Stereospecificity of L-myo-inositol-1-phosphate synthase for nicotinamide adenine dinucleotide.

Partially purified preparations of L-myo-inositol.-1-phosphate synthase (EC 5.5.1.4) from testis and mammary gland of laboratory rats (Rattus norvegicus) were used to show that this enzyme is specific for the pro-S hydrogen at C-4 of its cofactor, nicotinamide adenine dinucleotide (NAD). pro-S specificity of the first step (reversible oxidation of glucose 6-phosphate to 5-ketoglucose 6-phosphate) was proved by showing that tritium is transferred from [pro-S-4-3H]NADH but not from [pro-R-4-3H]NADH to glucose 6-phosphate when they are incubated with enzyme. That the stereospecificity in the second oxidation--reduction step (reduction of myo-inosose-2 1-phosphate to myo-inositol 1-phosphate) is the same as in the first step was shown by demonstrating that tritium from [5-3H]glucose 6-phosphate is incorporated into myo-inositol but not into NAD+ and that tritium from [4-3H]NAD+ is not incorporated into myo-inositol