The thiol group and amino group at the active site of actinidin were modified by o-phthaldehyde to form a fluorescence group with excitation and emission wavelength maxima at 345 um and 416 nm respectively. The fluorescence group was then used to probe the conformational changes of the modified enzyme in guanidine hydrochloride (GuHCl) solution. Results were compared with the changes of activity as well as fluorescence and CD spectra of unmodified actinidin in GuHCl sloutions. It is shown that the conformational changes of the enayme active site parallel the enzyme inactivation, and both of them precede the conformatvonal changes of the enzyme as a whole revealed by fluorescence and CD spectra.
