Multiple active conformers of mouse ornithine decarboxylase

Purified recombinant mouse ornithine decarboxylase (ODC) was denatured with urea or with guanidinium chloride. Enzymic activity was efficiently recovered upon dilution of the denaturing agent. ODC renatured after urea treatment was further characterized. Kinetics of decarboxylation of the natural substrate ornithine or of the suicide substrate α-difluoromethylornithine (DFMO) were not significantly changed by denaturation/ renaturation. Surprisingly, the renatured enzyme was not stably labelled with radioactive DFMO, in contrast with the native enzyme not subjected to denaturation. Native and renatured ODC did not differ in their c.d. spectra, but the former contained four reactive cysteine residues and the latter seven. These data indicate that a conformational change results from denaturation/ renaturation that does not alter decarboxylation of substrates, but does change the accessibility or stability of the cysteine-360 residue modified by decarboxylated DFMO.