Different NH2-terminal form with 12 additional residues of alpha 2-plasmin inhibitor from human plasma and culture media of Hep G2 cells.

alpha 2-Plasmin inhibitor (alpha 2PI) was purified from plasma or from the culture media of Hep G2 cells by one-step immunoaffinity chromatography procedure. Majority of alpha 2PI purified from plasma was the previously recognized plasma alpha 2PI with NH2-terminal Asn (Asn-alpha 2PI), whereas majority of alpha 2PI purified from the culture media was retaining the "pro" peptide of 12 amino acids with NH2-terminal Met (Met-alpha 2PI). When Hep G2 cells were cultured in serum-free media, the alpha 2PI secreted to the media was totally in a form of Met-alpha 2PI. Incubation of Met-alpha 2PI with human plasma induced the complete conversion of Met-alpha 2PI to Asn-alpha 2PI. The results indicate that alpha 2PI is synthesized and secreted from liver cells as Met-alpha 2PI and Met-alpha 2PI is converted to Asn-alpha 2PI by proteolytic cleavage in plasma during the circulation.