Mechanisms of methionine metabolism and toxicity: Roles of the transmethylation, transamination, and sulfoxidation pathways

L-Methionine (Met) is hepatotoxic in laboratory animals when present at high levels and may play a role in total parenteral nutrition-associated cholestasis in infants. While the mechanisms leading to Met hepatotoxicity are unclear, excessive formation of the Met metabolites 3-methylthiopropionic acid and homocysteine via the Met transamination (TA) and transmethylation (TM) pathways, respectively, may play a role in Met toxicity. Furthermore, the toxicological role of the Met sulfoxidation (SO) pathway is not known despite the increased detection of L-methionine-dl -sulfoxide in hypermethionemic laboratory animals and humans. To test the hypothesis that the Met TM, TA, and SO pathways play significant roles in Met metabolism and hepatotoxicity, in vivo Met metabolic studies in mice and in vitro Met metabolism and toxicity studies in freshly isolated mouse hepatocytes (FIMHs) were carried out. Male and female mice given a single i.p. dose of Met (400 mg/kg) had higher levels of Met TM and SO metabolites in the liver compared to mice given vehicle alone confirming the importance of these pathways in Met metabolism. In FIMHs, Met toxicity was characterized by increased lactate dehydrogenase (LDH) leakage, decreased trypan blue (TB) exclusion, and GSH depletion without GSSG formation in male hepatocytes whereas female hepatocytes were insensitive to Met toxicity. Met toxicity in male hepatocytes was potentiated by addition of 3-deazaadenosine, an inhibitor of the Met TM pathway, and was partially prevented by addition of aminooxyacetic acid (AOAA), an inhibitor of the Met TA pathway. Male FIMHs were also much more sensitive to L-methionine-dl-sulfoxide (MetO) toxicity compared to female FIMHs, which was also characterized by increased LDH leakage, decreased TB exclusion, and GSH depletion without GSSG formation. Addition of AOAA also reduced MetO toxicity in male hepatocytes. Studies examining the potential metabolic mechanisms responsible for gender differences in Met toxicological sensitivity provided evidence for increased Met uptake, decreased Met TM, and increased Met and MetO TA in male compared to female hepatocytes. Collectively, these data provide toxicological and metabolic evidence for the Met TM pathway playing a detoxification role in Met toxicity in FIMHs with the Met TA and SO pathways being involved in Met bioactivation.