UTILIZATION OF S-METHYLCYSTEINE AND METHYLMERCAPTAN BY METHIONINELESS MUTANTS OF NEUROSPORA AND PATHWAY OF THEIR CONVERSION TO METHIONINE .2. ENZYME STUDIES

1. 1. Methylcysteine lyase, which catalyzes the breakdown of methylcysteine to methylmercaptan, pyruvate and ammonia, was purified from extracts of wild-type Neurospora mycelia. The presence of methylcysteine lyase in me-2 methionineless mutants indicates that this activity is not due to β-cystathioanse. 2. 2. Ammonium sulfate fractionation of wild-type Neurospora extracts resulted in separation of the methylcysteine lyase from the β- and γ-cystathionases. The methylcysteine lyase, which was further purified by DEAE-cellulose chromatography, is apparently able to cleave methionine and cystathionine; the latter by either β- or γ-elimination. 3. 3. Methionine synthase, which catalyzes the reaction between O-acetylhomoserine and methylmercaptan, was shown to be present in extracts prepared from wild-type Neurospora and in extracts from me-7 methionineless mutant. The latter result implies that this activity is not due to cystathionine-γ-synthase. 4. 4. Methionine synthase was separated from cystathionine-γ-,ynthase showing that methionine synthase is not the result of an abnormal activity of these known enzymes. 5. 5. The results indicated that the two enzyme activites, methylcysteine lyase and methionine synthase, are not catalyzed by enzymes of the normal methionine biosynthetic pathway. The role of the alternate pathway of methionine biosynthesis by thiomethyl transfer from methylcysteine to O-acetylhomoserine is discussed. © 1969.