In thiamine deficiency, activation of erythrocyte transketolase by thiamine in vivo exceeds activation by cofactor in vitro.

In 60 thiamine deficient patients, the mean erythrocyte transketolase activity after activation by thiamine diphosphate cofactor in vitro, representing the apparent sum of holoenzyme and apoenzyme activities, was 0.609 (SD 0.166) U/g Hb before thiamine therapy and rose to 0.772 (SD 0.152) U/g Hb immediately after the administration of thiamine to the patients. The difference between these values, 0.163 (SD 0.130) U/g, is the mean activity of transketolase protein which can be activated by thiamine in vivo but not by thiamine diphosphate in vitro. This difference correlated with low initial erythrocyte transketolase activity in these patients, but not with their alcohol intake, liver function or diagnoses.