Hollander M M, Reiter A J, Horner W H, Cooper A J
Department of Biochemistry, Georgetown University Medical Center, Washington, D.C. 20007.
Arch Biochem Biophys. 1989 May 1;270(2):698-713. doi: 10.1016/0003-9861(89)90553-5.
It was observed previously that hydroxyguanidine is formed in the reaction of canavanine(2-amino-4-guanidinooxybutanoate) with amino acid oxidases. The present work shows that hydroxyguanidine is formed by a nonenzymatic beta,gamma-elimination reaction following enzymatic oxidation at the alpha-C and that the abstraction of the beta-H is general-base catalyzed. The elimination reaction requires the presence in the alpha-position of an anion-stabilizing group--the protonated imino group (iminium ion group) or the carbonyl group. The iminium ion group is more activating than the carbonyl group. Elimination is further facilitated by protonation of the guanidinooxy group. The other product formed in the elimination reaction was identified as vinylglyoxylate (2-oxo-3-butenoate), a very highly electrophilic substance. The product resulting from hydrolysis following oxidation was identified as alpha-keto-gamma-guanidinooxybutyrate (ketocanavanine). The ratio of hydroxyguanidine to ketocanavanine depended upon the concentration and degree of basicity of the basic catalyst and on pH. In the presence of semicarbazide, the elimination reaction was prevented because the imino group in the semicarbazone derivative of ketocanavanine is not significantly protonated. Incubation of canavanine with 5'-deoxypyridoxal also yielded hydroxyguanidine. Since the elimination reactions take place under mild conditions, they may occur in vivo following oxidation at the alpha-C of L-canavanine (ingested or formed endogenously) or of other amino acids with a good leaving group in the gamma-position (e.g., S-adenosylmethionine, methionine sulfoximine, homocyst(e)ine, or cysteine-homocysteine mixed disulfide) by an L-amino acid oxidase, a transaminase, or a dehydrogenase. Therefore, vinylglyoxylate may be a normal metabolite in mammals which at elevated concentrations may contribute to the in vivo toxicity of canavanine and of some of the other above-mentioned amino acids.
先前观察到，在刀豆氨酸（2-氨基-4-胍基氧基丁酸酯）与氨基酸氧化酶的反应中会形成羟基胍。目前的研究表明，羟基胍是在α-C处进行酶促氧化后通过非酶促β,γ-消除反应形成的，并且β-H的夺取是由广义碱催化的。消除反应要求在α-位存在一个阴离子稳定基团——质子化的亚氨基（亚胺离子基团）或羰基。亚胺离子基团比羰基更具活化作用。胍基氧基的质子化进一步促进了消除反应。消除反应中形成的另一种产物被鉴定为乙烯基乙醛酸酯（2-氧代-3-丁烯酸酯），一种亲电性很强的物质。氧化后水解产生的产物被鉴定为α-酮-γ-胍基氧基丁酸（酮刀豆氨酸）。羟基胍与酮刀豆氨酸的比例取决于碱性催化剂的浓度和碱度以及pH值。在存在氨基脲的情况下，消除反应被阻止，因为酮刀豆氨酸的氨基脲衍生物中的亚氨基没有明显质子化。刀豆氨酸与5'-脱氧吡哆醛一起孵育也会产生羟基胍。由于消除反应在温和条件下发生，它们可能在体内L-刀豆氨酸（摄入的或内源性形成的）或其他在γ-位带有良好离去基团的氨基酸（例如S-腺苷甲硫氨酸、甲硫氨酸亚砜亚胺、同型半胱氨酸或半胱氨酸-同型半胱氨酸混合二硫化物）在α-C处被L-氨基酸氧化酶、转氨酶或脱氢酶氧化后发生。因此，乙烯基乙醛酸酯可能是哺乳动物中的一种正常代谢产物，在浓度升高时可能导致刀豆氨酸和上述一些其他氨基酸在体内的毒性。
