Thematic review series: Patient-Oriented Research. Design and analysis of lipoprotein tracer kinetics studies in humans

Abstract

Lipoprotein tracer kinetics studies have for many years provided new and important knowledge of the metabolism of lipoproteins. Our understanding of kinetics defects in lipoprotein metabolism has resulted from the use of tracer kinetics studies and mathematical modeling. This review discusses all aspects of the performance of kinetics studies, including the development of hypotheses, experimental design, statistical considerations, tracer administration and sampling schedule, and the development of compartmental models for the interpretation of tracer data. In addition to providing insight into new metabolic pathways, such models provide quantitative information on the effect of interventions on lipoprotein metabolism. Compartment models are useful tools to describe experimental data but can also be used to aid in experimental design and hypothesis generation. The SAAM II program provides an easy-to-use interface with which to develop and test compartmental models against experimental models. The development of a model requires that certain checks be performed to ensure that the model describes the experimental data and that the model parameters can be estimated with precision.In addition to methodologic aspects, several compartment models of apoprotein and lipid metabolism are reviewed. Years of clinical investigation have provided valuable insight into the complexity of human lipoprotein metabolism. The measurement of plasma lipoprotein concentrations provides useful information, but from a functional viewpoint these concentrations reflect the balance between input and output in the lipoprotein system in plasma. The only way to quantify input and output in this system is by undertaking kinetics studies, typically using tracer methodology. These studies, however, are time-consuming and difficult to perform. Nevertheless, they are important to characterize the pathways that result in dyslipidemic states and to describe the in vivo mechanisms of action of treatments designed to regulate dyslipidemia and cardiovascular disease risk in clinical practice. There are many steps to consider when planning such metabolic studies. This review touches on all aspects of the design and analysis of lipoprotein kinetics studies, including philosophical angles, tracer methodology, mathematical modeling, and statistical considerations related to experimental design and analysis.With the advent of commercial stable isotopes and endogenous labeling protocols, some aspects of performing kinetics studies are now easier than when radioactive tracers and laborious exogenous labeling of lipoproteins were required. However, the number of laboratories and research groups undertaking these types of metabolic studies has changed little, highlighting the complexity of the methods involved.In addition to the design and modeling aspects of lipoprotein kinetics studies, what we have learned from apolipoprotein B (apoB) and HDL apoA-I and apoA-II tracer kinetics studies is covered in the present series of reviews by Parhofer and Barret...

Multiplexed peptide analysis for kinetic measurements of major human apolipoproteins by LC/MS/MS

“…This is another limitation of LC/MS/MS, but the comparison with the other methods used for concentration measurements is reassuring and suggests that this pitfall is probably not critical. The protocol used (primed and constant infusion of the tracer) is another limitation, as tracer boluses are often preferred for proteins with slow turnover rates ( 29 ). With this latter study design, the peak enrichments are different compared with the range we have analyzed.…”

Reproducibility of stable isotope-labeled tracer measures of VLDL-triglyceride and VLDL-apolipoprotein B-100 kinetics

“…A variety of tracer techniques are available for evaluating VLDL-TG and VLDL-apoB-100 kinetics (3)(4)(5)(6)(7)(8). Most contemporary methods make use of stable isotope-labeled tracers to measure the rate of incorporation and/or loss of the tracer into TG or apoB-100 in VLDL, often in combination with compartmental modeling analysis.…”

Proteome Kinetics: Coupling the Administration of Stable Isotopes with Mass Spectrometry-Based Analyses

“…Although the mathematics surrounding tracer kinetics have been described in detail Wolfe and Chinkes 2005), there are certain caveats that apply in different fields. For example, investigators working in the area of lipoprotein kinetics have recognized the need to add delays in the modeling (Barrett, Chan, and Watts 2006;Foster et al 1993;Patterson et al 2002). Namely, although proteins such as apolipoprotein B are continuously synthesized within liver and/or intestine, they are not immediately secreted into the circulation.…”