Structural basis for the anticoagulant activity of heparin. 2. Relationship of anticoagulant activity to the thermodynamics and fluorescence fading kinetics of acridine orange-heparin complexes

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Abstract
Complexing heparin or dermatan sulfate with the fluorescent probe acridine orange provides a means of studying electrostatic, as static and dynamic conformational aspects of the glycosaminoglycans via the thermodynamic and photochemical (fluorescence fading) properties of the complexes. The cooperative binding constants (Kq), fluorescence fading rate parameters (r"), and anticoagulant activities of heparins fractionated according to anionic density all showed qualitatively the same dependence upon anionic density. When Kq and r" plotted against anticoagulant activity, empirical relationships were observed. The corresponding values for unfractionated dermatan sulfate fell on the lines defined by the heparin fractions. Temperature-dependence studies showed that differences in fading rate seen for heparins of different anionic densities are entropic in origin and reflect differences in the ability to assume a special configuration. Differences in activation entropy for fluorescence fading can be empirically correlated with anticoagulant activity. The latter correlation suggests a physical similarity in the roles played by anionic density in both fluorescence fading and anticoagulant activity.