Studies on the uptake of xanthine by Schizosaccharomyces pombe from the stationary growth phase

Abstract

1. Cells of Schizosaccharomyces pombe from the stationary growth phase show only a low capacity for the uptake of xanthine. In the presence of 2 muM xanthine, the initial uptake rate is found in the range of 6-10- minus 21 Moles times Cells- minus 1 times Min- minus 1 at 30 degrees C. 2. Preincubation in glucose solution results in a pronounced stimulation of the uptake rate. 3. The stimulating effect of the glucose pretreatment is repressed by cycloheximide, ammonium ions and amino acids. 4. The substrate dependency of the uptake rate shows a saturation kinetics. The Km-value has been determined to 6 muM. The temperature optimum is found in the range of 30 degrees C. 5. Xanthine is only slightly accumulated within the cells against the external concentration. Most of the external xanthine label is stored up in form of xanthosine and guanosine as well as incorporated in nucleic acids.