Quantitative determination of the binding of epsilon-aminocaproic acid to native plasminogen

The binding of epsilon-amino[14C]caproic acid (6-aminohexanoic acid, EACA) to native human plasminogen was determined using the ultrafiltration technique of Paulus (Paulus, H. (1969) Anal. Biochem. 32, 91-100) at free ligand concentrations ranging from 2 micrometer to 16 mM. One strong binding site (Kd = 0.009 mM) and approximately five weaker ones (Kd = 5 mM) were found. The constants were obtained by fitting the experimental points to the simple assumption of two sets of noninteracting sites. The distinct separation of the two kinds of sites allowed the correlation of the well known epsilon-aminocaproic acid-induced conformational transition in plasminogen with the saturation of the weaker group of binding sites by this ligand. The conformational transition was monitored by measurements of the sedimentation coefficient, as was done by others earlier. The midpoint of the transition occurred at approximately 3.3 mM free ligand. A dissociation constant of 0.32 mM was also obtained for L-lysine by measurements of the competition between this compound and labeled epsilon-aminocaproic acid for the strong binding site. The correlation between epsilon-aminocaproic acid binding and effects of the compound on various physical and functional properties is discussed. A discussion of the possible sources of error encountered in the technique used is also included.