CHARACTERIZATION OF A GLYCEROL H-SORBITOPHILA( SYMPORT IN THE HALOTOLERANT YEAST PICHIA)

Pichia sorbitophila is a halotolerant yeast capable of surviving to ex tracellular NaCl concentrations up to 4 M in mineral medium when gluco se or glycerol are the only carbon and energy sources. Evidence is pre sented here that glycerol, the main compatible solute this yeast accum ulates so as to maintain osmotic balance, is actively co-transported w ith protons. This transport system was shown to be constitutive, not n eeding induction by either glycerol or salt, and was not repressible b y glucose. In glucose- or glycerol-grown cells, a simple diffusion was detectable, and iterative calculations were performed to calculate ki netic parameters, in the presence and in the absence of NaCl. At 25 de grees C, pH 5.0, in glucose-grown cells these were: K-m = 0.81 +/- 0.1 1 mM and V-max = 634.2 +/- 164.8 mu mol h(-1) per g (glycerol); K-m 1. 28 +/- 0.6 mM and V-max = 558.6 +/- 100.6 mu mol h(-1) per g (protons) . Correspondent stoichiometry was approximately 1, either for these co nditions or in the presence of 1 M-NaCl. An increase in acumulation ca pacity was evident when different concentrations of NaCl were present. This capacity was shown to be dependent on Delta pH and membrane pote ntial, consistently with an electrogenic character. We suggest that th e main role of this system is in osmoregulation, by keeping glycerol a ccumulated inside the cells, compensating for leakage, due to its lipo soluble character.