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We have previously shown that a functional free apolipoproteina (apoa) can be isolated from its parent lipoproteina (Lpa) by a mild reductive procedure. To shed further light on the properties of Lpa and apoa we subjected them to a limited proteolysis by porcine pancreatic elastase. This enzyme cleaved both at the Ile3520-Leu3521 bond in the linker between kringles IV-4 and IV-5 of apoa generating two fragments F1 and F2. In contrast to F1, which represented the N-terminal portion of apoa and was functionally inert, F2, representing the C-terminal domain, bound to lysine-Sepharose, fibrinogen, and fibronectin and formed a miniLpa particle when incubated with LDL. The proteolytic pattern by pancreatic elastase was also exhibited by human leukocyte elastase. F1, injected intravenously into normal mice, was rapidly cleared (Ty2, 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 residence time (Ty2, 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 after F1 injection were also present in the urine after injection of apoa or Lpa. Moreover, apoa fragments of the size seen in mouse urine were spontaneously present in normal human urine and appeared derived from larger apoa fragments in the plasma. Our results indicate that enzymes of the elastase family cleave human apoa in vitro into two main fragments that differ in structural and functional properties and metabolic behavior. The comparable size of apoa 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 Lpa in vivo.
Edelstein et al. (Mon,) studied this question.