Amino acids downregulate SIRT4 to detoxify ammonia through the urea cycle

Ammonia production via glutamate dehydrogenase is inhibited by SIRT4, a sirtuin that displays both amidase and non-amidase activities. The processes underlying the regulation of ammonia removal by amino acids remain unclear. Here, we report that SIRT4 acts as a decarbamylase that responds to amino acid sufficiency and regulates ammonia removal. Amino acids promote lysine 307 carbamylation (OTC^CP-K307) of ornithine transcarbamylase (OTC), which activates OTC and the urea cycle. Proteomic and interactome screening identified OTC as a substrate of SIRT4. SIRT4 decarbamylates OTC^CP-K307 and inactivates OTC in an NAD⁺-dependent manner. SIRT4 expression was transcriptionally upregulated by the amino acid insufficiency-activated GCN2-eIF2α-ATF4 axis. SIRT4 knockout in cultured cells caused higher OTC^CP-K307 levels, activated OTC, elevated urea cycle intermediates and urea production via amino acid catabolism. Sirt4 ablation decreased male mouse blood ammonia levels and ameliorated CCl₄-induced hepatic encephalopathy phenotypes. We reveal that SIRT4 safeguards cellular ammonia toxicity during amino acid catabolism.