Human high-density lipoproteins (HDL) are a complex mixture of structurally-related nanoparticles that perform distinct physiological functions. We previously showed human HDL containing apolipoprotein A-I (APOA1) but not apolipoprotein A-II (APOA2), designated LpA-I, is composed primarily of two discretely sized populations. Here, we isolated these particles directly from human plasma by antibody affinity chromatography, separated them by high-resolution size exclusion chromatography and performed a deep molecular characterization of each species. The large and small LpA-I populations were spherical with mean diameters of 109 Å and 91 Å, respectively. Unexpectedly, isotope dilution MS/MS with [¹⁵N]-APOA1 in concert with quantitation of particle concentration by calibrated ion mobility analysis demonstrated that the large particles contained fewer APOA1 molecules than the small particles; the stoichiometries were 3.0 and 3.7 molecules of APOA1 per particle, respectively. MS/MS experiments showed that the protein cargo of large LpA-I particles was more diverse. Human HDL and isolated particles containing both APOA1 and APOA2 exhibit a much wider range and variation of particle sizes than LpA-I, indicating that APOA2 is likely the major contributor to HDL size heterogeneity. We propose a ratchet model based on the trefoil structure of APOA1 whereby the helical cage maintaining particle structure has two 'settings' - large and small - that accounts for these findings. This understanding of the determinants of HDL particle size and protein cargo distribution serves as a basis for determining the roles of HDL subpopulations in metabolism and disease states.

中文翻译：

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载脂蛋白 AI 在没有载脂蛋白 A-II 的情况下调节 HDL 粒径。

人类高密度脂蛋白 (HDL) 是结构相关的纳米颗粒的复杂混合物，具有不同的生理功能。我们之前展示了含有载脂蛋白 AI (APOA1) 但不含载脂蛋白 A-II (APOA2) 的人类 HDL，称为 LpA-I，主要由两个离散大小的群体组成。在这里，我们通过抗体亲和色谱直接从人血浆中分离出这些颗粒，通过高分辨率尺寸排阻色谱分离它们并对每个物种进行了深入的分子表征。大和小的 LpA-I 种群呈球形，平均直径分别为 109 埃和 91 埃。出乎意料的是，同位素稀释 MS/MS 与 [ ¹⁵N]-APOA1 与校准离子淌度分析的粒子浓度定量相一致，表明大粒子比小粒子含有更少的 APOA1 分子；每个粒子的化学计量分别为 3.0 和 3.7 个 APOA1 分子。MS/MS 实验表明，大 LpA-I 颗粒的蛋白质货物更加多样化。人类 HDL 和同时包含 APOA1 和 APOA2 的分离颗粒表现出比 LpA-I 更广泛的粒径范围和变化，表明 APOA2 可能是 HDL 大小异质性的主要贡献者。我们提出了一个基于 APOA1 三叶结构的棘轮模型，其中螺旋笼保持粒子结构有两个“设置”——大的和小的——这解释了这些发现。