Department of Laboratory Medicine and Pathology, University of Washington, Seattle, WA, USA; Kidney Research Institute, University of Washington, Seattle, WA, USA.
Department of Pharmaceutics, University of Washington, Seattle, WA, USA.
J Steroid Biochem Mol Biol. 2023 Feb;226:106206. doi: 10.1016/j.jsbmb.2022.106206. Epub 2022 Oct 28.
Circulating 25-hydroxyvitamin D [25(OH)D] concentration is used to monitor vitamin D status. Plasma protein binding may influence the 25(OH)D dose-response to vitamin D treatment through a direct relationship between the plasma unbound ("free") fraction and clearance of 25(OH)D. We previously evaluated 25(OH)D clearance in relation to kidney function using intravenous administration of deuterium labeled 25(OH)D. In this follow up study, we determined the free fraction of 25(OH)D in plasma (i.e., percent free 25(OH)D) and the serum concentration and haplotype of vitamin D binding protein in these participants. We hypothesized that the percent free 25(OH)D would be positively associated with 25(OH)D clearance and would mediate associations between clearance and vitamin D binding protein (GC) haplotypes. Participants were mean (SD) age 64 (10) years and included 42 individuals with normal kidney function (controls), 24 individuals with chronic kidney disease, and 19 individuals with kidney failure on hemodialysis. Free plasma 25(OH)D and 25(OH)D concentrations were quantified with a new liquid chromatography-tandem mass spectrometry (LC-MS/MS) method. Because there is no reference measurement procedure for free 25(OH)D, we compared the new method with a widely-used predictive equation and a commercial immunoassay. The percent free 25(OH)D determined by predictive equation was weakly associated with 25(OH)D clearance (R = 0.27; P = 0.01). However, this association was absent when percent free 25(OH)D was determined using LC-MS/MS-measured free and total 25(OH)D concentrations. Method comparison uncovered a negative bias in immunoassay-measured free 25(OH)D concentrations among participants with kidney failure, so immunoassay results were not used to evaluate the association between percent free 25(OH)D and clearance. GC2 haplotype carriage was associated with 25(OH)D clearance. Among individuals with 2 relative to no GC2 alleles, clearance was 87 (95% CI: 15-158) mL/d greater. However, in contrast with the literature, GC2 carriage was not significantly related to DBP concentration or the percent free 25(OH)D (either predicted or measured). In conclusion, the free fraction of 25(OH)D is not strongly associated with 25(OH)D clearance but may explain small differences in clearance according to GC haplotype.
循环 25-羟维生素 D [25(OH)D] 浓度用于监测维生素 D 状态。血浆蛋白结合可能通过 25(OH)D 清除率与血浆未结合（“游离”）部分之间的直接关系影响维生素 D 治疗的 25(OH)D 剂量反应。我们之前使用氘标记的 25(OH)D 静脉给药评估了 25(OH)D 清除率与肾功能的关系。在这项后续研究中，我们确定了这些参与者血浆中 25(OH)D 的游离分数（即游离 25(OH)D 的百分比）以及维生素 D 结合蛋白的血清浓度和单倍型。我们假设游离 25(OH)D 的百分比与 25(OH)D 清除率呈正相关，并介导清除率与维生素 D 结合蛋白（GC）单倍型之间的关联。参与者的平均（标准差）年龄为 64（10）岁，包括 42 名肾功能正常（对照组）、24 名慢性肾脏病和 19 名血液透析的肾衰竭患者。游离血浆 25(OH)D 和 25(OH)D 浓度采用新的液相色谱-串联质谱（LC-MS/MS）方法进行定量。由于没有游离 25(OH)D 的参考测量程序，我们将新方法与广泛使用的预测方程和商业免疫测定进行了比较。预测方程确定的游离 25(OH)D 百分比与 25(OH)D 清除率呈弱相关（R = 0.27；P = 0.01）。然而，当使用 LC-MS/MS 测量的游离和总 25(OH)D 浓度确定游离 25(OH)D 百分比时，这种关联就不存在了。方法比较揭示了肾衰竭患者中免疫测定法测量的游离 25(OH)D 浓度存在负偏倚，因此免疫测定结果未用于评估游离 25(OH)D 百分比与清除率之间的关系。GC2 单倍型携带与 25(OH)D 清除率相关。与没有 2 个 GC2 等位基因的个体相比，清除率增加 87（95%CI：15-158）mL/d。然而，与文献相反，GC2 携带与 DBP 浓度或游离 25(OH)D 的百分比（预测或测量）均无显著关系。总之，25(OH)D 的游离分数与 25(OH)D 清除率没有很强的关联，但可能根据 GC 单倍型解释清除率的微小差异。
