Wray J W, Abeles R H
Institute of Molecular Biology, University of Oregon, Eugene 97403.
J Biol Chem. 1995 Feb 17;270(7):3147-53. doi: 10.1074/jbc.270.7.3147.
The 5-methylthio-D-ribose moiety of 5'-(methylthio)-adenosine is converted to methionine in a wide variety of organisms. 1,2-Dihydroxy-3-keto-5-methylthiopentene anion (an aci-reductone) is an advanced intermediate in the methionine salvage pathway present in the Gram-negative bacterium Klebsiella pneumoniae and rat liver. This metabolite is oxidized spontaneously in air to formate and 2-keto-4-methylthiobutyric acid (the alpha-keto acid precursor of methionine). Previously, we had purified an enzyme (E2) from Klebsiella which catalyzes the oxidative degradation of the aci-reductone to formate, CO, and methylthiopropionic acid. To further characterize the reactions of the aci-reductone we used its desthio analog, 1-2-dihydroxy-3-ketohexene anion (III), which was described previously. This molecule undergoes the analogous enzymatic and non-enzymatic reactions of the natural substrate, namely the formation of formate, CO, and butyrate from III. Experiments with 18O2 show that E2 is a dioxygenase which incorporates one molecule of 18O into formate and butyric acid. No cofactor has been identified. We were unable to find an enzyme which catalyzes the conversion of 1,2-dihydroxy-3-keto-5-methylthiopentane to a keto acid precursor of methionine. The keto acid is probably produced non-enzymically in Klebsiella. We have, however, identified and purified an enzyme (E3) from rat liver, which catalyzes the formation of formate and 2-oxopentanoic acid from III. This enzyme has a monomeric molecular mass of 28,000 daltons, and no chromophoric cofactor has been identified. Experiments with 18O2 show that E3 is a dioxygenase which incorporates an 18O molecule into formate and the alpha-keto acid. In rat liver CO formation was not detected.
5'-（甲硫基）-腺苷的5-甲硫基-D-核糖部分在多种生物体中可转化为甲硫氨酸。1,2-二羟基-3-酮-5-甲硫基戊烯阴离子（一种酸式还原酮）是革兰氏阴性菌肺炎克雷伯菌和大鼠肝脏中甲硫氨酸补救途径的一种高级中间体。这种代谢物在空气中会自发氧化形成甲酸和2-酮-4-甲硫基丁酸（甲硫氨酸的α-酮酸前体）。此前，我们从克雷伯菌中纯化出一种酶（E2），它催化酸式还原酮氧化降解为甲酸、一氧化碳和甲硫基丙酸。为了进一步表征酸式还原酮的反应，我们使用了其脱硫类似物1,2-二羟基-3-酮己烯阴离子（III），该类似物此前已有描述。该分子会发生与天然底物类似的酶促和非酶促反应，即从III形成甲酸、一氧化碳和丁酸。用18O2进行的实验表明，E2是一种双加氧酶，它将一分子18O掺入甲酸和丁酸中。尚未鉴定出辅助因子。我们未能找到一种催化1,2-二羟基-3-酮-5-甲硫基戊烷转化为甲硫氨酸酮酸前体的酶。这种酮酸可能在克雷伯菌中通过非酶促方式产生。然而，我们已经从大鼠肝脏中鉴定并纯化出一种酶（E3），它催化从III形成甲酸和2-氧代戊酸。这种酶的单体分子量为28,000道尔顿，尚未鉴定出发色辅助因子。用18O2进行的实验表明，E3是一种双加氧酶，它将一个18O分子掺入甲酸和α-酮酸中。在大鼠肝脏中未检测到一氧化碳的形成。
