Effect of NaCl on kinetics of D-glucosamine uptake in yeasts differing in halotolerance

The initial rate of D-glucosamine uptake by the non-halotolerant yeast Saccharomyces cerevisiae was approximately halved as the apparent half saturation constant (Km) and the apparent maximum velocity (Vmax) changed from 6.6 mM to 16.4 mM and from 22 mumol.g-1.min-1 to 16 mumol.g-1.min-1, respectively, when the salinity in the medium was increased from zero M to 0.68 M NaCl. Corresponding changes in a high affinity transport system in the halotolerant yeast Debaryomyces hansenii were from 1.1 mM to 4.6 mM and from 3.1 mumol.g-1.min-1 to 4.5 mumol.g-1.min-1, implying a practically unchanged transport capacity. In 2.7 M NaCl, Km and Vmax in this system were 24.5 mM and 1.1 mumol.g-1.min-1, respectively, representing a marked decrease in transport capability. Nevertheless, the degree of affinity in this extreme salinity must still be regarded as noteworthy. In addition to the high affinity transport system in D. hansenii, a low affinity system, presumably without relevance in D-glucosamine transport, was observed.