Effects of pH on the conformation and activation of plasminogen

Glu-plasminogen (plg) or Lys-plg solution was kept at pH 2.0 for various time intervals, and then readjusted to pH 7.0. The activation rate and conformational changes of such acid-treated plg were measured using the hydrolysis of S-2251, spectrofluorometry and spectropolarimetry (circular dichroism). The activation rate of acid-treated Glu-plg increased after readjustment to pH 7.0. The increase was not reversible even after keeping acid-treated Glu-plg for 24 hrs at neutral pH. The activation rate of acid-treated Lys-plg rather decreased. The fluorescence intensity at 340 nm of acid-treated plg decreased, and the intensity of fluorescence induced by the interaction of plasminogen with ANS (1-anilino-8-naphthalene sulfonate) increased after keeping plasminogen for longer than 24 hrs at pH 2.0. Circular dichroism spectra indicated that the secondary and tertiary structure of Glu-plg changed after acidification, and that only the tertiary structure of Lys-plg changed.