Schizosaccharomyces pombe showed maximum pyridoxine uptake activity around 10 h after starting cultivation. High concentrations of thiamine and pyridoxine in the medium did not affect the activity or the time but changed intracellular levels of vitamin B6 compounds. Pyridoxine was taken up by a saturable mechanism with two kinds of affinity (K(m) 22.4 microM and 118 microM). The uptake depended on the energy produced anaerobically with an optimum pH of 4.5. The uptake was completely inhibited by amiloride, sodium azide or 2,4-dinitrophenol. The uptake system of the fission yeast was different in various respects from that of a budding yeast.

Active transport activities of free B-6 vitamers in 35 strains of 8 genera of yeast were measured by isocratic reverse-phase HPLC. Many but not all strains transported pyridoxamine and/or pyridoxine. The active transport activities in some yeast strains tested were completely inhibited by amiloride (0.5 mM). In contrast to cells so far studied, yeast cells showed a novel character in metabolism of accumulated B-6 vitamers: the phosphorylation of the free B-6 vitamers was regulated at a low level. There was no apparent correlation between the presence of the active transport activity in yeasts and the requirement of vitamin B-6 for their growth.