Zager P G, Frey H J, Spalding C T, Wengs W J, Brittenham M C
J Clin Endocrinol Metab. 1986 Jan;62(1):84-9. doi: 10.1210/jcem-62-1-84.
Plasma 18-hydroxycorticosterone (18-OH-B) to aldosterone (aldo) concentration ratios reflect adrenal corticosterone methyloxidase type II activity. This ratio is determined not only by the relative secretion rates of the two steroids but also by differences in binding, distribution, and metabolism. Plasma cortisol alters the distribution of aldo between red blood cells (RBC) and plasma. We postulated that the distribution of 18-OH-B, like that of aldo, is determined by the availability of high affinity binding sites on plasma transcortin. Double equilibrium dialyses demonstrated that 18-OH-B, aldo, and cortisol compete for binding sites on transcortin. Increasing amounts of each of the three unlabeled steroids produced progressive decrements in the binding of all three labeled steroids to transcortin. The affinity of 18-OH-B (2 X 10(6) M-1) for transcortin was intermediate between those of cortisol (3 X 10(7) M-1) and aldo (0.9 X 10(6) M-1). Heat treatment of plasma decreased the binding of 18-OH-B and cortisol to transcortin by 82% and 75%, respectively. Gel filtration of plasma revealed that protein-bound [3H]18-OH-B and [14C]cortisol eluted in the same fractions. The addition of increasing quantities of unlabeled cortisol to whole blood in vitro produced similar increments in RBC to plasma concentration ratios of [3H]18-OH-B and [14C]aldo. The ratio of the percentage of circulating 18-OH-B in plasma to the percentage of circulating aldo in plasma was constant in blood containing low and high cortisol concentrations. Therefore, changes in plasma cortisol have similar effects on the distribution of 18-OH-B and aldo between RBC and plasma.
血浆18 - 羟皮质酮（18 - OH - B）与醛固酮（aldo）的浓度比反映了肾上腺皮质酮甲基氧化酶II型的活性。该比值不仅取决于这两种类固醇的相对分泌率，还取决于它们在结合、分布和代谢方面的差异。血浆皮质醇会改变醛固酮在红细胞（RBC）和血浆之间的分布。我们推测，18 - OH - B的分布与醛固酮一样，由血浆皮质转运蛋白上高亲和力结合位点的可利用性决定。双平衡透析表明，18 - OH - B、醛固酮和皮质醇会竞争皮质转运蛋白上的结合位点。三种未标记类固醇中每种的量增加，都会使所有三种标记类固醇与皮质转运蛋白的结合逐渐减少。18 - OH - B对皮质转运蛋白的亲和力（2×10⁶ M⁻¹）介于皮质醇（3×10⁷ M⁻¹）和醛固酮（0.9×10⁶ M⁻¹）之间。血浆经热处理后，18 - OH - B和皮质醇与皮质转运蛋白的结合分别减少了82%和75%。血浆的凝胶过滤显示，与蛋白质结合的[³H]18 - OH - B和[¹⁴C]皮质醇在相同的组分中洗脱。体外向全血中添加越来越多的未标记皮质醇，会使[³H]18 - OH - B和[¹⁴C]醛固酮的红细胞与血浆浓度比产生类似的增加。在含有低和高皮质醇浓度的血液中，血浆中循环的18 - OH - B百分比与血浆中循环的醛固酮百分比之比是恒定的。因此，血浆皮质醇的变化对18 - OH - B和醛固酮在红细胞和血浆之间的分布具有相似的影响。
