Privalle C T, Fridovich I
Department of Biochemistry, Duke University Medical Center, Durham, North Carolina 27710.
J Biol Chem. 1988 Mar 25;263(9):4274-9.
Escherichia coli growing anaerobically respond to NO3- with a 3-fold induction of the iron-containing superoxide dismutase. Mutants lacking nitrate reductase do not show this response. Anaerobically grown cells also contain an inactive form of the manganese-containing superoxide dismutase (MnSOD) which can be activated by addition of Mn(II) salts in the presence of acidic guanidinium chloride, followed by dialysis against neutral buffer. Direct addition of Mn(II) to a neutral solution of the inactive MnSOD does not impart activity. This inactive MnSOD thus behaves as would the apoenzyme or the enzyme bearing a metal other than Mn(II) at its active sites. Terminal electron acceptors, such as NO3- or trimethylamine N-oxide, increase the amount of inactive MnSOD produced by anaerobic E. coli. Paraquat, which is itself ineffective in this regard, markedly augments the effect of these terminal electron acceptors. It appears that flow of electrons to sinks such as NO3- or trimethylamine N-oxide, facilitated by paraquat, is sufficient to elicit biosynthesis of the MnSOD protein and that O2- is not needed for this process. Yet, oxygenation and concomitant O2- production do appear important for the insertion of manganese into the growing MnSOD polypeptide, possibly because O-2 oxidizes Mn(II) to Mn(III), and the latter is the valence state most effective in combining with the apoenzyme.
厌氧生长的大肠杆菌对硝酸根离子（NO₃⁻）作出反应，含铁超氧化物歧化酶的表达量提高3倍。缺乏硝酸还原酶的突变体则不表现出这种反应。厌氧生长的细胞还含有一种无活性形式的含锰超氧化物歧化酶（MnSOD），在酸性胍盐存在下加入锰（II）盐，随后用中性缓冲液透析，可使其激活。直接向无活性MnSOD的中性溶液中加入锰（II）并不能赋予其活性。因此，这种无活性的MnSOD表现得如同脱辅基酶或在其活性位点带有除锰（II）以外其他金属的酶。末端电子受体，如硝酸根离子（NO₃⁻）或三甲胺N-氧化物，会增加厌氧大肠杆菌产生的无活性MnSOD的量。百草枯本身在这方面无效，但能显著增强这些末端电子受体的作用。似乎在百草枯促进下，电子流向硝酸根离子（NO₃⁻）或三甲胺N-氧化物等受体就足以引发MnSOD蛋白的生物合成，且该过程不需要超氧阴离子（O₂⁻）。然而，氧化作用以及随之产生的O₂⁻对于将锰插入正在生长的MnSOD多肽中似乎很重要，这可能是因为超氧阴离子（O₂⁻）将锰（II）氧化为锰（III），而后者是与脱辅基酶结合最有效的价态。
