Pyruvic acid from cystine

The actual formation of pyruvic acid from various mercapturic acids upon which these formulae for cysteine and cystine were founded, was only shown later by Baumann s pupils, Konigs, Brenzinger and Schmitz, and in conjunction with Suter s observation that a-thiolactic acid was formed by the hydrolysis of horn, this formula for cystine was accepted. The results obtained, however, scarcely justified this formula as pointed out by Friedmann in 1902, who showed conclusively that the cystine, obtained from proteins, had not this constitution. 

Cysteine and cystine are unstable towards hot base. The reaction rate and the decomposition products are very much dependent on the presence or absence of oxygen. Presumably, cysteine is more stable than cystine but as with the stability in acid this fact is of minor importance when considering the reaction of cysteine in plant extracts. The production of alanine from cysteine has been demonstrated (Wieland and Wirth, 1949), but again this formation is probably due to secondary transamination with pyruvic acid formed initially. The formation of ammonia, hydrogen sulfide, and pyruvic acid from both cysteine and cystine has been demonstrated. With cysteine the decomposition probably follows a route parallel to that for serine described above ... 

As shown in Equations 1 and 2, cystine lyase produces cysteine, pyruvate, ammonia and sulfur from cystine. The cysteine is further acted upon to give pyruvate, hydrogen sulfide and ammonia. Hydrogen sulfide and ammonia formation probably account for the sulfhydryl compound and ammonia aromas detected in the unblanched sample. The pyruvic acid (at the pH of the samples) may account for the sour aroma detected in the unblanched sample, as well as samples treated with lyase and lipase separately. 

An enzsone system, present mainly m liver, forms hydrogen sulfide, pyruvic acid, and ammonia from cystine. The same enzyme system was... 

Our laboratory has studied the stereochemistry of methyl group formation in a number of a, 0 elimination reactions of amino acids catalyzed by pyridoxal phosphate enzymes. The reactions include the conversions of L-serine to pyruvate with tryptophan synthase 02 protein (78) and tryptophanase (79), of L-serine and l-tyrosine with tyrosine phenol-lyase (80), and l-cystine with S-alkylcysteine lyase (81). In the latter study, the stereospecific isotopically labeled L-cystines were obtained enzymatically by incubation of L-serines appropriately labeled in the 3-position with the enzyme O-acetyl serine sulfhy-drase (82). The serines tritiated in the 3-position were prepared enzymatically starting from [l-3H]glucose and [l-3H]mannose by a sequence of reactions of known stereochemistry (81). The cysteines were then incubated with 5-alkyl-cysteine lyase in 2H20 as outlined in Scheme 19. The pyruvate was trapped as lactate, which was oxidized with K2Cr202 to acetate for analysis. Similarly, Cheung and Walsh (71) examined the conversion of D-serine to pyruvate with... 

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