O-Acetylhomoserine Sulfhydrylase from Neurospora

An enzyme was purified 500-fold from Neurospora which catalyzes the formation of homocysteine from O-acetylhomoserine and sulfide or methionine from O-acetylhomoserine and methylmercaptan. The two activities were not separated by purification. The rate with methylmercaptan is 1.8 times that with sulfide. No cystathionine is formed from cysteine, and there is essentially no α-ketobutyrate formed from O-acetylhomoserine or cystathionine. The enzyme is highly specific for O-acetylhomoserine, having negligible activity with homoserine, serine, and several of their other acyl derivatives. The apparent Michaelis constants are 7 x 10-3 m for O-acetylhomoserine in both reactions, 7 x 10-4 m for sulfide, and 8 x 10-4 m for methylmercaptan. A requirement for pyridoxal phosphate can be shown after dialyzing the enzyme with cysteine. Moderate inhibition was observed with methionine, S-adenosylmethionine, and S-adenosylhomocysteine, but not with cystathionine. No repression by methionine was observed. No mutants were found which are defective in this activity. Previous evidence led to the conclusion that these reactions are not the primary mechanisms in vivo for homocysteine and methionine synthesis. A possible temporary function for this enzyme may be formation of methionine from methylmercaptan at times of exhaustion of an exogenous source of sulfur.