Expression analysis of two mutant human ornithine transcarbamylases in COS-7 cells

Ornithine transcarbamylase (OTC) is located in the mitochondrial matrix of the liver and small intestine and catalyzes the second step of the urea cycle. OTC deficiency (OTCD) is an X-linked inborn error of metabolism and causes hyperammonemia. We reported in 1992 the A152V and G195R mutations in patients with OTCD. These mutant OTC cDNAs were prepared by site-directed mutagenesis using the polymerase chain reaction (PCR). The wild-type and mutant cDNAs were transiently expressed in COS-7 cells. The wild-type cDNA gave an OTC activity of 1180 +/- 47 nmol/min per mg protein. The OTC activities of the A152V and the G195R mutants were 3.7% and 2.5% of that of wild-type, respectively. Immunoblot analysis showed that the quantities of OTC proteins in the A152V and G195R mutants were 29% and 12% of that of wild-type, respectively. In pulse-labeling and pulse-chase experiments, the precursor form of OTC was synthesized and processed to the mature form. The A152V mutant OTC was processed to the mature form as rapidly as the wild-type precursor. However, the processed, mature form of the mutant OTC was rapidly degraded, presumably in the mitochondrial matrix. These results indicate that OTCD with the A152V mutation is due both to rapid degradation of the processed, mature form, and to a lower specific activity of the remaining protein.