Primerano D A, Burns R O
J Bacteriol. 1982 Jun;150(3):1202-11. doi: 10.1128/jb.150.3.1202-1211.1982.
Salmonella typhimurium strain DU501, which was found to be deficient in acetohydroxy acid synthase II (AHAS II) and to possess elevated levels of transaminase B and biosynthetic threonine deaminase, required isoleucine, methionine, or pantothenate for growth. This strain accumulated alpha-ketobutyrate and, to a lesser extent, alpha-aminobutyrate. We found that alpha-ketobutyrate was a competitive substrate for ketopantoate hydroxymethyltransferase, the first enzyme in pantothenate biosynthesis. This competition with the normal substrate, alpha-ketoisovalerate, limited the supply of pantothenate, which resulted in a requirement for methionine. Evidence is presented to support the conclusion that the ambivalent requirement for either pantothenate or methionine is related to a decrease in succinyl coenzyme A, which is produced from pantothenate and which is an obligatory precursor of methionine biosynthesis. The autointoxification by endogenously produced alpha-ketobutyrate could be mimicked in wild-type S. typhimurium by exogenously supplied alpha-ketobutyrate or salicylate, a known inhibitor of pantothenate biosynthesis. The accumulation of alpha-ketobutyrate was initiated by the inability of the residual AHAS activity provided by AHAS I to efficiently remove the alpha-ketobutyrate produced by biosynthetic threonine deaminase. The accumulation of alpha-ketobutyrate was amplified by the action of transaminase B, which decreased the isoleucine pool by catalyzing the formation of alpha-keto-beta-methylvalerate and aminobutyrate from isoleucine and alpha-ketobutyrate; this resulted in release of threonine deaminase from end product inhibition and unbridled production of alpha-ketobutyrate. Isoleucine satisfied the auxotrophic requirement of the AHAS II-deficient strain by curtailing the activity of threonine deaminase. Additional lines of evidence based on genetic and physiological experiments are presented to support the basis for the autointoxification of strain DU501 as well as other nonpolarigenic ilvG mutant strains.
鼠伤寒沙门氏菌菌株DU501被发现缺乏乙酰羟酸合酶II（AHAS II），且转氨酶B和生物合成苏氨酸脱氨酶水平升高，其生长需要异亮氨酸、蛋氨酸或泛酸盐。该菌株积累了α-酮丁酸，在较小程度上还积累了α-氨基丁酸。我们发现α-酮丁酸是泛酸生物合成途径中第一种酶——酮泛解酸羟甲基转移酶的竞争性底物。这种与正常底物α-酮异戊酸的竞争限制了泛酸盐的供应，从而导致对蛋氨酸的需求。有证据支持这样的结论：对泛酸盐或蛋氨酸的矛盾需求与琥珀酰辅酶A的减少有关，琥珀酰辅酶A由泛酸盐产生，是蛋氨酸生物合成的必需前体。内源性产生的α-酮丁酸的自身中毒现象可通过外源供应α-酮丁酸或水杨酸（一种已知的泛酸生物合成抑制剂）在野生型鼠伤寒沙门氏菌中模拟。α-酮丁酸的积累是由AHAS I提供的残余AHAS活性无法有效去除生物合成苏氨酸脱氨酶产生的α-酮丁酸引发的。转氨酶B的作用放大了α-酮丁酸的积累，它通过催化异亮氨酸和α-酮丁酸形成α-酮-β-甲基戊酸和氨基丁酸，减少了异亮氨酸库；这导致苏氨酸脱氨酶从终产物抑制中释放出来，无节制地产生α-酮丁酸。异亮氨酸通过降低苏氨酸脱氨酶的活性满足了AHAS II缺陷菌株的营养缺陷需求。基于遗传和生理学实验的其他证据支持了DU501菌株以及其他非极性ilvG突变菌株自身中毒的基础。
