1. This study was designed to investigate the transamination pathway of methionine in humans.

2. Evidence is provided that methanethiol and its metabolites are formed via transamination of methionine.

3. Gas-liquid chromatography was used to measure serum and urinary transamination metabolites of methionine: 2-keto-4-methylthiobutyrate, 3-methylthiopropionate and methanethiol, and the metabolites of methanethiol, dimethylsulphide, protein—S—S—CH3 (a mixed disulphide of blood proteins and methanethiol) and X—S—S—CH3 (a mixed disulphide of methanethiol and another thiol with an unknown component X).

4. Methionine and the transamination intermediates were measured in 10 normal subjects, in six normal subjects after L-methionine loading (0.1 g/kg body weight) and in a male patient with hepatic methionine adenosyltransferase (EC deficiency.

5. In the patient with methionine adenosyltransferase deficiency, at least 20% of methionine was degraded via transamination. In normal subjects transamination of methionine did exist but was quantitatively not important in methionine catabolism, not even after methionine loading.

6. The results of this study might be of importance for future studies on the role of methanethiol in hepatic encephalopathy.

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