Inhibition of thiamine transport in baker's yeast by methylene blue

Methylene blue was found to inhibit thiamine transport competitively (Ki = 0.63 microM) in baker's yeast. The dye was also effective in abolishing the growth inhibition of Saccharomyces cerevisiae by pyrithiamine which is known to be taken up by a common transport system for thiamine in yeast cells. A possible mechanism for the inhibition by methylene blue of the thiamine transport system in baker's yeast is discussed.