Identification of cis-ethanesemidione as the organic radical derived from glycolaldehyde in the suicide inactivation of dioldehydrase and of ethanolamine ammonia-lyase

The hydrate of glycolaldehyde is a substrate analogue that induces the formation of cob(II)alamin and 5'-deoxyadenosine from adenosylcobalamin at the active site of dioldehydrase, and the resulting complex is inactive. The carbon atoms of glycolaldehyde hydrate remain bound to this complex, and it has been postulated that the first step or steps of the catalytic process on glycolaldehyde hydrate generate an intermediate that undergoes a destructive side reaction leading to inactivation of the enzyme [Wagner, O. W., Lee, H. A., Jr., Frey, P. A., and Abeles, R. H. (1966) J. Biol. Chem. 249, 1751- 1762]. All evidence suggests that dioldehydrase reaction proceeds by a radical mechanism, and the glycolaldehyde hydrate is expected to be converted initially into a radical. Electron paramagnetic resonance (EPR) spectroscopic analysis of the inactivated complex shows that glycolaldehyde is transformed into a cis-ethanesemidione radical that is weakly spin-coupled to the cob(II)alamin in the active site of the enzyme. This radical has been identified by analysis of EPR spectra obtained from samples with ¹³C- and ²H-labeled forms of glycolaldehyde. The analysis shows that the stable radical associated with the inactive complex is symmetrical and that it contains a single solvent-exchangeable proton, consistent with a cis- ethanesemidione. Glycolaldehyde also inactivates ethanolamine ammonia-lyase (EAL). EPR studies of ethanolamine ammonia-lyase reveal that treatment with glycolaldehyde also results in formation of an ethanesemidione radical bound in the active site. The suicide inactivation in both enzymatic reactions is postulated to result from formation of this stable radical, which cannot react further to abstract a hydrogen atom from 5'-deoxyadenosine. Analysis of the electron spin-spin coupling between the semidione radicals and cob(II)alamin in both enzymes indicates that the distance between the radical and Co²⁺ is ~11 Å in each case.