[Blood transketolase activity and the TDP effect as indices of thiamine body allowance].

Protein-bound and free forms of thiamin diphosphate (TDP) (separated by dialysis and gel filtration) were found in rat erythrocytes. Content of TDP in blood did not correlate with the transketolase activity at the initial steps of B1 avitaminosis. Decrease of the DTP total amount in blood by more than 80% did not affect distinctly the transketolase activity. As shown by binding of 14C-TDP during the equilibrium dialysis and gel filtration, the apotransketolase did not occur in erythrocytes of thiamin-deficient rats. Activation of transketolase, which occurred after addition of 50 mg TDP into the whole blood lysates (TDP-effect), was characteristic for the later steps of the avitaminosis; it depended rather on leukocyte than on erythrocyte transketolase. Estimation of TDP concentration in blood was the most suitable assay for a body providing with thiamin at the early steps of the avitaminosis. The accuracy of the coenzyme estimation was decreased within 15-30 days of the avitaminosis due to its drastic lowering. In this case, determination of the transketolase activity was the most suitable criterion. The thiamin-binding protein, found in erythrocytes, appears to participate in transport of the vitamin across erythrocyte membranes.