The mechanism of the triosephosphate isomerase reaction.

The interconversion of dihydroxyacetone phosphate and glyceraldehyde 3-phosphate catalyzed by the enzyme triosephosphate isomerase was first demonstrated by Meyerhof and Kiessling (1). Since then studies with the enzyme have disclosed little about the mechanism of this reaction. Such an interconversion may be envisioned as occurring by formation of an enediol as, based on chemical analogy (2)) has been proposed for the hexosephosphate isomerase reaction (3) or by a hydride shift such as occurs in the isomerization catalyzed by glyoxylase I (4, 5). The technique of using isotopic hydrogen to label either the water of the medium or the substrates and of determining the disposition of this isotope after reaction was employed in studying this mechanism. This approach has proved useful in demonstrating the formation of an enzyme-dihydroxyacetone phosphate compound before condensation in the aldolase reaction (6). In the present experiments it is shown that no direct hydride shift occurs during the triosephosphate isomerase reaction and that there is complete stereospecificity relative to the carbinol hydrogens of dihydroxyacetone phosphate in the conversion to glyceraldehyde 3-phosphate. These findings are consistent with the concept of an enediol intermediate only if it is assumed not to be freely dissociable from the enzyme. A preliminary report of these experiments has been published (7) and some of the findings have also been made by Bloom and Topper (8).