Incubations of testis myo-inositol-1-phosphate synthase with D-(5-18O)glucose 6-phosphate and with H218O show no evidence of Schiff base formation.

myo-Inositol-1-P synthase (EC 5.5.1.4) purified from rat testis and from bovine testis was allowed to react with D-[5-18O]glucose-6-P. myo-Inositol, obtained in these reactions, retained all of the 18O originally in the glucose-6-P. When these enzyme preparations were incubated with unlabeled glucose-6-P in a medium enriched in H2 18O no uptake of the oxygen isotope occurred that could be ascribed to the myo-inositol-1-P synthase reaction. By these criteria this enzyme, which is considered to use an aldolase mechanism in the cyclization step, cannot form a Schiff base during the reaction. In addition, these enzymes are not inhibited by 10 mM EDTA. One interpretation of this evidence is that the myo-inositol-1-P synthases we have studied are neither Class I nor Class II aldolases, and simply use base catalysis in the cyclization step.