MULTIPLE ACTIVE CONFORMERS OF MOUSE ORNITHINE DECARBOXYLASE

Purified recombinant mouse ornithine decarboxylase (ODC) was denatured with urea or with guanidinium chloride. Enzymic activity was efficien tly recovered upon dilution of the denaturing agent. ODC renatured aft er urea treatment was further characterized. Kinetics of decarboxylati on of the natural substrate ornithine or of the suicide substrate alph a-difluoromethylornithine (DFMO) were not significantly changed by den aturation/renaturation. Surprisingly, the renatured enzyme was not sta bly labelled with radioactive DFMO, in contrast with the native enzyme not subjected to denaturation. Native and renatured ODC did not diffe r in their c.d. spectra, but the former contained four reactive cystei ne residues and the latter seven. These data indicate that a conformat ional change results from denaturation/renaturation that does not alte r decarboxylation of substrates, but does change the accessibility or stability of the cysteine-360 residue modified by decarboxylated DFMO.