Purification and properties of transketolase from fresh rat liver

Transketolase has been purified from rat liver. The final product is judged to be homogeneous by the criteria of gel filtration on Sephadex G-200 and Sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. The estimated Mr's are 139,000 for the native protein and 69,000 for the dissociated subunits. The purified enzyme does not require either Mg2+ or thiamine pyrophosphate for optimum activity. The best C2-donor substrate is D-xylulose 5-phosphate, with Km = 25 microM. D-Fructose 6-phosphate is an active C2-donor, but beta-hydroxypyruvate was found to be inactive, even at high concentration. The product of the C2-transfer from D-xylulose 5-phosphate or D-fructose 6-phosphate to D-ribose 5-phosphate has been identified as D-sedoheptulose 7-phosphate. Neither D-glucose 6-phosphate nor D-arabinose 5-phosphate showed C2-acceptor activity. This enzyme does not appear to be responsible for the formation of octulose 8-phosphates which have been detected in liver extracts.