The clinically variable R40H mutant ornithine carbamoyltransferase shows cytosolic degradation of the precursor protein in CHO cells

• Published in: Journal of inherited metabolic disease Vol. 24; no. 6; pp. 614 - 622
• Main Authors: Mavinakere, M., Morizono, H., Shi, D., Allewell, N. M., Tuchman, M.
• Format: Journal Article
• Language: English
• Published: Dordrecht Kluwer Academic Publishers 01-11-2001; Springer; Blackwell Publishing Ltd
• Subjects: Adult; Aminoacid disorders; Animals; Biological and medical sciences; CHO Cells; Cricetinae; Cytosol - enzymology; Errors of metabolism; Genetic Vectors; Humans; In Vitro Techniques; Medical sciences; Metabolic diseases; Methionine - metabolism; Mitochondria, Liver - metabolism; Mutation; Ornithine Carbamoyltransferase - genetics; Ornithine Carbamoyltransferase - metabolism; Precipitin Tests; Protein Biosynthesis; Protein Precursors - genetics; Protein Precursors - metabolism; Rats; Transcription, Genetic; Human; Pathophysiology; Enzyme; Pathogenesis; EC 2.1.3.3; Transferases; Deficiency; Metabolic diseases; Biosynthesis; Enzymopathy; Precursor; Protein; Genetic disease; Ornithine carbamoyltransferase; Degradation; Heterogeneity; Phenotype; Aminoacid disorder
• ISSN: 0141-8955; 1573-2665
• DOI: 10.1023/A:1012726207870

Ornithine carbamoyltransferase (OCT) deficiency is now frequently found in adults with hyperammonaemia affected by mutations that cause partial deficiency of this urea cycle enzyme. One of these mutations (R40H) has occurred in several families and has been found also in asymptomatic relatives. To better understand the phenotypic heterogeneity of this recurrent mutation, we investigated the biological properties of the mutant protein. Using 35S 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 deficiency. The R40H OCT protein seems to be imported and processed by the mitochondria 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 deficiency in OCT enzymatic function conferred 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.