Metabolic effects of inhibitors of two enzymes of the branched-chain amino acid pathway in Salmonella typhimurium
Metabolic effects of inhibitors of two enzymes of the branched-chain amino acid pathway in Salmonella typhimurium
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DOI：
10.1128/jb.178.4.1187-1196.1996
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发表时间：
1996-02-01
影响因子：
3.2
通讯作者：
Barak, Z
中科院分区：
文献类型：
--
作者：
Epelbaum, S;Chipman, DM;Barak, Z
研究了表达单一乙酰羟基酸合成酶(AHAs)同工酶II的鼠伤寒沙门氏菌TV105菌株在支链氨基酸生物合成途径中两种酶抑制剂的代谢效应。一种是抑制该同工酶和其他生物AHAs的磺酰脲除草剂甲基磺脲(SMM)，另一种是抑制酮酸还原异构酶(KARI)的N-异丙基草酰羟甲酸酯(IpOHA)。测定了这些抑制剂对生长的影响、该途径中几种酶的水平以及该途径的中间产物水平。加入SMM后，AHAs底物2-酮丁酸的胞内浓度增加，但增加的酮丁酸与生长抑制之间缺乏相关性，表明前者不是后者的直接原因。SMM可显著降低缬氨酸的酮酸前体水平，但不能显著降低异亮氨酸前体的酮酸水平，并能部分克服SMM的抑制作用。SMM对支链氨基酸途径流入Valine臂的明显更强的影响，而不是异亮氨酸臂，可以通过AHAS反应的动力学以及丙酮酸、酮丁酸酯和Valine前体在代谢中的不同作用来解释。因此，该途径的组织加强了SMM的抑制作用，IpOHA在较低浓度下具有比SMM更强的初始效应，并导致KARI的乙酰羟基酸底物中的增加，令人惊讶的是，在酮丁酸中也是如此。在MIC处，Valine完全保护菌株TV105不受IpOHA的影响。这种影响可以排除许多解释，所以必须提出一些未知的安排所涉及的酶，IpOHA导致了最初的生长停止，并在一段时间后部分恢复，其持续时间随着抑制剂浓度的增加而增加。这种恢复显然是由于诱导了新的Kari合成，以及IpOHA从培养液中消失。
The metabolic effects of inhibitors of two enzymes in the pathway for biosynthesis of branched-chain amino acids were examined in Salmonella typhimurium mutant strain TV105, expressing a single isozyme of acetohydroxy acid synthase (AHAS), AHAS isozyme II. One inhibitor was the sulfonylurea herbicide sulfometuron methyl (SMM), which inhibits this isozyme and AHAS of other organisms, and the other was N-isopropyl oxalylhydroxamate (IpOHA), which inhibits ketol-acid reductoisomerase (KARI). The effects of the inhibitors on growth, levels of several enzymes of the pathway, and levels of intermediates of the pathway were ere measured. The intracellular concentration of the AHAS substrate 2-ketobutyrate increased on addition of SMM, but a lack of correlation between increased ketobutyrate and growth inhibition suggests that the former is not the immediate cause of the latter. The levels of the keto acid precursor of valine, but not of the precursor of isoleucine, were drastically decreased by SMM, and valine, but not isoleucine, partially overcame SMM inhibition. This apparent stronger effect of SMM on the flux into the valine arm, as opposed to the isoleucine arm, of the branched-chain amino acid pathway is explained by the kinetics of the AHAS reaction, as well as by the different roles of pyruvate, ketobutyrate, and the valine precursor in metabolism. The organization of the pathway thus potentiates the inhibitory effect of SMM, IpOHA has strong initial effects at lower concentrations than does SMM and leads to increases both in the acetohydroxy acid substrates of KARI and, surprisingly, in ketobutyrate. Valine completely protected strain TV105 from IpOHA at the MIC. A number of explanations for this effect can be ruled out, so that some unknown arrangement of the enzymes involved must be suggested, IpOHA led to initial cessation of growth, with partial recovery after a time whose duration increased with the inhibitor concentration. The recovery is apparently due to induction of new KARI synthesis, as well as disappearance of IpOHA from the medium.