Serine dehydrases

There is a general requirement for pyridoxal-5-phosphate (24, 25, 27, 44) although not all of the activity lost on dialysis is restored by adding the cofactor. This requirement explains the inhibition by hydroxylamine and hydrazine (24, 25). The reaction is a typical pyridoxal-5-phosphate catalyzed a,/ -elimination with a mechanism similar to serine dehydrase and cysteine desulfhydrase (45). The coenzyme is probably bound as a Schiff base with an amino group of the enzyme since there is an absorption maximum at 415 nm in solutions of the purified garlic enzyme (40). The inhibition by L-cysteine is presumably caused by formation of a thiazolidine with the coenzyme (46). Added pyridoxal-5-phosphate also combines directly with the substrate. The dissociation constant for the complex is about 5 X lO M. When this is taken into account, the dissociation constant of the holoenzyme can be shown to be about 5 X 10 M (47). The higher enzyme activity in pyrophosphate buflFer than in Tris or phosphate may be explained by pyrophosphate chelation of metal ions which otherwise form tighter complexes with the substrate and coenzyme (47). This decreases the availability of added coenzyme. 

Brief reference will be made to the following papers which have appeared since this chapter was written. Partially purified Neurospora L-serine dehydrase has been studied. Both L-serine and L-threonine appear to be deaminated by the same enzyme. The enzymatic pathway of histidine degradation in liver has been investigated. Soluble gluta-minase I has been prepared. Additional eiddence for the Avide scope of transamination has appeared. The presence of an ornithine-a-keto-glutarate transaminase in neurospora has been demonstrated. Transamination of non-a-amino acids has been demonstrated in brain in... 

More important evidence, however, is the widespread distribution and high activity of dehydrases that deaminate serine in nature. The anaerobic deamination of DL-serine by cell-free extracts of many animal species proceeds extensively with the formation of pyruvic acid upon addition of Mg++. os Similar serine dehydrases occur in many bacterial species (E. colt, P. pyocyanea, P. Ox-19, and Cl. welchit). - ... 

Two distinct serine dehydrases (deaminases), one active on the L and the other on the d form, have been prepared from E. and from... 

Neurospora. Both enzyme preparations attack the corresponding enantiomorphs of threonine and, presumably, the capacity to dehydrate serine and threonine is a property of the same enzyme. The L-serine dehydrase is comparatively unstable, the n-serine dehydrase quite stable. 

D-Serine dehydrase from both E. coli and Neurospora required pyridoxal phosphate as a coenzyme. In contrast to L-serine dehydrase, threonine is attacked very poorly by the D-enz3une. Magnesium ions had no activating affect on the E. coli enzyme, but it partially reversed the inhibition caused by the metal-binding reagents, phosphate, citrate, cysteine, cyanide, and 8-hydroxyquinoline of the Neurospora enzyme. ... 

Binkley and Okeson purified the enzyme system that cleaves cystathionine and found that neither phosphate nor ATP was required for activity, thus correcting the previous report that ATP was required. In addition to splitting cystathionine, this enzyme preparation also produced H2S from cysteine. The authors suggest that their enzyme may be identical with cysteine desulfhydrase. Binkley also reported that he had been able to synthesize cystathionine enzymatically from homocysteine and serine by a fractionated liver preparation which had been freed from the cystathionine cleavage enzyme, serine dehydrase and homoserine deaminase. The activity of the enzyme synthesizing cystathionine was either inhibited or unaffected by ATP, DPN, AMP, and various metal ions. 

Yanofsky, C. (1952) D-serine dehydrase of Neurospora, J. Biol. Chem. 198,343-352. 

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). 

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. 

Cheung, Y., and C. Walsh Stereospecific Synthesis of Isotopically Labeled Serine at Carbon 3 and Stereochemical Analysis of D-Serine Dehydrase Reaction. J. Amer. Chem. Soc. 98, 3397 (1976). 

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