Partial characterization of lipoproteins containing apo[a] in human atherosclerotic lesions

Previously we quantified the amounts of immunoreactive apo[a] found in human atherosclerotic lesions extracted sequentially with phosphate-buffered saline (PBS) and guanidine hydrochloride (GuHCl). In this study we have attempted to characterize lipoproteins containing apo[a] in such PBS and GuHCl fractions, obtained from autopsy samples, in order to eventually determine their structure-function relationships critical for evaluating the mechanisms that make them atherogenic. Apo[a] in the PBS extracts migrated slightly ahead of plasma Lp[a] on agarose electrophoresis. Although apo[a] in extracts showed the same isoforms as in plasma in SDS-PAGE, it was also highly fragmented. When a d < 1.10 g/ml ultracentrifugation fraction of the PBS extract was subjected to gel filtration, a major part of the immunoreactive apo[a] in this fraction co-isolated with plasma Lp[a]. When the Lp[a]-sized fraction was further separated by density gradient ultracentrifugation, a subpopulation was isolated containing apo[a] in the 1.06 < d < 1.08 g/ml density range that was free of lesion-derived low density lipoprotein (LDL) (A-LDL). This fraction contained immunoreactive apo[a] and apoB, had a total cholesterol to protein ratio of about 1, and demonstrated increases in fluorescence (360 ex/430 em) and conjugated dienes that were even greater than values obtained for the corresponding A-LDL sample. The void volume fraction following gel exclusion chromatography of the d < 1.10 g/ml fractions contained both apo[a] and apoB that comigrated on nondenaturing PAGE, suggesting that they were present on the same particle. Apo[a] in GuHCl extracts comigrated with plasma Lp[a] on agarose electrophoresis and contained apo[a] isoforms of similar molecular weights as those found in corresponding plasma samples. When the GuHCl extract was subjected directly to gel filtration in the presence of 6 M GuHCl, two included peaks of apo[a] immunoreactivity were present, one eluting slightly ahead of plasma Lp[a], the other slightly ahead of plasma LDL. Collectively, these data indicate that apo[a] is present in human atherosclerotic lesions in forms resembling intact but oxidized plasma Lp[a], as larger particles possibly representing Lp[a] complexed to itself or other plaque components, and as slightly smaller particles possibly representing degraded Lp[a].