Regulation of cysteine dioxygenase degradation is mediated by intracellular cysteine levels and the ubiquitin-26 S proteasome system in the living rat

Mammalian metabolism of ingested cysteine is conducted principally within the liver. The liver tightly regulates its intracellular cysteine pool to keep levels high enough to Meet the many catabolic and anabolic pathways for which cysteine is needed, but low enough to prevent toxicity. One of the enzymes the liver uses to regulate cysteine levels is CDO (cysteine dioxygenase). Catalysing the irrevensible oxidation of cysteine, CDO protein is up-reaulated in the liver in response to the dietary intake of cysteine. In the present Study, we have evaluated the contribution of the ubiquitin-26 S proteasome pathway to the diet-induced changes in CDO half-life. In the living rat, inhibition of the proteasome with PSI (proteasome inhibitor 1) dramatically stabilized CDO in the liver under dietary conditions that normally favour its degradation. Ubiquitinated CDO intermediates were also seen to accumulate in the liver. Metabolic analyses showed that PSI had a significant effect oil sulphoxidation flux secondary to the stabilization of CDO but no Significant effect oil the intracellular cysteine pool. Finally, by a combination of in vitro hepatocyte culture and in vivo whole animal studies, We were able to attribute the changes in CDO stability specifically to cysteine rather than the metabolite 2-mencaptoethylamine (cysteamine). The present study represents the first demonstration of regulated ubiquitination and degradation of a protein in a living mammal, inhibition of which had dramatic effects oil cysteine catabolism.