Control of Human Plasminogen Activation

Abstract

The activation of GlU₁-plasminogen (Glu-Pg) by streptokinase (SK), urokinase (UK) and tissue plasminogen activator (tPA) is under rigorous control by molecules such as ε-aminocaproic acid (EACA), fibrinogen (Fg), fibrin (Fn) and, as we have recently discovered, anions. This presentation will focus on the biochemical mechanisms that are involved in these processes. In the case of activation by SK, a species of activator complex, composed of Glu-Pg and SK, can be identified that is inhibited by anions, such as C1^–, and stimulated by Fg and Fn. This species rapidly decays to another activator complex, also consisting of Glu-Pg and SK, that is much less sensitive to control by these effector molecules. The most stable activator complex, containing equimolar SK and plasmin, is not affected to a great extent by anions, Fg or Fn. In the overall activation of Glu-Pg by SK, C1^– behaves as a mixed inhibitor, with a Ki of 6.4–9.2 mM, and Fg functions as a mixed activator, displaying a Ka of 110–240 nM. These results show that activation of Glu-Pg by SK in physiological samples would be considerably inhibited by C1^– in the absence of Fg. The activation of Glu-Pg by both high- and low-molecular weight UK is also inhibited by C1^–, but is stimulated by EACA. The inhibition by C1^– does not occur in the presence of concentrations of EACA that saturate its weak binding sites on Glu-Pg, and the stimulation by EACA is maximally exhibited in the presence of C1^–. The anion inhibitory effect follows a Hofmeister series, as does the binding (stimulatory) constant of EACA to Glu-Pg. Based upon structural studies of Glu-Pg in the presence of a variety of anions, we find that Glu-Pg can adopt a conformation that is optimal to activation and one that is not. The former occurs in the presence of weakly bound anions, or in the presence of any salt, plus saturating levels of EACA. The latter conformation is adopted in the presence of tightly bound anions. Anions also influence the rate of Glu-Pg activation by tPA, and the stimulation of this process by Fg and Fn. The mechanism of this effect is currently under investigation.

A Ligand-induced Conformational Change in Apolipoprotein(a) Enhances Covalent Lp(a) Formation

“…Plasminogen is kept in a closed conformation via an intramolecular interaction between the N-terminal tail domain and kringle V (27). ⑀-ACA replaces the internal ligand and leads to a transition from a closed to open conformation, resulting in a change in tryptophan fluorescence (28), an increase in the radius of gyration (18), and an enhancement of tPA-or uPA-mediated plasminogen activation to plasmin (29). Binding of plasminogen to fibrin, an important cofactor for plasminogen activation, has been postulated to elicit this conformational change (30).…”

Glycosylation of Asparagine-28 of Recombinant Staphylokinase with High-Mannose-type Oligosaccharides Results in a Protein with Highly Attenuated Plasminogen Activator Activity

“…The collected fractions from B were placed over a concanavalin A-Sepharose column. SakSTAR u was collected in the flow-through and wash in fractions [11][12][13][14][15][16]. After this, the column was washed with a solution of 10 mM sodium phosphate, 5 mM MgCl 2 , and 400 mM NaCl, pH 7.0.…”

Resistance of porcine blood clots to lysis relates to poor activation of porcine plasminogen by tissue plasminogen activator

“…These involve injury [11][12][13], surgical procedures [14][15][16] and chronic disease conditions such as arteriosclerosis [17]. Accordingly, it is imperative that the coagulation and fibrinolytic systems of the pig should approximately mimic those of the human.…”