Haber E, Haupert G T
Hypertension. 1987 Apr;9(4):315-24. doi: 10.1161/01.hyp.9.4.315.
Accumulating experimental evidence suggests that natriuresis in response to intravascular volume expansion is promoted by an endogenous regulator of Na+,K+-adenosine triphosphatase (ATPase). Efforts to purify this substance by a number of laboratories have as yet been unsuccessful. The properties of partially purified inhibitors from plasma, urine, and tissue often fail to possess the characteristics thought to be consistent with those of a physiological regulator. These include potency (Ki of approximately 1 nM), reversibility of inhibition, specificity for Na+,K+-ATPase, and responsiveness to relevant physiological stimuli. Two rather different candidate substances, extracted from urine and hypothalamus, have been purified to a high degree. Neither is a peptide, and both are of low molecular weight and resistant to acid hydrolysis. The substance from urine is rather nonpolar and interacts with digoxin-specific antibodies, while that from hypothalamus is polar and does not appear to share epitopes with the cardiac glycosides. On the serosal surface of the toad urinary bladder, the hypothalamic substance causes a reversible inhibition of Na+ transport, inhibits rubidium uptake in red blood cells by acting on the membrane's exterior surface, inhibits binding of ouabain to purified Na+,K+-ATPase, and reversibly inhibits hydrolysis of adenosine 5'-triphosphate by the enzyme with a Ki of 1.4 nM. The hypothalamic inhibitor may be differentiated from ouabain by their respective ionic requirements for optimal inhibition of enzymatic activity, and although both ouabain and the hypothalamic inhibitor fix Na+,K+-ATPase in its E2 conformation, the hypothalamic inhibitor does not promote phosphorylation of the enzyme by inorganic phosphate in the presence of Mg2+. Ionic requirements for inhibition also differentiate the hypothalamic inhibitor from vanadate ion, as does the inhibitor's activity in the presence of norepinephrine. Further enzymological and physiological studies will be facilitated by structural characterizations of the inhibitory substances and by the availability of a method to measure their concentrations in physiological fluids.
越来越多的实验证据表明，血管内血容量扩张引起的利钠作用是由一种内源性钠钾三磷酸腺苷（ATP酶）调节剂所促进的。多个实验室试图纯化这种物质，但至今尚未成功。从血浆、尿液和组织中部分纯化的抑制剂的特性，往往不具备被认为与生理调节剂特性相符的特征。这些特征包括效力（抑制常数Ki约为1纳摩尔）、抑制的可逆性、对钠钾ATP酶的特异性以及对相关生理刺激的反应性。从尿液和下丘脑提取的两种截然不同的候选物质已被高度纯化。两者都不是肽，且分子量低，耐酸水解。尿液中的物质极性较低，能与地高辛特异性抗体相互作用，而下丘脑的物质极性较高，似乎与强心苷没有共同的表位。在蟾蜍膀胱的浆膜表面，下丘脑物质可引起钠转运的可逆性抑制，通过作用于红细胞膜外表面抑制铷的摄取，抑制哇巴因与纯化的钠钾ATP酶的结合，并以1.4纳摩尔的抑制常数可逆地抑制该酶对三磷酸腺苷的水解。下丘脑抑制剂与哇巴因可通过各自对酶活性最佳抑制的离子需求来区分，尽管哇巴因和下丘脑抑制剂都能使钠钾ATP酶固定在其E2构象，但在镁离子存在的情况下，下丘脑抑制剂不会促进该酶被无机磷酸磷酸化。抑制的离子需求也将下丘脑抑制剂与钒酸盐离子区分开来，下丘脑抑制剂在去甲肾上腺素存在时的活性也是如此。抑制物质的结构表征以及测量其在生理流体中浓度的方法的可用性，将有助于进一步的酶学和生理学研究。
