Enzyme Kinetics and Pharmacological Characterization of Nucleotidases Released from the Guinea Pig Isolated Vas Deferens during Nerve Stimulation: Evidence for a Soluble Ecto-Nucleoside Triphosphate Diphosphohydrolase-Like ATPase and a Soluble Ecto-5′-Nucleotidase-Like AMPase
作者:Svetlana T. Mihaylova-Todorova、Latchezar D. Todorov、David P. Westfall
DOI:10.1124/jpet.102.033332
日期:2002.9.1
Previously, we have demonstrated that stimulation of the sympathetic nerves of the guinea pig vas deferens evokes release not only of the cotransmitters ATP and norepinephrine but also of soluble nucleotidases that break down extracellular ATP, ADP, and AMP into adenosine. In this study we show that the apparent K m values of the releasable enzyme activity vary depending on which of these adenine nucleotides is used as initial substrate. The K m value for ATP was 33.6 ± 2.3 μM, 21.0 ± 2.3 μM for ADP, and 10.0 ± 1.1 μM for AMP. The ratios of the V maxvalues for each enzyme reaction were 4:2:3. We have also found a different sensitivity of the metabolism of ATP and AMP by releasable nucleotidases to known nucleotidase inhibitors. Suramin inhibited the breakdown of ATP by releasable nucleotidases in a noncompetitive manner and with a K i value of 53 μM, but had no effect on the breakdown of AMP. The 5′-nucleotidase inhibitor α,β-methylene ADP inhibited the breakdown of AMP but not that of ATP. Concanavalin A inhibited the breakdown of AMP but had neither inhibitory nor facilitatory effects on the breakdown of ATP. 6- N , N -Diethyl-β,γ-dibromomethylene-d-ATP (ARL67156), an ecto-ATPase inhibitor, suppressed ATPase and AMPase activities, whereas NaN3 (10 mM) affected neither reaction, but inhibited the ADP metabolism. Phosphatase- and phosphodiesterase inhibitors did not affect the activity of the releasable nucleotidases. This evidence suggests that the soluble nucleotidases released during neurogenic stimulation of the guinea pig vas deferens combine an ecto-5′-nucleotidase-like and an ecto-nucleoside triphosphate diphosphohydrolase-like activity.
之前,我们已经证明刺激豚鼠输精管的交感神经不仅引发ATP和去甲肾上腺素等共同传递物质的释放,还引发可溶性核苷酸酶的释放,这些酶将细胞外ATP、ADP和AMP分解为腺苷。在本研究中,我们展示了可释放酶活性的表观K_m值取决于所用的腺苷酸作为起始底物。ATP的K_m值为33.6 ± 2.3 μM,ADP为21.0 ± 2.3 μM,AMP为10.0 ± 1.1 μM。每种酶反应的V_max值比率为4:2:3。我们还发现可释放核苷酸酶对已知核苷酸酶抑制剂的ATP和AMP代谢有不同的敏感性。苏黎世抑制了可释放核苷酸酶对ATP的分解,属于非竞争性抑制,K_i值为53 μM,但对AMP的分解没有影响。5′-核苷酸酶抑制剂α,β-亚甲基ADP抑制了AMP的分解,但对ATP则没有影响。刀豆凝集素A抑制了AMP的分解,但对ATP的分解既没有抑制作用也没有促进作用。6-N,N-二乙基-β,γ-双溴亚甲基-d-ATP(ARL67156),一种外源ATP酶抑制剂,抑制了ATP酶和AMP酶活性,而NaN3(10 mM)对两种反应均没有影响,但抑制了ADP的代谢。磷酸酶和磷酸二酯酶抑制剂对可释放核苷酸酶的活性没有影响。这些证据表明,在豚鼠输精管的神经刺激过程中释放的可溶性核苷酸酶结合了类似外源5′-核苷酸酶和外源核苷三磷酸二磷酸水解酶的活性。