Heparin-induced structural and functional alterations of bovine trypsin.

To investigate the mechanism whereby heparin can modulate the activity of serine proteinases, bovine trypsin was chosen as reference and treated with heparin at 10, 100 and 200 micrograms/ml, in buffer solvents, with and without incubation at 37 degrees C. Heparin caused rapid, buffer- and pH-dependent decrease in trypsin solubility due to the generation of insoluble fragments from proteinase. Desalting treatments variously restored solubility by removing insoluble material. UV absorption and fluorescence emission spectra revealed significant heparin-induced conformational alterations in the trypsin molecule, the maximal effect being apparent at a proteinase-to-heparin molar ratio ranging from 1.6 to 1.0. The involvement of the catalytic sites of trypsin by heparin was further confirmed by the significant reduction in the difference absorption spectra of proflavine. Both proteolytic and esterolytic activities of trypsin were shown to be markedly decreased by heparin, especially after 5 h incubation at 37 degrees C. However, when the proteolytic and esterolytic activities of trypsin were measured on fresh solutions not submitted to desalting treatments, variable activation instead of inhibition of both activities was observed in the presence of heparin, this effect waning spontaneously in time or after desalting treatment. The paradoxical increase in functional activities was not inhibited by soybean trypsin inhibitor and was accompanied by denaturation and fragmentation of the proteinase as demonstrated by spectroscopic analyses and SDS-PAGE of fresh solutions. The results obtained indicated that heparin causes a rapid, time- and temperature-dependent conformational alteration of trypsin with irreversible denaturation and degradation of the proteinase. The underlying mechanism appears to be heparin-catalyzed oxidative degradation of trypsin due to liberation of oxygen radicals which are also responsible for the temporary increase in catalytic functions.