Desulfhydrase liver

Cysteine Desulfhydrase. Cysteine undergoes a desulfuration that is believed to be analogous to the dehydration of serine. The enzyme cysteine desulfhydrase requires pyridoxal phosphate and forms pyruvate, H2S, and NH3. This enzyme has been found in animal liver and presumably occurs in microorganisms that release H2S from cysteine. Evidence has been obtained with H2S that indicates some reversibility of the reaction, but no thermodynamic data are available. A similar enzyme has been reported to form a-ketobutyrate, NH3, and H2S from homocysteine. ... 

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. 

Partially purified cysteine desulfhydrase from rat liver is inhibited by cyanide, arsenous oxide, and the carbonyl reagents phenylhydrazine, semicarbazide, hydroxylamine, and sodium bisulfite. 

An exocystine desulfhydrase has been found in rat liver by Greenstein and Leuthardt. This enzyme acts on peptides containing cystine in terminal linkage. ... 

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. 

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. 

IV. THE DESULFHYDRASE REACTION Enzyme preparations from rat and dog liver are capable of liberating H2S from cysteine directly. The reaction proceeds anaerobically. Ammonia is liberated as well as H2S. The reaction that occurs appears to be represented by equation 4. 

Support for this enzymic desulfhydration came from the work of Metzler and Snell (72), who proposed a detailed chemical mechanism of desulfhydration in model experiments in the presence of pyridoxine and a metal ion. Similar enzyme models were proposed by others (73). The enzymic aspect of this reaction is not entirely satisfactory however, ce relatively little is known of the desulfhydrase enzyme, either derived from the liver (74) or from bacteria (75). In addition several pyridoxal phosphate-protein enzymes can catalyze cysteine desulfhydration (see Section IV). 

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.

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