[Lag period as affected by the adrenal pyruvate dehydrogenase complex containing residual endogenous thiamine...

The pyruvate dehydrogenase complex from bovine adrenals does not completely lose thiamine pyrophosphate (TPP) in the course of purification. The preparations contain firmly bound residual TPP in the amount providing for one third of the maximal activity of the complex. A lag period (tau) is observed during catalysis by the pyruvate dehydrogenase complex with the residual TPP in the absence of exogenous coenzyme. The duration of the lag period depends on concentration of the complex: tau is decreased as the concentration increases. The decrease of the lag period occurs after preincubation of the complex with pyruvate, but it is the most remarkable after preincubation with pyruvate and CoA. The tetrahydrothiamine pyrophosphate at low concentrations completely inhibits the activity. However, the residual TPP is retained as is evidenced by the recovery of the complex activity after gel-filtration through Sephadex G-25. The development of the lag period may be interpreted in terms of partial limited dissociation of the residual TPP in the region of the catalytic center.