Hydrogen Atom Exchange between 5‘-Deoxyadenosine and Hydroxyethylhydrazine during the Single Turnover Inactivation of Ethanolamine Ammonia-Lyase

Abstract

The early steps in the single turnover inactivation of ethanolamine ammonia-lyase (EAL) from Salmonella typhimurium by hydroxyethylhydrazine (HEH) have been probed by rapid-mixing sampling techniques, and the destiny of deuterium atoms, present initially in HEH, has been investigated by mass spectrometry. The inactivation reaction produces acetaldehyde, the hydrazine cation radical, 5‘-deoxyadenosine, and cob(II)alamin (B_12r) in amounts stoichiometric with active sites. Rapid-mix freeze-quench EPR spectroscopy and stopped-flow rapid-scan spectrophotometry revealed that the hydrazine cation radical and B_12r appeared at a rate of ∼3 s^-1 at 21 °C. Analysis of 5‘-deoxyadenosine isolated from a reaction mixture prepared in ²H₂O did not contain deuteriuma result which demonstrates that solvent-exchangeable sites are not involved in the hydrogen-transfer processes. In contrast, all of the 5‘-deoxyadenosine, isolated from inactivation reactions with [1,1,2,2-²H₄]HEH, had acquired at least one ²H from the labeled inactivator. Significant fractions of the 5‘-deoxyadenosine acquired two and three deuteriums. These results indicate that hydrogen abstraction from HEH by a radical derived from the cofactor is reversible. The distribution of 5‘-deoxyadenosine with one, two, and three deuteriums incorporated and the absence of unlabeled 5‘-deoxyadenosine in the product are consistent with a model in which there is direct transfer of hydrogens between the inactivator and the 5‘-methyl of 5‘-deoxyadenosine. These results reinforce the concept that the 5‘-deoxyadenosyl radical is the species that abstracts hydrogen atoms from the substrate in EAL.