Reduced stability of rat brain transketolase after conversion to the apo form.

The stability of rat brain transketolase, whether measured at 37 or 0 degree C, was reduced after conversion to the apo form by removal of thiamine diphosphate, as shown by a decline in the activity recovered when assayed in the presence of thiamine diphosphate. Both the shape of the breakdown curve and the failure to recover the full activity, even after incubation with thiamine diphosphate, showed that the breakdown of the apotransketolase was complex. The initial rate of breakdown of the apoenzyme was sharply pH dependent, being minimal at 37 degrees C at a pH value of 7.6, close to that likely to exist in vivo. The rate rose sharply with deviation of the pH in either direction. The stability of the enzyme on storage at 0 degree C showed a similar pattern of pH dependence, provided that allowance is made for temperature effects on dissociation constants. These findings provide further support for the hypothesis that differences in brain transketolase may play a part in the etiology of Wernicke-Korsakoff's syndrome.