FUNCTIONAL AND METABOLIC DIFFERENCES BETWEEN ELASTASE-GENERATED FRAGMENTS OF HUMAN LIPOPROTEIN[A] AND APOLIPOPROTEIN[A]

We have previously shown that a functional free apolipoprotein[a] (apo [a]) can be isolated from its parent lipoprotein[a] (Lp[a]) by a mild reductive procedure. To shed further light on the properties of Lp[a] and apo[a] we subjected them to a limited proteolysis by porcine pancr eatic elastase. This enzyme cleaved both at the Ile3520-Leu3521 bond i n the linker between kringles IV-4 and IV-5 of apo[a] generating two f ragments F1 and F2. In contrast to F1, which represented the N-termina l portion of apo[a] and was functionally inert, F2, representing the C -terminal domain, bound to lysine-Sepharose, fibrinogen, and fibronect in and formed a miniLp[a] particle when incubated with LDL. The proteo lytic pattern by pancreatic elastase was also exhibited by human leuko cyte elastase. F1, injected intravenously into normal mice, was rapidl y cleared (T-1/2, 2.9 h) and after 1 h fragments in the size range of 100-33 kDa were observed in the urine. In turn, F2 had a longer reside nce time (T-1/2, 5 h) and was excreted in the urine only after 5 h as fragments of 70-45 kDa. Fragments in the same size range as found afte r F1 injection were also present in the urine after injection of apo[a ] or Lp[a]. Moreover, apo[a] fragments of the size seen in mouse urine were spontaneously present in normal human urine and appeared derived from larger apo[a] fragments in the plasma. Our results indicate that enzymes of the elastase family cleave human apo[a] in vitro into two main fragments that differ in structural and functional properties and metabolic behavior. The comparable size of apo[a] fragments observed in the urine of humans and injected mice invites the speculation that enzymes of the elastase family may play a role in the biology of Lp[a] in vivo.