The mechanism of action of ethanolamine ammonia-lyase, an adenosylcobalamin-dependent enzyme. The source of the third methyl hydrogen in the 5'-deoxyadenosine generated from the cofactor during catalysis.

Abstract

Ethanolamine ammonia-lyase is an adenosylcobalamin-dependent enzyme which catalyzes the conversion of ethanolamine and propanolamine to ammonia and the corresponding aldehydes. A mechanism has been proposed for this and other adenosylcobalamin-dependent reactions which involves cleavage of the carbon-cobalt bond of the cofactor followed by abstraction of a substrate hydrogen atom by the adenosyl fragment to form 5'-deoxyadenosine. In support of this proposal, a previous study demonstrated that the deamination of propanolamine by ethanolamine ammonia-lyase is accompanied by the reversible cleavage of the carbon-cobalt bond of the cofactor, with the production of 5'-deoxyadenosine (Babior, B.M., Carty, T.J., and Abeles, R.H. (1974) J. Biol. Chem. 249, 1689-1695). The present study is concerned with the origin of the third hydrogen atom on the methyl group of the 5'-deoxyadenosine produced in that reaction. The 5'-deoxyadenosine isolated from an incubation mixture initially containing enzyme, [5',5'-D2]adenosylcobalamin, and [1,1-D2]propanolamine was chemically degraded so that the 4' and 5' carbon atoms were, respectively, converted to the carbonyl and methyl carbons of acetaldehyde. Analysis of the p-nitrophenylhydrazone of the acetaldehyde by gas-liquid chromatography-mass spectroscopy revealed 3 deuterium atoms/molecule, indicating that two of the methyl hydrogens originated from adenosylcobalamin and the third was donated by substrate. This observation provides further support for the participation of 5'-deoxyadenosine in the mechanism of adenosylcobalamin-dependent reactions.