Desulfhydrase activity

L-Cysteine desulfhydrase in leaves of cucurbit plants is a constitutive enzyme whose activity can be enhanced by preincubation of leaf discs with L-cysteine, D-cysteine, or structural analogs of L-cysteine at millimolar concentrations preincubation with cystine does not affect the activity of the enzyme (20.261. Although the stimulation of L-cysteine desulfhydrase activity is... 

Enzymes that possess the same catalytic activity but are evolutionarily unrelated and play very diverse biological functions can be found, for example, in a group of PLP-dependent enzymes with desulfhydrase activity, that is, enzymes that release H2S from thiol amino acids. Bacteria employ desulfhydrases not only in the metabolism of sulfurated amino acids and in the adaptation to new nutrient sources, but also, sometimes, as virulence factors. An E. coli enzyme with desulfhydrase activity is even known to act as a modulator of gene expression, although this function seems to be unrelated to catalysis. Sulfide production by PLP-dependent enzymes is also important in vertebrates, where H2S has been shown to act as a neuromodulator. " ... 

Since even a slightly elevated L-cysteine concentration is inhibitory or possibly toxic for the cells E. coli possesses another mechanism for detoxification of this compound in addition to excretion degradation of L-cysteine. Five enzymes with L-cysteine desulfhydrase activity have been identified so far in this organism L-tryptophanase (TnaA), L-cystathionine p-lyase (MetC),... 

O-acetyl-L-serine (thiol)-lyase A (CysK), O-acetyl-L-serine (thiol)-lyase B (CysM) and MalY. Thus in order to engineer a potent L-cysteine over-producer excessive degradation of the amino acid must be prevented by inactivation of the genes encoding the major L-cysteine desulfhydrase activities. 

Kallio has partially purified the cysteine and homocysteine desulfhydrases from P. morganii and found that pyridoxal phosphate was active as cofactor. The possibility that threonine might be an intermediate in reaction 21 was ruled out by the finding that threonine was relatively inactive as a substrate. The role of pyridoxal phosphate as cofactor for animal desulfhydrases is suggested from the findings of Braunstein and Azarkh" that liver homogenates from pyridoxine-deficient animals had lower cysteine desulfhydrase activity than those from normal animals. Dietary supplementation with pyridoxine raised the activity to the normal level. 

Corroborative evidence has been obtained from experiments with isolated tissue preparations. The in vitro formation of cystine by liver slices (and a saline extract) was observed in nuxtures containing dl-homocysteine and DL-serine. Neither substance was effective when incubated alone. The L-forms of the two amino acids were implicated in the reaction, as D-homocysteine and o-serine did not substitute for the DL-forms and the isolated cystine had the L-configuration. The reaction proceeded best under anaerobic conditions and in the present of O.OOlAf CN , as the latter inhibits cysteine desulfhydrase activity. Methionine substituted very poorly for homocysteine in vitro. 

Loss of sulfur by desulfhydrase activity, which is generally now considered to be of little or no metabolic significance, leads to the formation of pyruvate. More important, the pathway of oxidation of the sulfur of cysteine (see Chapter 16) also results in the formation of pyruvate. The subsequent metabolism of pyruvic acid should then be along the well-known pathways of carbohydrate metabolism. 

Fig. 4. The effects of dietary protein on the activities of cysteine dioxygenase, cysteine desulfhydrase in liver and the urinary taurine excretion of intact rat. Plotted are cysteine dioxygenase activity(0). Cysteine desulfhydrase activity (O )> urinary taurine contents (I j). Results are expressed as the mean S.D.(represented by vertical line) of six and three animals fed on basal diet (20% protein diet) and other diets, respectively. Animals were fed on experimental diets for 2 days prior to sacrifice.

Plants also contain D-cysteine desulfhydrase activity. This enzyme, first re-... 

The enantiomers of this drug differ in their efficacy and activity, with (D)-penicilla-mine being the enantiomer required for pharmaceutical preparations. The (l)-enantiomer is toxic, and its absorption by the human body is more than the (D)-enantiomer. While both enantiomers of penicillamine are desulfhydrated by (r.)-cysteine desulfhydrase, only the (l)-isomer inhibits the action of this enzyme [2], The reported optical rotation values for (D)-penicillamine are ... 

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. 

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. 

If L-cysteine desulfhydrase is responsible for the H2S emitted by plants in the presence of L-cysteine, the lower rates of H2S evolution obtained with D-cysteine seem anomalous in view of the much higher activity of the D-specific enzyme in the tissues examined. At least part of this apparent anomaly can be attributed to the more rapid uptake of the L-isomer, which is about fourfold greater than with D-cysteine. Although L-cysteine desulfhydrase has not been purified and the kinetics examined in detail, the available information suggests... 

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