Ramakrishnan R, Arad Y, Wong S, Ginsberg H N
Department of Medicine, College of Physicians and Surgeons, Columbia University, New York, NY 10032.
J Lipid Res. 1990 Jun;31(6):1031-42.
Radiolabeling of whole lipoproteins or individual apolipoproteins has been an essential tool for the determination of the kinetics of apolipoprotein metabolism in vivo. Mathematical analysis of specific radioactivity (SA) or total radioactivity data has demonstrated the existence of significant complexity in the plasma decay curves of several apolipoproteins. Results obtained during development of methods to study the metabolism of apolipoprotein B (apoB) in very low density lipoprotein (VLDL) subclasses isolated according to flotation (Sf) rates from whole radiolabeled (d less than 1.006 g/ml) VLDL suggested nonuniform radiolabeling of apoB in the three Sf subclasses being studied. We therefore determined apoB SA in VLDL Sf subclasses in ten hypertriglyceridemic and five normal subjects. After radioiodination of apoB in whole VLDL, different apoB SA were found in Sf 400-100, Sf 100-60, and Sf 60-20. The pattern of labeling was quite variable among subjects. On average, apoB SA in the VLDL tracer was greatest in Sf 400-100, and least in Sf 60-20. Nonuniform labeling could also be demonstrated in five studies in which samples were obtained 3 min after intravenous injection of the tracer into subjects with a wide range of plasma triglycerides. Nonuniform labeling of apoB in whole VLDL was also demonstrated in two of the subjects by isolating subclasses of their VLDL that did not bind to an anti-apolipoprotein E immunoaffinity column. These results indicate that the usual assumption of homogeneous labeling of apoB may be erroneous. We have derived a simple mathematical formula to study the consequences of this assumption in estimating kinetic parameters. It is shown that an erroneous assumption of homogeneous tracer labeling may significantly underestimate or overestimate the true production rate, even in a simple two-pool model. Identification of labeling characteristics and incorporation of this information into the mathematical analysis of the plasma radioactivity data can improve the accuracy of the analysis as well as the sensitivity of compartmental models generated by such data.
对全脂蛋白或单个载脂蛋白进行放射性标记一直是测定体内载脂蛋白代谢动力学的重要工具。对比放射性（SA）或总放射性数据的数学分析表明，几种载脂蛋白的血浆衰变曲线存在显著的复杂性。在开发从全放射性标记的（d小于1.006 g/ml）极低密度脂蛋白（VLDL）中根据漂浮（Sf）率分离的极低密度脂蛋白亚类来研究载脂蛋白B（apoB）代谢的方法过程中所获得的结果表明，在所研究的三个Sf亚类中，apoB的放射性标记不均匀。因此，我们测定了10名高甘油三酯血症患者和5名正常受试者极低密度脂蛋白Sf亚类中的apoB SA。对全极低密度脂蛋白中的apoB进行放射性碘化后，在Sf 400 - 100、Sf 100 - 60和Sf 60 - 20中发现了不同的apoB SA。各受试者之间的标记模式差异很大。平均而言，极低密度脂蛋白示踪剂中的apoB SA在Sf 400 - 100中最高，在Sf 60 - 20中最低。在五项研究中也证实了标记不均匀，这些研究是在向血浆甘油三酯水平范围广泛的受试者静脉注射示踪剂3分钟后采集样本。通过分离两名受试者的极低密度脂蛋白亚类（这些亚类不与抗载脂蛋白E免疫亲和柱结合），也证实了全极低密度脂蛋白中apoB的标记不均匀。这些结果表明，通常认为apoB标记均匀的假设可能是错误的。我们推导出了一个简单的数学公式来研究这一假设在估计动力学参数时的影响。结果表明，即使在简单的双池模型中，示踪剂标记均匀这一错误假设也可能显著低估或高估真实生成率。识别标记特征并将此信息纳入血浆放射性数据的数学分析中，可以提高分析的准确性以及由此类数据生成的房室模型的敏感性。
