Normal muscle CPT 1 and CPT 2 activities in hepatic presentation patients with CPT 1 deficiency in fibroblasts : Tissue specific isoforms of CPT 1?

Abstract

Human carnitine palmitoyltransferase (CPT) deficiency results in 2 clinical forms: a more common “muscular form” with myoglobinuria with or without delayed or impaired ketogenesis and a rare “hepatic form” with hypoketotic hypoglycemia, encephalopathy and seizures without muscular manifestations. We present 2 patients, a male (patient 1) and a female (patient 2) with infantile “hepatic” CPT deficiency and previously documented CPT 1 deficiency in fibroblasts. In patient 2, a deficiency of “total” CPT activity in liver had also been previously documented. We set up an isotope exchange assay system that effectively differentiated CPT 1 and CPT 2 activities in muscle. We found normal CPT 1 and CPT 2 activities in our patients under near saturating substrate conditions. The CPT 1 and CPT 2 activities were suppressed to a strikingly similar degree under different kinetic conditions as compared to control muscle and were found to have similar K m values for carnitine and PCoA. With K m concentrations of carnitine, the mean residual activities of CPT 1 for patients 1 and 2 were 49 and 44%, respectively (control range 40–53%); the mean residual activities of CPT 2 were 60 and 46%, respectively (control range 49–59%). With K m concentrations of PCoA, the mean residual activities of CPT 1 for patients 1 and 2 were 52 and 58%, respectively (control range of 52–59%); mean residual activities of CPT 2 were 54% and 56%, respectively (control range of 51–68%). When the V max concentration of PCoA was doubled and bovine serum albumin reduced to 0.1%, the mean residual activities of CPT 1 for patients 1 and 2 were 69 and 63%, respectively (control range 60–80%). In “muscular” patients, a marked absolute deficiency of CPT 2 activity (< 12% residual) was found with an apparent increased sensitivity to suppression of enzymatic activity when the K m concentration of carnitine was used. We suggest that CPT 1 and CPT 2 may be separate proteins. Furthermore, CPT 1 itself may exist as tissue-specific isoforms being the same protein in liver and fibroblasts and a different protein in muscle. Either could be encoded for by the same or closely related genes.