Urea cycle regulation: I. Coupling of ornithine metabolism to mitochondrial oxidative phosphorylation

Ornithine metabolism is coupled to oxidative phosphorylation in isolated rat liver mitochndria. The pathway involving ornithine:a-ketoglutarate transaminase (OKT), glutamic semialdehyde dehydrogenase (GSDH), and glutamate dehydrogenase (GDH) with cycling of a-ketoglutarate-glutamate at the OKT reaction appears to be involved. Ornithine may be utilized by this pathway to sustain ATP levels during mitochondrial energy-defìciency states with resultant decreased urea-cycle flux and increased ammonia production. This pathophysiologic mechanism suggests that hyperammonemia is a consequence of an energy-defìciency state. Therapy directed toward alleviating the energy-defìciency state may be more beneficial than efforts to reduce ammonia levels.