Oxidative events cause degradation of apoB-100 but not of apo[a] and facilitate enzymatic cleavage of both proteins

Lipoprotein [a] (Lp[a]) contains equimolar amounts of apoB-100 and apolipop rotein [a] (apo[a]). Both proteins are amenable to degradation in vivo by m echanisms yet to be clearly defined. In this study, we examined the in vitr o susceptibility of LDL and Lp[a], obtained from the same donor, to oxidati on by either Cu2+ or the combined Crotalus adamanteus phospholipase A(2) an d soy bean lipoxygenase system, monitoring the course of the reaction by th e generation of conjugated dienes and fatty acids. In some experiments, tre atment with leukocyte elastase (LE) or matrix metalloproteinase 12 (MMP-12) was administered before and after the oxidative step. In the case of Lp[a] we found that with both oxidizing systems, conditions that caused the brea kdown of apoB-100 did not degrade apo[a] although oxidation-mediated change s were detected in the latter by intrinsic tryptophan fluorescence spectros copy. Similar results were obtained with a reassembled Lp[a] obtained by in cubating free apo[a] with LDL. Both apo[a] and apoB-100 were cleaved by LE and AMP-12 but the enzymatic cleavage was more marked when the preoxidized proteins were used as a substrate. jlr Taken together, our in vitro studies indicate that apo[a] but not apoB-100 resists oxidative fragmentation, whe reas both proteins are cleaved by enzymes of the serine and metalloproteina se families. We speculate that the fragments of apo[a] observed in vivo may be preferentially generated by proteolytic rather than oxidative events, w hereas apoB-100 can be degraded by both mechanisms.