Studies on the activation of lysine-2,3-aminomutase by (−)-S-adenosyl- l-methionine

Abstract

To investigate the physiological significance of the activation of lysine-2,3-aminomutase by S-adenosylmethionine in Clostridium SB 4, the content of the sulfonium compound has been measured in this microorganism with a new isotope dilution technique. 0.57 μmole of S-adenosylmethionine per g of wet bacterial cell have been detected, indicating saturating cellular concentrations of the sulfonium compound with respect to lysine mutase (the apparent K m of the enzyme with respect to S-adenosylmethionine is 2.8·10 −8 M). To study the mechanism of such activation, several S-adenosylmethionine analogues have been tested: among them only S-adenosyl-(5′)-3-methylthiopropylamine (decarboxylated S-adenosylmethionine) exerts an appreciable effect on the enzyme activity, while S-adenosyl- l-(2-hydroxy-4-methylthio)butyric acid, S-inosyl- l-methionine, S-inosyl- l-(2-hydroxy-4-methylthio)butyric acid, S-methyl- l-methionine and its deaminated derivative did not modify the reaction rate to any extent. The data suggest that both amino groups of the molecule are involved in the activation. In addition, experiments with (−)- S- and (±)- S-adenosylmethionine demonstrate that the steric configuration of the sulfonium pole is not related to the activation mechanism. Equilibrium dialysis experiments between purified enzyme and S-adenosyl-[ carboxy- 14C]methionine or S-adenosyl[ Me- 14C]methionine failed to show any measurable binding between the sulfonium compound and the protein. Precipitation of the enzyme with trichloroacetic acid in the presence of S-adenosyl[ Me- 14C]methionine confirmed the apparent absence of a tight binding between the labeled carbon of the sulfonium compound and the enzyme.