The reciprocal effects of epsilon-aminohexanoic acid and chloride ion on the activation of human [Glu1]plasminogen by human urokinase.

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Abstract
The activation of human [Glu1]plasminogen ([Glu1]Pg) by high-molecular-weight two-chain human urinary urokinase [EC 3.4.21.31) and low-molecular-weight two-chain human urinary urokinase is inhibited by Cl- at physiological concentrations and stimulated by .epsilon.-aminohexanoic acid (.epsilon.Ahx; .epsilon.-aminocaproic acid). The inhibition by Cl- does not occur in the presence of concentrations of .epsilon.Ahx that saturate the acid''s weak binding sites on [Glu1]Pg, and the stimulation by .epsilon.Ahx is maximally exhibited in the presence of Cl-. We have used intrinsic fluorescence measurements with [Glu1]Pg to show that the conformational alteration and the concomitant increase in activation rate that accompanies .epsilon.Ahx-binding to [Glu1]Pg in the presence of Cl- does not occur in the same manner without Cl-. Further, the decrease in the intrinsic fluorescence that is attendant to Cl- binding to [Glu1]Pg in the absence of .epsilon.Ahx is not observed in the presence of this effector molecule. Analyses of the results of this manuscript strongly indicate that a conformation of [Glu1]Pg that is not optimal for its activation by urokinase is adopted in the presence of Cl-, and this is relieved by .epsilon.Ahx. This has important implications in the inhibition of [Glu1]Pg activation in the solution phase of blood plasma and in the large acceleration of this process when plasminogen is bound to physiological positive effectors via its .epsilon.Ahx-binding site(s).

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