Studies on the Conformational Changes of Plasminogen Induced during Activation to Plasmin and by 6‐Aminohexanoic Acid

The hydrodynamic properties of plasminogen are highly affected during the activation process as found by studies on the sedimentation‐velocity, sedimentation‐equilibrium behaviour, gelfiltration behaviour of plasminogen A, plasminogen B and of plasmin inactivated by diisopropylphosphorofluoridate (Dip‐plasmin). The molecular weight was estimated to 92 000, 84000 and 81 000 for plasminogen A, B and Dip‐plasmin, respectively and the frictional ratio, f/f 0 , was found to be 1.50, 1.56 and 1.55. The circular dichroism (CD) spectra of human plasminogen A, plasminogen B and Dip‐plasmin have been studied at pH 7.4 in 0.1 M KCI. The far‐ultraviolet spectra are almost identical for all the samples and indicate that they contain a large proportion of random structure (about 80%) and about 20%β‐structure, while no significant amounts of α‐helix could be detected. In the near‐ultraviolet region, large significant differences were seen, which are consistent with a stepwise activation of plasminogen A via plasminogen B to plasmin. The effects of 6‐aminohexanoic acid and l ‐lysine on the CD spectra of plasminogen A and of the activation peptide on that of plasminogen B indicate that the amino‐terminal peptidic material has great affinity for the rest of the protein at pH 7.4 and greatly affects the CD spectrum between 250–280 nm. In this area, the spectra of Dip‐plasmin and plasminogen B are almost identical and the same spectrum is also obtained with plasminogen A when treated 0.01 M 6‐aminohexanoic acid or 0.1 M l ‐lysine. The conformation around the amino‐terminal part of Dip‐plasmin and around the internal peptide bond cleaved during the activation to plasmin seems to be of great importance for the CD spectra at 280–305 nm.