Elevated plasma lipoprotein (a) [Lp(a)] contributes to the total risk of coronary heart disease (CHD) and is lowered by niacin and estrogens. Lp(a) has not been standardized as yet, due to difficulties with immunoassay standardizations and marked heterogeneity in apolipoprotein (a) [apo(a)] isoform size. Apo(a) is a glycoprotein that is unique to Lp(a) and shares homology (78-99%) with plasminogen. Unlike plasminogen, apo(a) is heavily glycosylated and binds tightly to wheat germ agglutinin (WGA). We propose to further develop a method to measure and standardize lp(a) by its cholesterol content [Lp(a)-C], a relatively stable component of Lp(a) that does not vary with isoform size. It has been determined in Phase I that it is feasible to separate Lp(a) from other plasma lipoproteins by lectin binding and to measure its cholesterol content by standard enzymatic methods. Phase I has provided information in better defining and optimizing the conditions of the assay. Due to homology with plasminogen, Lp(a) interferes with fibrinolysis and hence promotes thrombosis, a process intimately involved in atherosclerosis. Since lysine-binding isolates the fraction of Lp(a) that interferes with the activation of plasminogen in vitro, fractionation of the lectin-bound Lp(a) may result in an assay that is more predictive of atherosclerotic disease. In a small pilot study there was a trend for elevated amounts of the lysine-bound fraction in the case vs control subgroup. During Phase I the Lp(a)-C assay will be further refined to adapt to rapid cholesterol autoanalyzers and the lysine-bound fractionation method will be optimized. These assays will be applied to plasma samples from participants in the Framingham offspring study to determine normal ranges, correlation with Lp(a) by ELISA, and prediction of CHD and stroke.
The use of lectins as substitutes for immunoglobulins will be explored. This will potentially make a large number of assays that often require monospecific antibodies to be more affordable and more available to medical providers.
