Abstract:The activity of transketolase in mouse brain was 5.7 nmol/min/mg protein measured by an enzyme‐coupled spectrophotometric assay. The apparent Kmfor ribose‐5‐phosphate was 330 μM, ford‐xylulose‐5‐phosphate was 120 μM, and for thiamine pyrophosphate was 7 μM. However, thiamine pyrophosphate remained tightly bound to transketolase in homogenates in which it dissociated completely from another thiamine pyrophosphate‐ dependent enzyme, the pyruvate dehydrogenase complex. These data suggest that loss of transketolase activity is likely to be a later consequence of thiamine deficiency in mammalian brain than is decreased activity of pyruvate dehyd