Peña A, Pardo J P, Ramírez J
Arch Biochem Biophys. 1987 Mar;253(2):431-8. doi: 10.1016/0003-9861(87)90196-2.
Studies were performed to define the effects and mechanism of NH+4 transport in yeast. The following results were obtained. Glucose was a better facilitator than ethanol-H2O2 for ammonium transport; low concentrations of uncouplers or respiratory inhibitors could inhibit the transport with ethanol as the substrate. With glucose, respiratory inhibitors showed only small inhibitory effects, and only high concentrations of azide or trifluoromethoxy carbonylcyanide phenylhydrazone could inhibit ammonium transport. Ammonium in the free state could be concentrated approximately 200-fold by the cells. Also, the addition of ammonium produced stimulation of both respiration and fermentation; an increased rate of H+ extrusion and an alkalinization of the interior of the cell; a decrease of the membrane potential, as monitored by fluorescent cyanine; an immediate decrease of the levels of ATP and an increase of ADP, which may account for the stimulation of both fermentation and respiration; and an increase of the levels of inorganic phosphate. Ammonium was found to inhibit 86Rb+ transport much less than K+. Also, while K+ produced a competitive type of inhibition, that produced by NH4+ was of the noncompetitive type. From the distribution ratio of ammonium and the pH gradient, an electrochemical potential gradient of around -180 mV was calculated. The results indicate that ammonium is transported in yeast by a mechanism similar to that of monovalent alkaline cations, driven by a membrane potential. The immediate metabolic effects of this cation seem to be due to an increased [H+]ATPase, to which its transport is coupled. However, the carriers seem to be different. The transport system studied in this work was that of low affinity.
开展了多项研究以确定酵母中铵离子（NH₄⁺）转运的影响因素及机制。得到了以下结果。对于铵转运，葡萄糖比乙醇 - 过氧化氢是更好的促进剂；低浓度的解偶联剂或呼吸抑制剂能以乙醇为底物抑制转运。以葡萄糖为底物时，呼吸抑制剂仅表现出较小的抑制作用，只有高浓度的叠氮化物或三氟甲氧基羰基氰基苯腙能抑制铵转运。游离态的铵可被细胞浓缩约200倍。此外，添加铵会刺激呼吸和发酵；氢离子（H⁺）外排速率增加且细胞内碱化；用荧光花青监测发现膜电位降低；三磷酸腺苷（ATP）水平立即下降而二磷酸腺苷（ADP）增加，这可能解释了对发酵和呼吸的刺激作用；无机磷酸盐水平增加。发现铵对⁸⁶铷离子（⁸⁶Rb⁺）转运的抑制作用远小于钾离子（K⁺）。而且，虽然钾离子产生竞争性抑制类型，但铵离子产生的是非竞争性抑制类型。根据铵的分布比和pH梯度，计算出约 -180 mV的电化学势梯度。结果表明，铵在酵母中的转运机制与单价碱性阳离子类似，由膜电位驱动。这种阳离子的直接代谢效应似乎是由于与之转运偶联的[H⁺]ATP酶增加所致。然而，载体似乎不同。本研究中所研究的转运系统是低亲和力的。
