Brown H D, Satyanarayana T, Umbarger H E
J Bacteriol. 1975 Mar;121(3):959-69. doi: 10.1128/jb.121.3.959-969.1975.
The three enzymes in the leucine biosynthetic pathway of yeast do not exhibit coordinate repression and derepression in response to the carbon source available in the culture medium. Growth in an acetate medium results in derepression of the first enzyme in the pathway, alpha-isopropylmalate synthase, and repression of the second two enzymes, alpha-isopropylmalate isomerase and beta-isopropylmalate dehydrogenase, relative to the levels found in glucose-grown cells. The role of endogenous leucine pools as a mediator of these differences was investigated. The leucine pools did not differ significantly between acetate-grown and glucose-grown cells. However, an elevated endogenous leucine pool, caused by exogenous leucine in the growth medium, did decrease the rate of decay of alpha-isopropylmalate synthase activity observed when acetate-grown cells were shifted to glucose. Evidence is provided suggesting that an elevated endogenous leucine pool may increase the in vivo stability of alpha-isopropylmalate synthase under several different conditions. Studies on the kinetics of alpha-isopropylmalate synthase decay in vivo and sensitivity to leucine inhibition indicate that there are two classes of the enzyme in acetate-grown yeast cells.
酵母亮氨酸生物合成途径中的三种酶不会根据培养基中可用的碳源表现出协同阻遏和去阻遏。相对于在葡萄糖培养基中生长的细胞中发现的水平，在乙酸盐培养基中生长会导致该途径中第一种酶α-异丙基苹果酸合酶的去阻遏，以及后两种酶α-异丙基苹果酸异构酶和β-异丙基苹果酸脱氢酶的阻遏。研究了内源性亮氨酸库作为这些差异介导因子的作用。在乙酸盐培养基中生长的细胞和在葡萄糖培养基中生长的细胞之间，亮氨酸库没有显著差异。然而，生长培养基中外源性亮氨酸导致的内源性亮氨酸库升高，确实降低了乙酸盐培养基中生长的细胞转移到葡萄糖培养基时观察到的α-异丙基苹果酸合酶活性的衰减速率。有证据表明，内源性亮氨酸库升高可能会在几种不同条件下增加α-异丙基苹果酸合酶在体内的稳定性。对α-异丙基苹果酸合酶在体内衰减动力学和对亮氨酸抑制敏感性的研究表明，在乙酸盐培养基中生长的酵母细胞中有两类这种酶。
