ULTRASTRUCTURAL, IMMUNOCYTOCHEMICAL AND STEREOLOGICAL INVESTIGATION OF HEPATOCYTES IN A PATIENT WITH THE MUTATION OF THE ORNITHINE TRANSCARBAMYLASE GENE

We studied a male newborn suffering from deficiency of ornithine trans carbamylase (OTC) that is due to a G-to-A substitution in codon 269 of the OTC gene, This study intends to define the cell biological mechan isms in this naturally occurring OTC mutation which may explain the mi ld clinical course in spite of the very low residual enzyme activity, Using immunogold labeling of thawed thin frozen sections of liver from this patient and a control liver, we analyzed the quantitative distri bution of several mitochondrial proteins in the cytosol and the mitoch ondria of hepatocytes, In addition, the absolute volumes and surface d ensities of mitochondria and peroxisomes were determined, Our results show that the absolute volume of mitochondria in the patient's hepatoc ytes was increased to 141% (P < 0.001) without any change in the surfa ce density indicating an increased number of mitochondria, In the pati ent's hepatocytes the peroxisomes were increased in size but not in nu mber, The concentration of OTC was elevated in the cytosol (P < 0.001) and to a lesser extent in mitochondria (P < 0.01) of the patient's he patocytes thus indicating a doubling of OTC relative to control liver cells. The quantity of OTC in mitochondria was 63% higher in diseased liver cells. By conventional thin section electron microscopy, mitocho ndria-like structures with poorly defined cristae and an electron-dens e matrix were observed in the cytoplasm of the diseased hepatocytes. B y immunoelectron microscopy, they contained the cytochrome c oxidase I I subunit as well as DNA but lacked OTC, carbamylphosphate synthetase, F-1-ATPase beta submit and catalase. Thus it appears that these struc tures represent defective and probably degenerating mitochondria, Our data indicate that the reduced enzyme activity of the mutant OTC is pa rtly compensated by an increased amount of enzyme molecules in the cyt osol as well as mitochondria combined with an increase in the biogenes is of mitochondria.