The Mechanism of Action of Ethanolamine Deaminase: VI. ETHYLENE GLYCOL, A QUASI-SUBSTRATE FOR ETHANOLAMINE DEAMINASE

Abstract

Abstract Incubation of 5'-deoxyadenosylcobalamin with ethylene glycol in the presence of ethanolamine deaminase leads to the cleavage of the coenzyme at the carbon-cobalt bond, with the production of equivalent amounts of 5'-deoxyadenosine and acetaldehyde, together with a new corrinoid which appears to arise by a substitution in the coordination sphere of the cobalt whereby the 5'-deoxyadenosyl residue is replaced by another ligand. This reaction is accompanied by the transfer of a hydrogen from ethylene glycol to 5'-deoxyadenosine. After the reaction is over the acetaldehyde dissociates slowly from the enzyme, but both 5'-deoxyadenosine and the new corrinoid remain bound to the enzyme, where the latter is gradually converted to hydroxocobalamin. The cleavage of coenzyme, which is first order in enzyme-B12 complex, obeys saturation kinetics with respect to ethylene glycol, showing a Vmax of about 0.2 sec-1 and a Km for ethylene glycol of 0.02 m. The optical spectrum of the new B12 derivative resembles that of an alkyl cobalamin or a thiol cobalamin. However, spectral changes observed on treating the reaction mixture with urea or heat to denature the enzyme suggest that the latter possibility is more likely than the former. The possibility that the new compound is a hitherto unknown type of corrin derivative cannot be excluded. As a result of the reaction, the activity of the enzyme is substantially reduced, but not abolished. The enzyme is capable of promoting the cleavage of more than 1 mole of coenzyme per mole of active site. Both of these observations indicate that the enzyme participates catalytically in the cleavage reaction, and is not destroyed as a consequence of the reaction. These results are formulated in terms of a mechanism which relates the observed reaction to the catalytic reaction in which ethanolamine is substrate.