Two synchrotron-based techniques, synchrotron X-ray fluorescence (SXRF) and X-ray absorption spectroscopy (XAS), have demonstrated that Ca2+ and Zn2+ were the major metal ions distributed in the natural latex of Euphorbia cf. lactea. Both metal ions were found to affect the fibrinogenolytic activity of a homodimeric protease purified from the latex of this plant. The dimeric protein had an estimated molecular mass of about 82 kDa analyzed by SDS-PAGE. Therefore this protein was called as EuP-82. Based on the results of circular dichroism (CD) spectroscopy and the fibrinolytic activity measurement, it was found that Ca2+ could activate the proteolytic activity of the enzyme by stabilizing its backbone structure. The intact conformation of EuP-82 was predicted from CD spectrum, which consisted of 51 % alpha-helix and 9 % beta-sheet. Zn2+ (10 mM) could decrease the fibrinolytic activity of EuP-82 to 30 +/- 1 %. CD spectrum also supported that the inhibitory effect of Zn2+ on the enzyme activity occurred by the drastic change of the enzyme structure with increasing the random coil conformation and by switching between alpha-helix and beta-sheet structure. These results could be of first importance for further application to use EuP-82, the natural source protease as a potential drug for the thrombosis treatment. The fibrinolytic activity of EuP-82 may be enhanced by plasma Ca2+ which generally involves in human hemostasis system.