Y-cystathionase

NH2). The acid crystallises readily when 4g in 50mL H2O is treated with abs EtOH at 4°/ 3hrs, and is collected washed with cold abs EtOH and Et20 and dried in vac. Also recrystallises from aqueous Me2CO, Rp on Si02 TLC plates with n-BuOH-H20-AcOH (4 1 1) is 0.26. The racemate has m 238-240°. [Leukart et al. Helv Chim Acta 59 2181 1976 Eberle and Zeller Helv Chim Acta 68 1880 1985 Jansen et al. Reel Trav Chim Pays-Bas 88 819 7969.] It is a suicide inhibitor of y-cystathionase and other enzymes [Washtier and Abeles Biochemistry 16 2485 7977 Shinozuka et al. Eur J Biochem 124 377 7982]. 

This pyridoxal-phosphate-dependent enzyme [EC 4.4.1.1] (also referred to as homoserine deaminase, homoserine dehydratase, y-cystathionase, cystine desulfhy-drase, cysteine desulfhydrase, and cystathionase) catalyzes the hydrolysis of cystathionine to produce cysteine, ammonia, and a-ketobutanoate (or, 2-oxobutanoate). 

Gonversion of homocysteine to Gys occurs in two reactions catalyzed by two pyridoxal phosphate-requiring enzymes, cystathionine p-synthase and y-cystathionase. 

The subsequent cleavage of cystathionine to yield cysteine, a-ketobutyrate and NH4+ is catalyzed by y-cystathionase, a pyridoxal-phosphate-containing enzyme. This transsulfura-tion pathway is one of the routes used for methionine catabolism. 

Studies on the / ,-/-elimination reaction catalyzed by y-cystathionase showed that in the conversion of homoserine to a-ketobutyrate one atom of deuterium from the solvent is incorporated at C-/ , where it occupies the pro-S position [156]. The stereochemistry of protonation at C-/ in this reaction is thus the same as in the / -/-elimination catalyzed by cystathionine-y-synthase. 

Trapping of the aminoacrylate intermediate in the reactions catalyzed by cystathionine-y-synthase and y-cystathionase produced the same diastereomer of KEDB which was different from the one formed with bacterial L-threonine dehydratase. Unfortunately, this experiment has apparently not been done with threonine synthetase. 

Reminiscent of the irreversible inhibition of GABA-T by ethanolamine-O-sulfate (10). which involves enzyme-induced P elimination of sulfate to generate an electrophilic Michael acceptor, P-haloamino acids have been found to lead to irreversible inhibition via 8-elimination mechanisms. Thus bacterial alanine racemase is irreversibly inhibited by P-chloro-D-alanine (24), P-fluoroalanine (25) and by P,P,P-trifluoroalanine (26). P,P,P-Trifluoroalanine has also been found to be an irreversible inactivator of y-cystathionase (26, 27). the enzyme previously shown to be inactivated by propargylglycine (7). 

Figure 10.9. Metabolism of methionine. Methionine adenosyltransferase, EC 2.5.1.6 methionine synthetase, EC 2.1.1.13 (vitamin B12-dependent) and EC 2.1.1.5 (betaine as a methyl donor) cystathionine, 6-synthetase, EC 4.2.1.22 and y-cystathionase, EC 4.4.I.I.

There is evidence for the possible participation of a third sulphurtransferase, cystathionase (cystathionine y-lyase EC 4.4.1.1), a cytosolic enzyme, in CN detoxification in the kidney and rhombencephalon (Wrobcl et al., 2004). A product of the y-cystathionase reaction, bis(2-amino-2-carboxylethyl)trisulphide(thiocystine), may serve as a sulphur substrate donor for rhodanese. Another product of the reaction, 3-(thiosulpheno)-alanine (thiocysteine), may be an additional link between y-cystathionase and CN biodetoxification. In addition to its function of generating sulphane sulphur compounds, y-cystathionase also functions as a sulphane sulphur carrier. 

Cysteine synthesis is a primary component of sulfur metabolism. The carbon skeleton of cysteine is derived from serine (Figure 14.7). In animals the sulfhydryl group is transferred from methionine by way of the intermediate molecule homocysteine. (Plants and some bacteria obtain the sulfhydryl group by reduction of SOj to S2 as H2S. A few organisms use H2S directly from the environment.) Both enzymes involved in the conversion of serine to cysteine (cystathionine synthase and y-cystathionase) require pyridoxal phosphate. 

A number of enzymes involved in cysteine metabolism have been described in the tapeworm Hymenolepis diminuta (48). In addition to a high cystathionine /3-synthase activity it has low y-cystathionase activity, and contains cysteine aminotransferase, cysteine dioxygenase and cysteine sulphinate aminotransferase the latter two could be involved in the oxidation of cysteine. 

Since methionine has several pathways open to it, it is essential to know what factors control the direction that its metabolism takes. Studies in young adults have shown that the utilization of methyl groups is normally accounted for chiefly by creatinine formation. This reaction consumes more 5-adenosylmethionine than all other transmethylations together. However, examination of enzyme activities from these two pathways in fetal animals leads to the conclusion that remethylation preponderates over transsulfuration. Indeed, since y-cystathionase activity is immeasurable in human fetal liver and brain, not only is the remethylation sequence favored, but also cysteine then becomes an essential amino acid for the fetus and infant. 

Inhibitors of this type have been used in the irreversible inhibition of pyridoxal-linked aspartate aminotransferase, y-cystathionase, and tryptophan synthetase. These inhibitors are y-unsaturated amino acids. Aspartate aminotransferase is inhibited by molecules 1 and 2,... 

Propargylglycine is an irreversible inhibitor of both pyridoxal-linked y-cystathionase and aspartate aminotransferase. The mechanism of inhibition of y-cystathionase involves the following sequence of steps. 

Fig. 10.1. Defects of transmethylation (methioninehomocysteine), transsulfuration (methionine sulfate), and remethylation (homocysteine - methionine) enzymes of sulfur amino acid metabolism 10.1, methionine adenosyltransferase 10.2, cystathionine ) -synthase 10.3, y-cystathionase 10.4, sulfite oxidase 10.5, molybdenum cofactor 10.6, methylenetetrahydrofolate reductase 10.7 and 10.8, methionine synthase.

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