Problems with the measurement of apolipoproteins AI and AII

There is considerable evidence demonstrating that increased levels of low density lipoprotein (LDL) cholesterol and decreased levels of high density lipoprotein (HDL) cholesterol are associated with coronary artery disease (CAD). Yet, these lipoprotein markers are insensitive for identifying risk of CAD. Measurement of subcomponents of HDL may offer more sensitive markers. Investigators have focused on protein components (apolipoproteins) of HDL as a potentially important marker. Unfortunately, because much of the immunoreactivity of apolipoproteins is hidden as a result of an association with lipids, it is difficult to measure them accurately. Detergents and other denaturing agents have been used to expose immunoreactivity. Poor correlations among different methods suggest that some detergents presently used may not be adequate for effective measurement of apolipoprotein (APO) AI. Studies using denaturing agents to probe HDL particles indicate that APO AII immunoreactivity is more resistant to exposure than that of APO AI. Data presented here indicate that the immunoreactivity of APO AII can be increased up to 50 percent by treatment with 4M guanidine provided the concentration of guanidine is diluted to less than 50 mM in the assay system. Previous studies failed to notice this effect because high levels of guanidine inhibited the antibody-antigen reaction in the immunoassay, making it appear that APO AII had not been exposed. It is concluded that with our present state of knowledge, it is unclear which, if any apolipoprotein assays, are adequately designed to achieve optimal exposure of antigenic sites, but that pretreatment with guanidine may be a simple, effective way to optimize APO AI and APO AII assays for clinical purposes.