Mechanism of the irreversible inactivation of mouse ornithine decarboxylase by α-difluoromethylornithine: Characterization of sequences at the inhibitor and coenzyme binding sites

Mouse ornithine decarboxylase (ODC) was expressed in Escherichia coli and the purified recombinant enzyme used for determination of the binding site for pyridoxal 5′-phosphate and of the residues modified in the inactivation of the enzyme by the enzyme-activated irreversible inhibitor, α-difluoromethylornithine (DFMO). The pyridoxal 5′-phosphate binding lysine in mouse ODC was identified as lysine 69 of the mouse sequence by reduction of the purified holoenzyme form with NaB[³H], followed by digestion of the carboxymethylated protein with endoproteinase LysC, radioactive peptide mapping using reversed-phase high pressure liquid chromatography and gas-phase peptide sequencing. This lysine is contained in the sequence PFYAVKC, which is found in all known ODCs from eukaryotes. The preceding amino acids do not conform to the consensus sequence of SXHK, which contains the pyridoxal 5′-phosphate binding lysine in a number of other decarboxylases including ODCs from E. coli. Using a similar procedure to analyze ODC labeled by reaction with [5-¹¹C]DFMO, it was found that lysine 69 and cysteine 360 formed covalent adducts with the inhibitor. Cysteine 360, which was the major adduct accounting for about 90% of the total labeling, is contained within the sequence -WGPTCDGL(I)D-, which is present in all known eukaryote ODCs. These results provide strong evidence that these two peptides form essential parts of the catalytic site of ODC. Analysis by fast atom bombardment-mass spectrometry of tryptic peptides containing the DFMO-cysteine adduct indicated that the adduct formed in the enzyme was probably the cyclic imine S-((2-(1-pyrroline))methyl)cysteine. This is readily oxidized to S-((2-pyrrole)methyl)cysteine or converted to S-((2-pyrrolidine)methyl)cysteine by NaBH₄ reduction. This adduct is consistent with spectral evidence showing that inactivation of the enzyme with DFMO does not entail the formation of a stable adduct between the pyridoxal 5′-phosphate, the enzyme, and the inhibitor.