Medium Chain 3-Ketoacyl-Coenzyme A Thiolase Deficiency: A New Disorder of Mitochondrial Fatty Acid β-Oxidation

A Japanese male neonate died at 13 d of age after presenting at 2 d of age with vomiting, dehydration, metabolic acidosis, liver dysfunction, and terminal rhabdomyolysis with myoglobinuria. Multiple urine organic acid analyses consistently revealed a markedly elevated excretion of lactic acid, 3-hydroxybutyric acid, and saturated and unsaturated C6-C16 dicarboxylic acids, with predominant C12-C16 species. Oxidation of [1-14C]octanoic acid in cultured skin fibroblasts was significantly reduced (0.59 nmol/h/mg of protein; controls, 1.93 +/- 0.65), [1-14C]palmitic acid oxidation was 1.11 nmol/h/mg of protein (controls, 1.63 +/- 0.41). A systematic study of the catalytic activities of nine enzymes of the beta-oxidation cycle using the respective optimal substrate revealed a deficiency of a single enzyme not previously associated with a metabolic disorder, medium chain 3-ketoacyl-CoA thiolase (patient, 3.9 nmol/min/mg protein; controls (n = 6), 10.2 +/- 2.3). Immunoprecipitation with antibodies raised against medium chain 3-ketoacyl-CoA thiolase revealed a 60% decrease compared with controls.