Glutathione serine dehydrase

The preparation in a cell-free form of a D-serine dehydrase from E. coli which requires pyridoxal phosphate has been described by Metzler and Snell 218). This enzyme is readily separated from the L-serine (and threonine) dehydrase of Wood and Gunsalus 216). Unlike the latter enzyme, the D-serine dehydrase does not require AMP or glutathione. DL-Threonine was slowly deaminated by the system. 

Mammalian tissues contain enzymes that catalyze the nonoxidative deamination of serine, threonine, and homoserine. Since the postulated reaction mechanism involves a dehydration before the deamination, these enzymes are called dehydrases. L-Serine, L-threonine, and L-homoserine dehydrases have been partially purified and all are specific for the L-amino acid. Serine and threonine dehydrases require pyridoxal phosphate, ATP, and glutathione for activity. Pyridoxal phosphate requires the homoserine enzyme, but the need for ATP and glutathione has not been demonstrated. The reaction is likely to involve the formation of a Schiff base. The homoserine dehydrase has been... 

Umbarger and Brown 233) have observed L- and D-threonine dehy-drases in E. coli which are stimulated by pyridoxal phosphate. In a continuation of this work 234) they have presented evidence that E. coli extracts contain two distinct L-threonine dehydrases. One of the dehydrases is the L-threonine dehydrase of Wood and Gunsalus and requires pyridoxal phosphate, AMP, and glutathione. It was found to be an adaptive enzyme, and was active against L-serine. The second dehydrase is present in extracts of wild-type E. coli, but not in mutants which are unable to convert threonine to a-ketobutyrate as a step in isoleucine synthe. It requires only pyridoxal-5-phosphate and is inhibited by isoleucine. On the basis of kinetic studies it is proposed that the Wood-Gunsalus dehydrase combines with one molecule of substrate, whereas the other enzyme combines with two. L-Serine is a substrate for both enz3unes. Evidence is presented for a third deaminase in E. coli cells which is serine-specific. 

Both isomers of both serine and threonine are deaminated by Neuro-spora extracts 225-228). The enzymes involved have been purified by Yanofsky and his associates 225-228). A specific D-serine and D-threonine dehydrase has been purified thirty-five- to fortyfold from this mold 2f ). An absolute requirement for pyridoxal phosphate was demonstrated. No requirement for AMP or glutathione could be demonstrated. Some indication of a metal requirement was observed. The preparation was not active with the li-isomers of serine and threonine or DL-homoserine and DL-homo-cysteine. The rate of deamination of D-threonine is very slow compared to that with L-serine. Activity was observed with D-glutamic acid and D-as-partic acid. Since other D-amino acids were not deaminated by the preparation, these results could not be due to a contamination with D-amino acid oxidase. Furthermore, when either of these amino acids was incubated in the presence of D-serine, the keto acid production was a summation of that for each substrate alone. Pyridoxal phosphate had no effect on keto acid formation from the dicarboxylic amino acids. It is of interest that D-amino acid oxidase of Neurospora does not attack D-serine or D-threonine (77). 

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