322 Biochimica et Biophysica Acta, 552 (1979) 322--330 @)Elsevier/North-Holland Biomedical Press BBA 78326 CATION EXCHANGES OF YEAST IN THE ABSENCE OF MAGNESIUM A. RODRIGUEZ-NAVARRO and E.D. SANCHO Cdtedra de Microbiologfa, Escuela Tdcnica Superior de Ingenieros AgrOnomos, COrdoba (Spain) (Received August 1st, 1978) Key words: Cation exchange; Transport control; Antiport mechanism; Mg2*; (Yeast) Summary Environmental Mg2+ was found to influence the K+/Na÷ exchange rate of metabolizing yeast. Addition of EDTA increased the exchange rate and Mg2÷ reversed the effect of EDTA. Yeast starved in the absence of Mg2÷ exchanged cellular K + or Na* for external H ÷ when maintained at acidic pH. The exchange rate depended on cellular pH and showed the same kinetics for both K* and Na ÷. At acidic pH, the presence of external cations neither inhibited H ÷ absorption nor changed the cation/H* 1 : 1 stoichiometry. At neutral pH, external cations inhibited H ÷ influx but did not change the cation efflux. The K÷/Na÷ exchange is discussed as electrically coupled and the K+/H* and Na ÷/H÷ exchanges as electroneutral antiports. Introduction Yeast cells exchange K + for cellularH+ [1,2], accumulating K ÷ against steep gradients [3]. In media containing both Na + and K* the latteris preferentially accumulated [4], and if the yeast has been previously charged with Na ÷, Na ÷ is exchanged for K ÷, even against chemical potentials [5]. In addition to these heterologous exchanges, the cell maintains a considerable K÷/K * exchange, and net gain or loss of K ÷ can be explained as a result of the influx and efflux processes determined by the described exchanges [6]. The kinetics of these cation exchanges in yeast have been extensively studied [4,7,8], but there is still controversy about the mechanism. Conway et al. [5] and Ryan and Ryan [9] have proposed that two systems mediate the exchanges, while Rothstein [10] considers it unnecessary to postulate more than one cation transport system. In the present paper we show that Mg2+ affects cation exchanges in yeast,