Malonyl-CoA abnormal inhibition of residual enzyme activity in carnitine palmitoyltransferase deficiency.

The residual enzyme activity in tissues of a 6-year-old boy with myoglobinuria and carnitine palmitoyltransferase (CPT) deficiency was studied with malonyl-CoA, a specific inhibitor of CPT-I in rat tissues. In this patient the enzyme deficiency was limited to the CPT fraction insensitive to malonyl-CoA, since the residual activity was an increased amount of CPT sensitive to the inhibitor. CPT sensitivity to malonyl-CoA was also assayed in human liver mitochondria, and inhibition was similar to that found in rat liver. Moreover, comparative data on human liver mitochondria and biopsy specimens showed that, after freeze-thawing and homogenization, CPT sensitivity to malonyl-CoA was decreased in both these preparations, indicating that studies of CPT inhibition by malonyl-CoA in homogenates of frozen tissues may be equated to those in homogenates of frozen mitochondria.