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 4.4.1.8], also referred to as /3-cystathionase and cystine lyase, catalyzes the hydrolysis of cystathionine to yield homocysteine, pyruvate, and ammonia. 

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

Figure 9-5. Pathway for formation of cysteine from methionine. Only the enzymes involved in known diseases of this pathway are shown. Cystathionase is deficient in cysthioninuria, which leads to accumulation of cystathionine without producing frank symptoms. Cystathionine p-synthase deficiency causes homocystinuria.

Cystathionine (3-lyase (cystathionase) O-Acetylserine sulfhydrylase (cysteine synthase)... 

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. 

The vitamin B6-dependent enzyme CBS catalyzes the first step, in which homocysteine reacts with serine to form L-cystathionine. In the second step, L-cystathionine is converted to L-cysteine, a-ketobutyrate, and ammonia by the vitamin B6-dependent enzyme cystathionase (7). 

There are two pyridoxal phosphate-requiring enzymes in the homocysteine degradation pathway, which are associated with genetic diseases. In homo-cystinuria, cystathionine synthase is defective, and large amounts of homocystine are excreted in the urine. Some homocystinurics respond to the administration of large doses of vitamin B6. In cystathioninuria, cystathionase is either defective or absent. These patients excrete cystathionine in the urine. Cystathionase is often underactive in the newborns with immature livers, and cysteine and cystine become essential amino acids. Human milk protein is especially rich in cysteine, presumably to prepare the newborn for such a contingency. 

The carbon skeleton of methionine is converted to a-ketobutyrate (Figure 20.17), which is catabolized to propionyl-CoA and then to succinate. The sulfur atom is transferred to serine in the cystathionase reaction to yield cysteine. Cysteine is nonessential, because it can be derived from serine and methionine. 

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 9.5. Methionine load test for vitamin Be status. Methionine synthetase, EC 2.1.1.13 (vitamin Bi2-dependent) 2.1.1.5 (betaine as methyl donor) cystathionine synthetase, EC 4.2.1.22 and cystathionase, EC 4.4.1.1. Relative molecular masses (Mr) methionine, 149.2 homocysteine, 135.2 cystathionine, 222.3 and cysteine, 121.2.

The metabolism of methionine, shown in Figure 9.5, includes two pyri-doxal phosphate-dependent steps cystathionine synthetase and cystathionase. Cystathionine synthetase is litde affected by vitamin Bg deficiency,... 

As discussed in Section 10.3.4.2, the metabolic fate of homocysteine arising from methionine is determined not only by the activity of cystathionine synthetase and cystathionase, hut also the rate at which it is remethylated to methionine (which is dependent on vitamin B12 and folate status) and the requirement for cysteine. 

Sato A, Nishioka M, Awata S, Nakayama K, Okada M, Horinchi S, Okabe N, Sassa T, Oka T, and Natori Y (1996) Vitamin Be deficiency accelerates metabolic turnover of cystathionase in rat Vwei. Archives of Biochemistry and Biophysics 330,409-13. 

---