Hydrazine cation radical in the active site of ethanolamine ammonia-lyase: Mechanism-based inactivation by hydroxyethylhydrazine

A study has been made of the mechanism of inactivation of the adenosylcobalamin-dependent enzyme, ethanolamine ammonia-lyase (EAL), by hydroxyethylhydrazine. Incubation of EAL with adenosylcobalamin and hydroxyethylhydrazine, an analogue of ethanolamine, leads to rapid and complete loss of enzymic activity. Equimolar quantities of 5'-deoxyadenosine, cob(II)alamin (B-12r), hydrazine cation radical, and acetaldehyde are products of the inactivation. Inactivation is attributed to the tight binding of B-12r in the active site. Removal of B-12r from the protein by ammonium sulfate precipitation under acidic conditions, however, restores significant activity. This inactivation event has also been monitored by electron paramagnetic resonance (EPR) spectroscopy. In addition to EPR signals associated with B-12r, spectra of samples of inactivation mixtures reveal the presence of another radical. The other radical is bound in the active site where it undergoes weak magnetic interactions with the low spin Co2+ in B-12r. The radical species was unambiguously identified as a hydrazine cation radical by using [N-15(2)]hydroxyethylhydrazine, 2H(2)O, and quantitative interpretation of the EPR spectra. Homolytic fragmentation of a hydroxyethylhydrazine radical to acetaldehyde and a hydrazine cation radical is consistent with all of the observations. All of the experiments indicate that the mechanism-based inactivation of EAL by hydroxyethylhydrazine results from irreversible cleavage of the cofactor and tight binding of B-12r to the active site.