Different NH2-Terminal Form with 12 Additional Residues of α2-Plasmin Inhibitor from Human Plasma and Culture Media of HEP G2 Cells

α2-Plasmin inhibitor (α2PI) was purified from plasma or from the culture media of Hep G2 cells by one-step immunoaffinity chromatography procedure. Majority of α2PI purified from plasma was the previously recognized plasma α2PI with NH2-terminal Asn (Asn-α2PI), whereas majority of α2PI purified from the culture media was retaining the "pro" peptide of 12 amino acids with NH2-terminal Met (Met-α2PI). When Hep G2 cells were cultured in serum-free media, the α2PI secreted to the media was totally in a form of Met-α2PI. Incubation of Met-α2PI with human plasma induced the complete conversion of Met-α2PI to Asn-α2PI. The results indicate that α2PI is synthesized and secreted from liver cells as Met-α2PI and Met-α2PI is converted to Asn-α2PI by proteolytic cleavage in plasma during the circulation.