A novel immune-isolation method for direct quantification of triglycerides associated with lipoprotein(a)

Lipoprotein (a) [Lp(a)] is viewed as a cholesterol-rich, LDL-like particle, yet potential heterogeneity in its lipid composition is not well understood. We developed and validated a novel immune-isolation assay to directly quantify triglycerides (TGs) associated with Lp(a) [Lp(a)-TGs]. Lp(a) was selectively isolated from plasma using magnetic beads conjugated with monoclonal antibody LPA4 targeting apolipoprotein(a), followed by enzymatic quantification of TGs. Assay specificity was ensured using washing buffers to prevent nonspecific lipoprotein interactions. Spike-in experiments with purified VLDL/intermediate density lipoprotein lacking Lp(a) demonstrated no measurable interference. Lp(a)-cholesterol [Lp(a)-C] was measured using an established immune-isolation method. The ratio of Lp(a)-TG to Lp(a)-C was calculated to distinguish TG-enriched Lp(a) particles from the typical cholesterol-rich, LDL-like phenotype. Lp(a)-TG, Lp(a)-C, Lp(a) molar concentration, and estimated compositional ratios were quantified in 36 normotriglyceridemic individuals and 114 individuals with moderate hypertriglyceridemia (150-500 mg/dl). In normotriglyceridemic individuals, mean (SD) TGs were 98.4 (31.9) mg/dl, Lp(a)-TG 1.42 (2.83) mg/dl, Lp(a)-C 4.03 (4.01) mg/dl, and the Lp(a)-TG/Lp(a)-C ratio was 0.59 (1.27). Lp(a)-TG and Lp(a)-C accounted for mean (SD) 1.22% (0.10) of total plasma TGs and 2.62% (2.01) of total plasma cholesterol. In individuals with hypertriglyceridemia, mean (SD) TGs were 284 (85) mg/dl, Lp(a)-TG 53.7 (25.3) mg/dl, Lp(a)-C 14.4 (6.9) mg/dl, and the Lp(a)-TG/Lp(a)-C ratio was 3.99 (1.20). Lp(a)-TG and Lp(a)-C accounted for mean (SD) 19.9% (6.53) of total plasma TGs and 9.68% (4.41) of total plasma cholesterol. This immune-isolation assay is the first validated, high-throughput method for direct quantification of Lp(a)-TG. This study demonstrates that Lp(a) lipid composition is variable and enriched in triglycerides and cholesterol in hypertriglyceridemic states. It provides a platform for future mechanistic, epidemiologic, and pharmacologic studies of Lp(a)-triglyceride interactions. This immune-isolation assay is the first validated, high-throughput method for direct quantitation of Lp(a)-TG.