Khachatrian L, Howlett A, Klein C
J Biol Chem. 1987 Jun 15;262(17):8071-6.
(NH4)2SO4 was found to activate adenylate cyclase in Dictyostelium discoideum membranes. The effect of (NH4)2SO4 on the enzyme was observed after pretreatment of membranes but could not be observed if the salt was added to the assay mixture. Activation was seen when membranes were pretreated with 0.16 M (NH4)2SO4 and was maximal at 0.6-1.0 M. The maximal activation of the enzyme was observed within 3 min of pretreatment and was not readily reversible. The effect was specific for the NH+4 ion since pretreatment of membranes with other NH+4 salts could activate the enzyme, whereas pretreatment with NaCl or KCl could not. Pretreatment of plasma membranes with (NH4)2SO4 eliminated the sensitivity of the enzyme to the inhibitory effect of guanine nucleotides. (NH4)2SO4 pretreatment also significantly attenuated the inhibition by guanine nucleotides of cAMP binding to its plasma membrane receptor. The effect of (NH4)2SO4 on GTP inhibition of cAMP binding to its receptor was even more dramatic when the salt was present in the binding assay. (NH4)2SO4 also increased the ADP-ribosylation by cholera toxin of a 39,000-Da membrane protein. The data support the hypothesis that (NH4)2SO4-induced changes in adenylate cyclase and the cAMP receptor are due to an alteration of a putative G protein.
已发现硫酸铵（(NH4)2SO4）可激活盘基网柄菌（Dictyostelium discoideum）细胞膜中的腺苷酸环化酶。在对细胞膜进行预处理后可观察到(NH4)2SO4对该酶的作用，但如果将盐添加到测定混合物中则无法观察到这种作用。当用0.16 M (NH4)2SO4预处理细胞膜时可观察到激活作用，在0.6 - 1.0 M时激活作用最大。在预处理3分钟内观察到该酶的最大激活，且这种激活不易逆转。该作用对NH4+离子具有特异性，因为用其他NH4+盐预处理细胞膜可激活该酶，而用NaCl或KCl预处理则不能。用(NH4)2SO4预处理质膜消除了该酶对鸟嘌呤核苷酸抑制作用的敏感性。(NH4)2SO4预处理还显著减弱了鸟嘌呤核苷酸对cAMP与其质膜受体结合的抑制作用。当在结合测定中存在该盐时，(NH4)2SO4对GTP抑制cAMP与其受体结合的作用更为显著。(NH4)2SO4还增加了霍乱毒素对一种39,000道尔顿膜蛋白的ADP核糖基化作用。这些数据支持以下假说：(NH4)2SO4诱导的腺苷酸环化酶和cAMP受体的变化是由于一种假定的G蛋白的改变所致。
