Correlating crosslink formation with enzymatic activity in cysteine dioxygenase

Cysteine dioxygenase (CDO) from rat and other mammals exhibits a covalent post-translational modification between the residues C93 and Y157 that is in close proximity to the active site, and whose presence enhances the enzyme's activity. Protein with and without C93-Y157 crosslink migrates as distinct bands in SDS-PAGE, allowing quantification of the relative ratios between the two forms by densitometry of the respective bands. Expression of recombinant rat wild type CDO in Escherichia coli typically produces 40-50% with the C93-Y157 crosslink. A strategy was developed to increase the ratio of the non-crosslinked form in an enzyme preparation of reasonable quantity and purity, allowing direct assessment of the activity of non-crosslinked CDO and mechanism of formation of the crosslink. The presence of ferrous iron and oxygen is a prerequisite for C93-Y157 crosslink formation. Absence of oxygen during protein expression increased the fraction of non-crosslinked CDO, while presence of the metal chelator EDTA had little effect. Metal affinity chromatography was used to enrich non-crosslinked content. Both the enzymatic rate of cysteine oxidation and the amount of cross-linking between C93 and Y157 increased significantly upon exposure of CDO to air/oxygen and substrate cysteine in the presence of iron in a hitherto unreported two-phase process. The instantaneous activity was proportional to the amount of crosslinked enzyme present, demonstrating that the non-crosslinked form has negligible enzymatic activity. The biphasic kinetics suggest the existence of an as yet uncharacterised intermediate in crosslink formation and enzyme activation.