Rat brain apotransketolase: activation and inactivation.

Kinetic analysis of the combination of rat brain apotransketolase with thiamine diphosphate suggested that the enzyme exists in more than one form. One part of the apoenzyme reacted rapidly with thiamine diphosphate to reconstitute the holoenzyme, but another part appeared to combine only relatively slowly. In addition, an apparently irreversible further change took place, the apoenzyme being converted progressively to a form which apparently could not be activated by thiamine diphosphate. The relative proportions of the three forms i.e., that reacting rapidly, slowly, or not at all with thiamine diphosphate, were a function of the duration and conditions of storage, with the proportion of the apoenzyme form which reacted rapidly with thiamine diphosphate decreasing progressively. The findings reported here provide a possible explanation for problems various workers have encountered in attempting to evaluate Michaelis constants for the reaction of thiamine diphosphate with apotransketolase.