Abstract
Physiological roles of the transsulfuration pathway have been recognized by its contribution to the synthesis of cytoprotective cysteine metabolites, such as glutathione, taurine/hypotaurine, and hydrogen sulfide (H(2)S), whereas its roles in protecting against methionine toxicity remained to be clarified. This study aimed at revealing these roles by analyzing high-methionine diet-fed transsulfuration-defective cystathionine γ-lyase-deficient (Cth(-/-)) mice. Wild-type and Cth(-/-) mice were fed a standard diet (1 × Met: 0.44%) or a high-methionine diet (3 × Met or 6 × Met), and hepatic conditions were monitored by serum biochemistry and histology. Metabolome analysis was performed for methionine derivatives using capillary electrophoresis- or liquid chromatography-mass spectrometry and sulfur-detecting gas chromatography. The 6 × Met-fed Cth(-/-) (not 1 × Met-fed Cth(-/-) or 6 × Met-fed wild type) mice displayed acute hepatitis, which was characterized by markedly elevated levels of serum alanine/aspartate aminotransferases and serum/hepatic lipid peroxidation, inflammatory cell infiltration, and hepatocyte ballooning; thereafter, they died of gastrointestinal bleeding due to coagulation factor deficiency. After 1 week on 6 × Met, blood levels of ammonia/homocysteine and hepatic levels of methanethiol/3-methylthiopropionate (a methionine transamination product/methanethiol precursor) became significantly higher in Cth(-/-) mice than in wild-type mice. Although hepatic levels of methionine sulfoxide became higher in 6 × Met-fed wild-type mice and Cth(-/-) mice, those of glutathione, taurine/hypotaurine, and H(2)S became lower and serum levels of homocysteine became much higher in 6 × Met-fed Cth(-/-) mice than in wild-type mice. Thus, transsulfuration plays a critical role in the detoxification of excessive methionine by circumventing aberrant accumulation of its toxic transamination metabolites, including ammonia, methanethiol, and 3-methylthiopropionate, in addition to synthesizing cysteine-derived antioxidants to counteract accumulated pro-oxidants such as methionine sulfoxide and homocysteine.