Threonine dehydrase

Potassium is a cofactor and activates a large variety of enzymes, including glycerol dehydrogenase, pyruvate kinase, L-threonine dehydrase, and ATPase. Its acute toxicity is primarily due to its action as an electrolyte. Excessive or diminished potassium levels can disrupt membrane excitability and influence muscle cell contractility and neuronal excitability. 

Serine and threonine dehydrases. Serine and threonine are not substrates in transamination reactions. Their amino groups are removed by the pyridoxal phosphate-requiring hepatic enzymes serine dehydratase and threonine dehydratase. The carbon skeleton products of these reactions are pyruvate and a-keto-butyrate, respectively. 

The most useful, and thus far successful, examples have involved irreversible reactions of nucleophilic functions of an enzyme s reactive site with an enzymatically activated Kcat inhibitor of a Michael-type addition reaction. The activation invariably requires participation of the enzyme s prosthetic group (e.g., flavin of monoamine oxidase) or coenzymes such as pyridoxal (vitamin B) as its phosphate, which is associated with several enzymes (e.g., threonine dehydrase, ornithine decarboxylase, a-ketoglutarate transaminase). 

L-Threonine Dehydrase as a Model of Allosteric Control Involving Ligand-Induced Oligomerization... 

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... 

An absolute requirement for pyridoxal phosphate was found for the Neurospora enzyme, none for the E. coli enzyme. The failure to observe this in the latter may be connected with the difficulty of obtaining dehydrase preparations free of pyridoxal phosphate.Pyridoxamine phosphate could not substitute for the pyridoxal phosphate. That there is a requirement for pyridoxal phosphate is supported by the fact that the threonine dehydrase activity is strongly inhibited by 5 X 10 M hydroxylamine and cyanide. 

The mammalian enzymes, L-serine dehydrase and L-threonine dehy-drase, have been separated and purified from sheep liver by Sayre and Greenberg (230). L-Serine dehydrase was found to be specific for L-serine. No activity was observed with the n-isomer or with d- or L-threonine. L-Threonine dehydrase was active with L-threonine, but not the D-isomer, and to a slight extent with L-serine. It has not been established whether L-serine is a true substrate for L-threonine dehydrase or whether the enzyme preparation is still contaminated with the L-serine dehydrase. Neither enzjrme was active with DL-allothreonine, DL-homocjrsteine, DL-cysteine, or jS-phenylserine. 

Evidence has been presented that L-threonine dehydrase 8S1), but not L-serine dehydrase, is an inducible enzyme in mice and rats. The activity of threonine dehydrase in the livers of these animals is increased up to four times the normal value by the intraperitoneal injection of substrate. No change in serine dehydrase activity was observed. This is substantiating evidence that two separate enzymes are involved for the two substrates. Similar effects could be shown in liver perfusion studies. The inducible character of L-threonine dehydrase in E. colt 232) has been observed (see discussion below). 

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. 

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. 

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). 

Phillips, A. T., and Wood, W. A., 1964, Basis for AMP activation of biodegradative" threonine dehydrase from Escherichia coli, Biochem. Biophys. Res. Commun. 15 530. 

Peraino, C., Blake, R. L., and Pitot, H. C., 1965, Studies on the induction and repression of enzymes in rat liver. III. Induction of ornithine 8-transaminase and threonine dehydrase by oral intubation of free amino acids, /. Biol Chem. 240 3039. 

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