L-methionine catabolism in trichomonads

Trichomonas vaginalis growing in complex medium produced volatile thiols at a rate of 0.7 nmol min-1 (mg protein)-1 and the parasite suspended in PBS with L-methionine excreted volatile thiols, including methanethiol, and alpha-keto acid. Cell-free extracts of the parasite also produced volatile thiols from L-methionine, at the rate of 5.4 nmol min-1 (mg protein)-1. Thiol production was not detectable with living cells or cell-free extracts of Tritrichomonas foetus, Trichomitus batrachorum or Pentatrichomonas hominis and homogenates of a range of trypanosomatids and mouse liver also failed to produce volatile thiols from L-methionine. Approximately equimolar concentrations of alpha-keto acid and volatile thiols were produced from L-methionine by cell-free extracts of Trichomonas vaginalis; the release of ammonia, however, was not detectable. The parasite enzyme catabolised a range of substrates and was inhibited by several compounds, including bithionol and DL-propargylglycine. Parasites grown in the presence of 10(-5) M DL-propargylglycine had no detectable L-methionine-catabolising enzyme activity. These findings indicate that T. vaginalis is significantly different from other trichomonads, a range of trypanosomatids and mouse liver in L-methionine catabolism, and that the parasite enzyme responsible for the breakdown of L-methionine in T. vaginalis appears to be similar in several ways to bacterial L-methionine-gamma-lyase (EC 4.4.1.11) and trichomonal homocysteine desulphurase (EC 4.4.1.2).