Microbial transsulfuration: the mechanism of an enzymatic disulfide elimination reaction.

In mammalian tissues the formation of cysteine from homocysteine, a process which has been called “transsulfuration,” has been shown to be mediated by two separable enzymes (l-3). The first catalyzes a condensation between homocysteine and serine to yield the unsymmetrical thioether L-cystathionine. The latter is then cleaved, by a y elimination catalyzed by the second enzyme, to cysteine, cu-ketobutyrate, and ammonia. The cleavage enzyme also catalyzes the decomposition of homoserine to cr-ketobutyrate and ammonia. The ability of microorganisms to grow without exogenous methionine has generally been attributed to their capacity to carry out a reciprocal pair of reactions, in which cystathionine is formed from cysteine and homoserine, and cleaved, by a /3 elimination, to homocysteine, pyruvate, and ammonia. Some evidence has been reported that microorganisms also possess the two enzymes present in animal tissues (4, 5). I f this were correct, sulfur could be reversibly transferred from cysteine to homocysteine by a process mediated by four different enzymes, each catalyzing an essentially irreversible reaction. This scheme is based on studies of the nutritional requirements and metabolite accumulations of Neurospora mutants blocked in methionine biosynthesis (4, 6). At the enzyme level it remains largely hypothetical. We have begun a study of microbial transsulfuration by undertaking to isolate the cystathionine cleavage enzyme which catalyzes y elimination to yield cysteine and cr-ketobutyrate.1 To facilitate obtaining this enzyme free from the hypothetical second cleavage enzyme, we chose as enzyme source a Neurospora mutant that can grow when supplied with methionine or homocysteine, but not with cystathionine, which it accumulates (4). The mutant was expected to lack an enzyme which cleaves cystathionine to homocysteine and pyruvate. This expectation has not, in fact, been realized. Our present evidence suggests that wild type Neurospora may contain only one cleavage enzyme, which catalyzes a heterogeneous decomposition of cystathionine. We have not detected any modifications in the physical or catalytic properties of the enzyme from the mutant strain, or any difference in the amount of it present.2