The clinically variable R40H mutant ornithine carbamoyltransferase shows cytosolic degradation of the precursor protein in CHO cells M. MAVINAKERE 1 , H. MORIZONO 1 , D. SHI 1 , N. M. ALLEWELL 2 and M. TUCHMAN 1 * 1 Children's Research Institute, Children's National Medical Center, The George Washington University, Washington DC; 2 College of Life Sciences, University of Maryland, College Park, Maryland, USA * Correspondence: Children's National Medical Center, 111 Michigan Avenue NW, Washington DC 20010-2970, USA. E-mail: mtuchman@cnmc.org MS received 20.02.01 Accepted 25.05.01 Summary: Ornithine carbamoyltransferase (OCT) de¢ciency is now frequently found in adults with hyperammonaemia affected by mutations that cause partial de¢ciencyofthisureacycleenzyme.Oneofthesemutations(R40H)hasoccurred in several families and has been found also in asymptomatic relatives. To better understand the phenotypic heterogeneity of this recurrent mutation, we inves- tigated the biological properties of the mutant protein. Using 35 S labelling, the import and processing of the R40H mutant OCT protein was investigated in intact CHO cells and in isolated rat liver mitochondria and compared to the wild type and R141Q mutant that causes complete enzyme de¢ciency. TheR40HOCTproteinseemstobeimportedandprocessedbythemitochondria in a manner similar to that of wild type. However, it is consistently degraded to a smaller fragment in the intact cells, unlike the wild type and R141Q mutant. The mature form of the enzyme is not susceptible to degradation. These data, obtained in CHO cells, suggest that de¢ciency in OCT enzymatic function con- ferred by the R40H mutation is likely caused by enhanced degradation of the preprotein in the cytosol. We propose therefore that variation in the rate of OCT turnover is responsible for the heterogeneity of the clinical phenotype in these patients. Ornithine carbamoyltransferase (OCT) enzyme (EC 2.1.3.3) catalyses the conver- sion of ornithine and carbamoyl phosphate to citrulline and inorganic phosphate. Human OCT is expressed from a gene on the X-chromosome as an inactive precursor J. Inherit. Metab. Dis. 24 (2001) 614^622 # SSIEM and Kluwer Academic Publishers. Printed in the Netherlands. 614