We have synthesized and characterized O-bromoacetylthiamine (BrAcThiamine), a new reagent for inactivating the thiamine transport system in Saccharomyces cerevisiae. A Lineweaver-Burk plot of data from the transport kinetic measurements showed that BrAcThiamine was a competitive inhibitor of thiamine transport in S. cerevisiae with a Ki value of 0.60 microM. Incubating BrAcThiamine with yeast cells at 40 degrees C in 0.05 M potassium phosphate buffer, pH 5.0, caused concentration- and time-dependently a remarkable loss of thiamine transport activity. The inactivating reaction of yeast thiamine transport by BrAcThiamine proceeded most effectively at pH 5.0, coinciding with the optimal pH of the transport activity. Thiamine and thiamine analogs (pyrithiamine and O-acetylthiamine) protected yeast thiamine transport activity against the inactivation by BrAcThiamine. In addition, it was found that a membrane fraction prepared from yeast cells treated with BrAcThiamine had a thiamine-binding activity only 20% of that from control cells without inactivating the binding activity of the soluble fraction. These results suggest that BrAcThiamine inactivates the uptake activity by irreversible binding to the binding site of carrier protein(s) in the thiamine transport system.