Bégin-Heick N
J Biol Chem. 1985 May 25;260(10):6187-93.
In mice homozygous for the ob gene (ob/ob), the response of adipose tissue adenylate cyclase to stimulation by lipolytic hormones is abnormally low in comparison to that in lean mice (+/+). Studies on the kinetics of adenylate cyclase activation in white adipocyte membranes under a variety of conditions show the following differences between +/+ and ob/ob mice. 1) The inhibitory effects of GTP and guanyl-5'-yl imidodiphosphate, which were clearly seen in +/+ membranes, were absent in the ob/ob membranes. 2) Half-maximal activation by GTP (in the presence of isoproterenol) required at least 10 times more GTP in ob/ob than in +/+ membranes. 3) Increasing the magnesium concentration (up to 10 mM) of the assay medium facilitated the activation of cyclase by modulatory ligands proportionately more in ob/ob than in +/+ membranes; in the +/+ membranes, 10 mM Mg2+ abolished the inhibitory effects of GTP. 4) Treatment with pertussis toxin attenuated the inhibitory effects of guanine nucleotides in +/+ membranes; no effect of the treatment was seen in the ob/ob membranes. 5) Pretreatment of membranes with cholera toxin facilitated cyclase activation proportionately more in ob/ob than in +/+ membranes; in addition, this treatment led to a shift to the left of the GTP dose-response curve in the ob/ob membranes. Cholera and pertussis toxins catalyzed the incorporation of ADP-ribose into their respective substrates in both the +/+ and the ob/ob membranes, showing that the alpha subunits of the stimulatory and inhibitory proteins of the regulatory component Ns and Ni, respectively are present in both types of membranes. Taken together, the results are consistent with the hypothesis that an excess of beta subunit (either primary or secondary to an altered interaction between beta and Ni alpha or Ns alpha) is responsible for the altered sensitivity to activating ligands of the adipocyte adenylate cyclase of the ob/ob mouse. In addition to these findings, we report an effect of the ob gene on the expression of adenylate cyclase activity, since adipose tissue cyclase from heterozygous lean mice (+/ob) showed characteristics which were intermediate between those of +/+ and ob/ob membranes.
在肥胖基因（ob/ob）纯合的小鼠中，与瘦小鼠（+/+）相比，脂肪组织腺苷酸环化酶对脂解激素刺激的反应异常低。在各种条件下对白色脂肪细胞膜中腺苷酸环化酶激活动力学的研究表明，+/+和ob/ob小鼠之间存在以下差异。1）GTP和鸟苷-5'-基亚氨基二磷酸的抑制作用在+/+细胞膜中明显可见，而在ob/ob细胞膜中则不存在。2）在ob/ob细胞膜中，GTP（在异丙肾上腺素存在下）达到半数最大激活所需的GTP量至少是+/+细胞膜中的10倍。3）将测定介质的镁浓度提高到10 mM时，ob/ob细胞膜中调节性配体对环化酶的激活促进作用比+/+细胞膜中更大；在+/+细胞膜中，10 mM Mg2+消除了GTP的抑制作用。4）用百日咳毒素处理可减弱+/+细胞膜中鸟嘌呤核苷酸的抑制作用；在ob/ob细胞膜中未观察到该处理的效果。5）用霍乱毒素预处理细胞膜后，ob/ob细胞膜中环化酶的激活促进作用比+/+细胞膜中更大；此外，该处理导致ob/ob细胞膜中GTP剂量-反应曲线向左移动。霍乱毒素和百日咳毒素在+/+和ob/ob细胞膜中均催化ADP-核糖掺入各自的底物，表明调节成分Ns和Ni的刺激性和抑制性蛋白的α亚基分别存在于两种类型的细胞膜中。综上所述，这些结果与以下假设一致，即β亚基过量（要么是原发性的，要么是β与Niα或Nsα之间相互作用改变的继发性结果）是ob/ob小鼠脂肪细胞腺苷酸环化酶对激活配体敏感性改变的原因。除了这些发现外，我们还报告了ob基因对腺苷酸环化酶活性表达的影响，因为杂合瘦小鼠（+/ob）的脂肪组织环化酶表现出介于+/+和ob/ob细胞膜之间的特征。
