Human, rabbit and bovine plasminogens, having different sensitivity to streptokinase-activating action, differ, according to spectrophotometric titration, tryptophan fluorescence and circular dichroism spectroscopy, in the state of tyrosine and tryptophan residues, and in secondary and tertiary structures. Human plasminogen-streptokinase equimolar complex formation (according to gel chromatography) is accompanied by a differential ultraviolet spectrum. Difference spectroscopy is a convenient and adequate means of studying the formation of the said complexes. Streptokinase-human plasminogen complex formation is not hindered by partial substitution of water (20%) with ethanol or dimethylsulphoxide or by addition of O.OOIMsodium dodecylsulphate. The complex is not formed in 6 M urea, in solution, at pH < 2.0 or ~12.0-13.0, or with bovine plasminogen. Circular dichroism and tryptophan fluorescence spectral pattern changes during streptokinase-plasminogen complexformation enable us to conclude that streptokinase secondary and tertiary structures undergo certain rearrangements in theframework of the complex, while tryptophan-containing sites of the molecule are not drastically changed. The data obtained enable us to presuppose formation of streptokinase-rabbit plasminogen complexes which differ from human plasminogen complexes with streptokinase
