Improved determination of transketolase activity in erythrocytes.

We describe a new and better method for determining transketolase (EC 2.2.1.1) activity in human erythrocytes. Heating the hemolysate (55 degrees C, 5 min) inactivates transaldolase (EC 2.2.1.2), the enzyme that catalyzes the formation of fructose 6-phosphate and erythrose 4-phosphate from sedoheptulose 7-phosphate and glyceraldehyde 3-phosphate. The net effect is that the quantity of sedoheptulose 7-phosphate formed more precisely represents the transketolase activity, which is unaffected under these conditions. Use of ribosephosphate isomerase (EC 5.3.1.6) and ribulose-phosphate 3-epimerase (EC 5.1.3.1) to establish an equilibrium among the pentoses allowed us to confirm the stoichiometry of the transketolase reaction. We also discuss the effect of thiamin pyrophosphate, which is used to reflect thiamin deficiency.