Watanabe Y, Jakobs K H
Mol Pharmacol. 1986 Mar;29(3):258-63.
The diterpene, forskolin, increases basal adenylate cyclase activity in membranes of human platelets to more than 20-fold with an EC50 of about 5 microM. However, when the platelet adenylate cyclase was activated via the stimulatory coupling component, Ns, e.g., by the hormone, prostaglandin E1, or the stable GTP analog, guanosine 5'-[gamma-thio]triphosphate, added in combination with a protease, forskolin was able to inhibit the enzyme. The inhibition was half-maximal and maximal (40-50% inhibition) at 0.01 and 0.1 microM forskolin, respectively, and occurred without apparent lag phase. At a maximally inhibitory concentration, forskolin largely reduced the apparent affinity of the Ns-stimulated platelet adenylate cyclase for its substrate MgATP in a noncompetitive manner, which resulted in a pronounced inhibition by forskolin at low substrate concentrations and a further increase in activity at high MgATP concentrations. Treatment of intact platelets or platelet membranes with agents known to interfere with Ni-mediated adenylate cyclase inhibition did not diminish but even increased the forskolin-induced inhibition of the adenylate cyclase. However, inhibition of the prostaglandin E1-stimulated adenylate cyclase by forskolin and the inhibitory hormonal agents, thrombin and epinephrine, were not additive at maximally inhibitory concentrations. Furthermore, increasing concentrations of Mg2+ and Mn2+ reduced (Mg2+) or even reversed (Mn2+) the forskolin-induced inhibition. The data indicate that forskolin apparently has two distinct effects on the platelet adenylate cyclase, namely inhibition and stimulation. The data furthermore suggest that the adenylate cyclase inhibition by forskolin is not mediated by the inhibitory guanine nucleotide-binding protein Ni, but may be due to an action of the diterpene at the adenylate cyclase catalytic moiety, particularly when activated by Ns, or a closely related membrane component.
二萜类化合物毛喉素可使人类血小板膜中的基础腺苷酸环化酶活性增加20倍以上，其半数有效浓度（EC50）约为5微摩尔。然而，当血小板腺苷酸环化酶通过刺激性偶联成分Ns被激活时，例如通过激素前列腺素E1或稳定的鸟苷三磷酸类似物5'-[γ-硫代]三磷酸鸟苷，并与蛋白酶联合添加时，毛喉素能够抑制该酶。在0.01和0.1微摩尔毛喉素浓度下，抑制作用分别达到半数最大抑制和最大抑制（40 - 50%抑制），且无明显延迟期。在最大抑制浓度下，毛喉素以非竞争性方式大幅降低了Ns刺激的血小板腺苷酸环化酶对其底物MgATP的表观亲和力，这导致在低底物浓度时毛喉素产生明显抑制，而在高MgATP浓度时活性进一步增加。用已知可干扰Ni介导的腺苷酸环化酶抑制作用的试剂处理完整血小板或血小板膜，不仅不会减弱反而会增强毛喉素诱导的腺苷酸环化酶抑制作用。然而，在最大抑制浓度下，毛喉素与抑制性激素试剂凝血酶和肾上腺素对前列腺素E1刺激的腺苷酸环化酶的抑制作用并非相加。此外，Mg2+和Mn2+浓度的增加会降低（Mg2+）甚至逆转（Mn2+）毛喉素诱导的抑制作用。数据表明，毛喉素对血小板腺苷酸环化酶显然有两种不同的作用，即抑制和刺激。数据还表明，毛喉素对腺苷酸环化酶的抑制作用不是由抑制性鸟嘌呤核苷酸结合蛋白Ni介导的，而是可能由于该二萜类化合物作用于腺苷酸环化酶催化部分，特别是当被Ns或密切相关的膜成分激活时。
