Biochemistry of Amino Acids - A. Majster 1961

Intermediary Metabolism of Amino Acids
Methionine and Cysteine
Formation and Transformations of Cystathionine

Nutritional biochemistry experiments have revealed that dietary Cysteine (cystine) is replaceable. Specifically, it was demonstrated that the cysteine requirement in rats can be met by Methionine, and that the methionine requirement can be partially covered by dietary cystine (p. 122). These findings suggested that the sulfur of methionine is converted into the sulfur of cystine in the rat. The increased excretion of cystine in cystinuric patients following the administration of methionine [452], as well as the elevated formation of mercapturic acids after the co-administration of methionine and halogenated benzenes [453, 454], also pointed toward the possibility of such a conversion. Tarver and Schmidt [455], using S³⁵-labeled methionine, obtained direct evidence for The conversion of methionine sulfur into cystine sulfur in the rat Organism, whereas Stetten [222] showed that following the administration of N¹⁵-labeled Serine to rats, cystine containing a high concentration of the isotope is formed in the Tissues. Brand and coworkers [457] observed that when homocystine was administered to cystinuric patients, they excreted increased amounts of cystine; the authors hypothesized [458] that The transfer of homocysteine sulfur to the cysteine molecule proceeds via an intermediate compound, S-(ß-amino-ß-carboxyethyl)-homocysteine, later named cystathionine. Du Vigneaud and coworkers [459–464] confirmed this metabolic pathway, demonstrating that the serine molecule serves as the sulfur acceptor. All four diastereomers of cystathionine were synthesized and investigated in rat feeding experiments [459, 465]. The D-isomers proved to be inactive; L-cystathionine supported growth, replacing dietary cysteine in growth assays, whereas L-allo-cystathionine stimulated growth in the absence of methionine when Choline was present in the diet. The authors concluded that L-cystathionine is cleaved in the rat organism into cysteine and homoserine, whereas the Cleavage of L-allo-cystathionine yields homocysteine and serine:

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Sulfur-labeled L-cystathionine was converted into labeled cystine [464], and in vitro experiments with rat Liver preparations revealed that it yields cysteine and a-ketobutyric acid [460, 462, 466]. It is possible that a-ketobutyric acid originated from homoserine, since homoserine added to the enzyme preparation induced The formation of this keto acid. Apparently, the amination of a-ketobutyric acid to a-aminobutyric acid can take place in the organism [467]. The latter appears in elevated amounts in human urine after methionine ingestion [468, 469]. When studying The enzyme catalyzing the Condensation of homocysteine and serine, it was found to be distinct from the enzyme system that cleaves cystathionine. The reactions of cystathionine formation and cleavage can be represented as follows:

Liver preparations from B₆-deficient rats fail to catalyze the formation of cysteine from homocysteine and serine unless Pyridoxal phosphate is added to the homogenates [470]; this indicates the involvement of vitamin B₆ in sulfur transfer via cystathionine. Pyridoxal phosphate was found to be essential both for the system synthesizing cystathionine and for the one cleaving it [471]¹. For ß-substitution (or condensation) reactions of intermediate Schiff bases, Snell and coworkers formulated a mechanism that explains The Role of vitamin B₆ in these transformations (p. 255).

¹ The participation of pyridoxal phosphate in the synthesis of cystathionine and its cleavage into NH₃, a-ketobutyric acid, and cysteine was established simultaneously by Goryachenkova (DAN, 85, 603, 1952) and Binkley [471], whereas its role as a coenzyme for homoserine deaminase was demonstrated by Braunstein and Azarh (DAN, 85, 385, 1952). Greenberg and coworkers isolated a multifunctional pyridoxal enzyme in crystalline form from rat liver, which catalyzes the reactions described above—the cleavage of cystathionine yielding NH₃, H₂S, and Pyruvate, and the deamination of homoserine (Y. Matsuo, D. M. Greenberg, J. Biol. Chem., 230, 545, 561, 1958; 234, 507, 516, 1959); another purified pyridoxal enzyme isolated from rat liver apparently combines the function of cystathionine synthesis from serine and homocysteine with serine dehydratase or serine deaminase activity. — Ed. note.

It has been established that rat liver preparations effect the deamination of homoserine to form a-ketobutyric acid [472]; this reaction was found to be catalyzed by a specific deaminase that does not act upon serine or Threonine (see note on p. 366).

Binkley [473] succeeded in separating the cystathionine-cleaving enzyme from the cystathionine-synthesizing enzyme by selectively inactivating the latter through heating to 50°. In addition to L-cystathionine, the enzyme cleaves djenkolic acid, L-allo-cystathionine, and lanthionine [474].

Further proof of the role of cystathionine in the transsulfuration process was obtained in an experiment with a cystinuric patient who was administered S³⁵-methionine. The isotope was detected in urinary cystine [475]. Many Other Compounds also serve as sources of cysteine sulfur. L-Lanthionine has been found to support the growth of rats maintained on a cystine-deficient diet [476]. The sulfur of homolanthionine [477, 478] and ethionine [479] can also be converted into cystine sulfur.

In rats, this pathway of cysteine formation appears to be irreversible. However, in certain microorganisms, the synthesis of methionine from cysteine proceeds via a series of reactions representing the reverse of the process discussed above. Some organisms, such as Neurospora, catalyze these reactions in both directions. The synthesis of methionine in N. crassa has been studied using several mutants blocked at various steps of the biosynthetic pathway [235, 236, 480]. For instance, mutants capable of growing on media containing: a) methionine, b) methionine or homocysteine, c) methionine, homocysteine, or cystathionine, as well as mutants blocked at the homoserine synthesis stage or at the stage of cysteine condensation with homocysteine, were isolated. Similar Escherichia coli mutants have also been investigated; experiments using Enzymes and isotopes yielded data demonstrating that the pathways of cysteine-to-methionine conversion in this microorganism and in Neurospora are analogous [483–486]. Cystathionine has been isolated from the culture filtrates of certain Neurospora mutants [480] and from the urine of B₆-deficient rats [1104]. S³⁵-Cystathionine was also detected in mammalian tissues in experiments employing S³⁵-methionine [487, 488].

Thus, the carbon chain of cysteine in both mammals and microorganisms originates from serine. Serine METABOLISM reactions were discussed above (pp. 319–323).

The mechanism by which sulfur is introduced into the serine carbon chain remains unclear¹. Studies on mutants of certain microorganisms suggest the following pathway [489, 1105, 1106]:

It is possible that hydrogen sulfide is utilized via the reversal of Reactions Catalyzed by cysteine and homocysteine desulfhydrases (p. 376); sulfite may be utilized through the Reversal of the cleavage reaction of ß-sulfinylpyruvic acid (p. 380).

¹ An enzyme (serine sulfhydrase) has been obtained from Yeast which, in the presence of pyridoxal phosphate, synthesizes L-cysteine by replacing the OH group of serine with the HS group of hydrogen sulfide (K. Schlossmann, F. Lynen, Biochem. Z., 328, 591, 1957); another analogous yeast enzyme forms S-methylcysteine from serine and methyl mercaptan (E. C. Wolff, S. Black, P. F. Downey, J. Am. Chem. Soc., 78, 5958, 1956). — Ed. note.