Restoration of nitrogen homeostasis in a man with ornithine transcarbamylase deficiency.

We evaluated the hypothesis that sodium phenylbutyrate-induced phenylacetylglutamine biosynthesis in a man with partial ornithine transcarbamylase (OTC) deficiency has a dual effect; it provides an additional vehicle for waste nitrogen excretion, and in the process it suppresses the patient's residual urea N synthesis, which then may be available for N homeostasis if the need arises. A 38-year-old man was studied over three periods. Period I was a control period during which he received a fixed caloric and N intake plus L-citrulline. Phenylbutyrate was added in period II and was maintained during period III, during which his N intake was increased. Plasma levels of ammonium and glutamine and net urea N synthesis were measured in each period; phenylacetylglutamine N synthesis was measured in periods II and III. These studies demonstrated that phenylbutyrate administration led to a 73% decrease in net de novo urea N synthesis during period II, which subsequently increased threefold in period III in response to the increased N intake. Phenylacetylglutamine N synthesis was 2.27 g/d, similar to his estimated maximum net urea N synthesis of 2.65 g/d. During periods II and III, his plasma levels of ammonium and glutamine improved as compared with period I when they were abnormally high. We conclude that sodium phenylbutyrate treatment of patients with urea cycle disorders who have significant residual enzyme activity results in both an improvement in waste N excretion and improved N homeostasis as a result of the generation of a reserve urea N synthetic capacity. This therapeutic approach may be useful in other nitrogen accumulation decreases, eg, portal-systemic encephalopathy.