The mutual relationship between the two molecular forms of the major fibrinolysis inhibitor alpha-2-antiplasmin in blood

Abstract

Alpha-2-antiplasmin, a major inhibitor of fibrinolysis, is synthesized in the liver and occurs in blood in two molecular forms: a very active plasminogen-binding (PB) form and a less active nonplasminogen-binding (NPB) form. This study investigates the origin and mutual relationship of these two forms in vivo and in vitro. Despite wide variation in plasma concentration of the inhibitor (16% to 138%), the ratio between the two forms in vivo was found to be, in the main, constant among healthy volunteers, heterozygotes for a congenital deficiency of alpha- 2-antiplasmin, and patients with a stable liver cirrhosis: PB/NPB = 2.41 +/- 0.34 (SD). Resynthesis after depletion or increased synthesis in the acute-phase reaction showed a specific increase of the PB form of the molecule in blood after discontinuation of L-asparaginase or streptokinase therapy and after myocardial infarction. In vitro studies demonstrated that only the PB form was present after one day in the culture medium of the human cell line Hep G2, while the NPB form appeared after 11 days. Clearance after inhibition of synthesis by L- asparaginase therapy revealed a more rapid decrease in the PB form relative to the NPB form in blood, demonstrated by a change in the PB- NPB ratio from 2.86 +/- 0.55 to 1.74 +/- 0.24 (mean of 6, SD). An apparently spontaneous first order conversion from the PB to NPB form, with an apparent half-life of about eight days, was demonstrated at 37 degrees C in plasma and serum in vitro. The conversion was found to be temperature dependent and uninfluenced by the fibrinolytic components fibrinogen, fibrin, and plasminogen. Additions of a variety of enzymes or inhibitors did not interfere with the process. These results demonstrate that the PB form of alpha-2-antiplasmin is produced by the liver and that the NPB form is formed in the circulation.

Antiplasmin

“…Surprisingly, approximately 40% of circulating AP binds plasmin slowly [28]. Sasaki et al [29] made the same observation and demonstrated that the slow form was truncated at the C-terminus by at least the final 26 amino acids.…”

Increased N‐terminal cleavage of alpha‐2‐antiplasmin in patients with liver cirrhosis

“…In the circulation, the α2AP protein undergoes both N‐ and C‐terminal proteolytic cleavages, which significantly modify its activity. In approximately 65% of circulating α2AP the C‐terminus is intact so it can bind to plasmin(ogen) (plasminogen‐binding α2AP, PB‐α2AP), whereas the remainder (~35%) is cleaved and loses its ability to bind plasminogen (non‐plasminogen‐binding α2AP, NPB‐α2AP) . NPB‐α2AP remains an active plasmin inhibitor, but displays very slow binding kinetics to plasmin and is not significantly crosslinked into fibrin by activated factor XIII .…”

A novel missense mutation in the human plasmin inhibitor (alpha2‐antiplasmin) gene associated with a bleeding tendency

“…The C‐terminal region is a 51 amino acid long extension that is unique among serpins and contains a secondary binding site for plasminogen and plasmin (Sugiyama et al , 1988). About 30% of plasmin inhibitor in plasma exists in a proteolytically modified form that lacks the secondary C‐terminal plasmin(ogen) binding site (Kluft et al , 1986). Plasmin inhibitor in plasma may also be proteolytically modified in the N‐terminus resulting in formation of a 12 amino acids shorter form with N‐terminal methionine (Bangert et al , 1993).…”