Accumulation of hypotaurine in tissues and urine of rats fed an excess methionine diet

Systematic re-evaluation of the long-used standard protocol of urease-dependent metabolome sample preparation

In the urinary metabolomics for finding biomarkers in urine, owing to high concentrations of urea, for chromatography-based metabolomic analysis, urea needed to be degraded by urease. This urease pretreatment has been the key step of sample preparation for standard urinary metabolomics until today even for mass spectrometry-based analysis. The urease pretreatment involving incubation of urine with urease contradicts the concept of metabolome sampling, which should immediately arrest metabolic reactions to prevent alterations of a metabolite profile. Nonetheless, the impact of urease pretreatment has not been clearly elucidated yet. We found that activities of urease and endogenous urinary enzymes and metabolite contaminants from the urease preparations introduce artefacts into metabolite profiles, thus leading to misinterpretation.

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 irreversible oxidation of cysteine, CDO protein is up-regulated 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 PS1 (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 PS1 had a significant effect on sulphoxidation flux secondary to the stabilization of CDO but no significant effect on 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-mercaptoethylamine (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 on cysteine catabolism.