THE THIAMINE-DEPENDENT HYSTERETIC BEHAVIOR OF HUMAN TRANSKETOLASE - IMPLICATIONS FOR THIAMINE-DEFICIENCY

We have investigated the hysteretic properties of human transketolase with emphasis on its dependency on thiamine pyrophosphate concentratio n. As demonstrated previously, the reaction progress curves revealed a slow transition from an initial low velocity to a faster final steady -state velocity, characterized by the rate constant tau(-1). The rate of the transition was dependent on the concentration of the thiamine p yrophosphate cofactor, with progressively longer transition times foun d as the concentration of thiamine pyrophosphate was decreased. At phy siological thiamine pyrophosphate concentrations, the inverse rate con stant was in the range of 10 to 20 min for fibroblast-derived transket olase and increased dramatically with only small decreases from these levels of thiamine pyrophosphate. Variation in the lag was found when transketolase from different individuals was examined. Moreover, at lo w levels of thiamine, the rate of the transition was different between fibroblast- and lymphoblast-derived transketolase. The substantial la g in formation of active holoenzyme and the findings of interindividua l variation and cell type variation in the lag period suggest mechanis ms for the loss of transketolase activity during thiamine deficiency a nd may explain, at least in part, the differential sensitivity to defi ciency demonstrated by tissues and individuals.