Alteration of Polypeptide Backbone Structure as determined by Circular Dichroic Spectral Analysis in Relation to Change of Plasminogen Activator Activity of Two Forms of Human Urinary Urokinase denatured with Guanidine Hydrochloride

Abstract

Alterations of polypeptide backbone structure and plasminogen activator activity of human urinary urokinase (UK) denatured with 0-6 M guanidine hydrochloride (Gdn-HCl) were investigated. The fractions of helix, β-from, β-turn and unordered structure were determined by the best-curve-fitting method based on the circular dichroic (CD) spectra at 200-240 nm to be 0.14, 0.23, 0.26 and 0.37 for M.W. 55000 form (H-UK) and 0.12, 0.17, 0.32 and 0.40 for M.W. 36000 form (L-UK), respectively. In view of the variation with Gdn-HCl concentration of ellipticity at the negative extrema (204 nm for H-UK ; 202 nm for L-UK), the denaturation process was concluded not to be a two-state transition : at 2.0-3.0M Gdn-HCl, the helical and β-form fractions were immobilized within a narrow range of 0.00-0.01 for both UK forms, corresponding to almost unchanged minimal ellipticities even with the increase of Gdn-HCl concentration. This indicates that an intermediate state (I) exists besides the native (N) and denatured (D) states. At 0.75-1.25 M Gdn-HCl, an activated state (A) intervened between N and I for H-UK but not for L-UK. The N→A transition of H-UK was an exceptional transition in that the helicity steadily increased with the enhanced minimal ellipticity. This corresponded to the observation that the activity of H-UK was potentiated by 18% during this transition. At the same Gdn-HCl concentration, L-UK reduced both its activity by 27-38% and its minimal ellipticity in contrast to H-UK. These findings show that the denaturation processes consist of "four states-three transitions" for H-UK and "three states-two transitions" for L-UK.