Dynamism of Rat Liver Ornithine Metabolisms in Relation to Dietary High-Protein Stimulation of the Urea Cycle
Reports on hereditary deficiencies of the urea cycle enzyme, e.g. hyperammonemia with homocitrullinuria1 and ornithine carbamoyl transferase(OCT) deficiency2, led us to the view that ornithine transport into the mitochondrial matrix represents an important factor controlling citrulline synthesis and plays a role in the regulation of carbamoyl phosphate synthesis3. We have long been interested in whether ornithine is derived from arginine or proline, i.e., whether ornithine, produced from arginine in the cytosol, is transported into the mitochondria, or whether ornithine is derived from proline via Δ1-pyrroline-5-carboxylate(P5C) in the mitochondrial matrix. Once the urea cycle is activated, ornithine is continuously transported into the mitochondrial matrix from cytosol and released from mitochondria into the cytosol(“ornithine flux”); this transport system is composed of a citrulline-exchange transporter and an H+ antiporter4,5. We now present some in vitro mitochondrial studies on ornithine transport and citrulline formation from proline, and an analysis of in vivo arginine and ornithine metabolisms. This investigation was undertaken to elucidate the characteristics of “orni-thine flux” and its relation to dietary stimulation of urea cycle.
KeywordsUrea Cycle Carbamoyl Phosphate Casein Diet Citrulline Level Ornithine Carbamoyl Transferase
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