A large and highly glycosylated protein constituent of LIPOPROTEIN (A). It has very little affinity for lipids but forms disulfide-linkage to apolipoprotein b-100. Apoprotein(a) has serine PROTEINASE activity and can be of varying sizes from 400- to 800-kDa. It is homologous to plasminogen and is known to modulate thrombosis and fibrinolysis.