active CPT I, II or both. The mutant CPT would be most vulnerable to inhibition by its substrate and/or product when lipid metabolism is stressed. This could also explain why the symptoms differ from muscle carnitine deficiency, and why so little lipid accumulates in muscle in CPT deficiency.