[Purification and properties of rat liver transketolase].

Transketolase was isolated from wet tissue of rat liver and purified by ammonium sulfate and CM-cellulose fractionation and by adsorption chromatography on hydroxylapatite. The native enzyme is made up of two subunits with molecular weight of 70 000, is electrophoretically homogeneous and has a specific activity of 2.8 u. per mg of protein (30 degrees). The enzymatic and fluorimetric assays revealed the presence of two moles of thiamine diphosphate per mole of protein. The Arrhenius plots for the rate of the transketolase reaction with a pentose phosphate mixture as substrate are continuous at 10-32 degrees; the activation energy is 89.9 cJ/mole, temperature index is 3.3. The curve for the reaction rate versus substrate concentration is S-shaped; the apparent Km value for xylulose 5-phosphate is 2.2 x 10(-5) M. The ions with a tetraedric structure (arsenate, phosphate, sulfate) act as competitive inhibitors of the transketolase-catalyzed reaction.