Stereochemistry and kinetic isotope effects in the decarboxylation of S-adenosylmethionine catalyzed by the pyruvyl enzyme, S-adenosylmethionine decarboxylase.

S-Adenosyl-5'-3-methylthio[1-3H]propylamine was prepared by decarboxylation of S-adenosylmethionine in tritiated water, catalyzed by S-adenosylmethionine decarboxylase from Escherichia coli. Degradation of this product to 3-methylthio[1-3H]propylamine followed by oxidation in a coupled assay system involving pea seedling amine oxidase, catalase, and aldehyde dehydrogenase indicated less than 4% of total counts in water and greater than 75% in NADH plus NAD+. These results allow the assignment of tritium in 3-methylthiopropylamine to the 1-R configuration, establishing that S-adenosylmethionine decarboxylase, a pyruvate-containing decarboxylase, operates via a retentive mode. Tabulation of the available stereochemical results for two pyruvate-containing and three pyridoxal phosphate-dependent amino acid decarboxylases indicates that stereochemistry has been conserved both within each class of amino acid decarboxylases and between structurally distinct classes of decarboxylases. Comparison of the specific activity of 3-methylthio[1-3H]propylamine to the tritiated water employed in the decarboxylation reaction leads to a primary tritium isotope effect of 4.5 under conditions of substrate concentration far in excess of Km. Although a portion of the tritium in 3-methylthiopropylamine could have arisen through an enzyme-catalyzed exchange into decarboxylated adenosylmethionine (leading to an underestimate of the kinetic tritium isotope effect), we are unable to detect enzymatic loss of tritium from decarboxylated S-adenosylmethionine. The magnitude of the observed isotope effect is discussed in the context of a kinetically significant exchange of an active site residue with solvent in the present of enzyme-bound S-adenosylmethionine.