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The Mechanism of Action of Ethanolamine Ammonia-Lyase, a B₁₂-dependent Enzyme

EVIDENCE FOR TWO INTERMEDIATES IN THE CATALYTIC PROCESS

From the ¹ From the New England Medical Center Hospital and Tufts University School of Medicine, Boston, Massachusetts 02111, and the Graduate Department of Biochemistry, Brandeis University, Waltham, Massachusetts 02154

Ethanolamine ammonia-lyase, an enzyme catalyzing the adenosylcobalamin-dependent deamination of ethanolamine, also catalyzes the conversion of l-2-aminopropanol to propionaldehyde and ammonia. In this reaction, tritium is transferred from enzyme·[5'-³H]adenosylcobalamin to the C-1 position of l-2-aminopropanol, as well as to the carbon of the product aldehyde. The labeling pattern is consistent with the mechanism of hydrogen transfer deduced with other substrates. Tritium transfer also occurs between enzyme·[5'-³H]adenosylcobalamin and propionaldehyde in the presence of NH₄⁺. Unlike the deamination of ethanolamine, the conversion of 2-aminopropanol to propionaldehyde and NH₄⁺ is reversible, since tritiated 2-aminopropanol was isolated from reaction mixtures originally containing only propionaldehyde, ammonia, and enzyme·[5'-³H]adenosyl cobalamin.

The partitioning of tritium derived from enzyme·[5'-³H]-adenosylcobalamin between l-2-aminopropanol and propionaldehyde was determined in reactions begun with l-2-aminopropanol as well as in reactions begun with propionaldehyde and ammonia. In the first case, the ratio [³H]propanolamine to [³H]propionaldehyde is 1.8:1; in the second case, the ratio is 0.3:1. This difference in the partitioning of tritium from the tritiated enzyme complex is consistent with the notion that there are at least two intermediates in the catalytic process, each exchanging tritium with coenzyme, which interconvert slowly with respect to the rate of tritium exchange.

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