Chang E C, Kosman D J
Department of Biochemistry, School of Medicine, State University of New York, Buffalo 14214.
J Bacteriol. 1990 Apr;172(4):1840-5. doi: 10.1128/jb.172.4.1840-1845.1990.
Mutant strains of the yeast Saccharomyces cerevisiae which lack functional Cu,Zn superoxide dismutase (SOD-1) do not grow aerobically unless supplemented with methionine. The molecular basis of this O2-dependent auxotrophy in one of the mutants, Dscd1-1C, has been investigated. Sulfate supported anaerobic but not aerobic mutant growth. On the other hand, cysteine and homocysteine supported aerobic growth while serine, O-acetylserine, and homoserine did not, indicating that the interconversion of cysteine and methionine (and homocysteine) was not impaired. Thiosulfate (S2O3(2-] and sulfide (S2-) also supported aerobic growth; the activities of thiosulfate reductase and sulfhydrylase in the aerobic mutant strain were at wild-type levels. Although the levels of SO4(2-) and adenosine-5'-sulfate (the first intermediate in the SO4(2-) assimilation pathway) were elevated in the aerobically incubated mutant strain, this condition could be attributed to a decrease in protein synthesis caused by the de facto sulfur starvation and not to a block in the pathway. Therefore, the activation of SO4(2-) (to form 3'-phosphoadenosine-5'-phosphosulfate) appeared to be O2 tolerant. Sulfite reductase activity and substrate concentrations [( NADPH] and [SO3(2-)]) were not significantly different in aerobically grown mutant cultures and anaerobic cultures, indicating that SOD-1- mutant strains could reductively assimilate sulfur oxides. However, the mutant strain exhibited an O2-dependent sensitivity to SO3(2-) concentrations of less than 50 microM not exhibited by any SOD-1+ strain or by SOD-1- strains supplemented with a cytosolic O2(-)-scavenging activity. This result suggests that the aerobic reductive assimilation of SO4(2-) at the level of SO3(2-) may generate a cytotoxic compound(s) which persists in SOD-(1-) yeast strains.
酿酒酵母的突变菌株若缺乏功能性铜锌超氧化物歧化酶（SOD-1），则无法在有氧条件下生长，除非添加甲硫氨酸。已对其中一个突变体Dscd1-1C中这种氧气依赖性营养缺陷型的分子基础进行了研究。硫酸盐支持厌氧条件下的突变体生长，但不支持有氧条件下的生长。另一方面，半胱氨酸和高半胱氨酸支持有氧生长，而丝氨酸、O-乙酰丝氨酸和高丝氨酸则不支持，这表明半胱氨酸和甲硫氨酸（以及高半胱氨酸）的相互转化未受损害。硫代硫酸盐（S2O3(2-)）和硫化物（S2-）也支持有氧生长；有氧突变菌株中硫代硫酸盐还原酶和巯基酶的活性处于野生型水平。尽管在有氧培养的突变菌株中硫酸根离子（SO4(2-)）和腺苷-5'-硫酸盐（SO4(2-)同化途径中的第一个中间体）的水平有所升高，但这种情况可能归因于事实上的硫饥饿导致的蛋白质合成减少，而不是该途径中的阻断。因此，SO4(2-)的活化（形成3'-磷酸腺苷-5'-磷酸硫酸盐）似乎对氧气具有耐受性。在有氧生长的突变体培养物和厌氧培养物中，亚硫酸盐还原酶活性以及底物浓度（[NADPH]和[SO3(2-)]）没有显著差异，这表明SOD-1突变菌株能够还原性同化硫氧化物。然而，该突变菌株对浓度低于50微摩尔的亚硫酸根离子（SO3(2-)）表现出氧气依赖性敏感性，这是任何SOD-1+菌株或具有胞质O2(-)清除活性的SOD-1-菌株所未表现出的。这一结果表明，在亚硫酸根离子（SO3(2-)）水平上SO4(2-)的有氧还原性同化可能会产生一种细胞毒性化合物，该化合物在SOD-(1-)酵母菌株中持续存在。
