Discrimination between Alkali Metal Cations by Yeast

Abstract

Extracellular pH markedly influences the ability of yeast cells to discriminate between K+ and Na+, with K+ favored to a greater degree at low pH. Studies of the kinetics of uptake of individual alkali metal cations by fermenting yeast indicate three zones relative to pH. Between pH 6 and 8, H + has no effect. Below pH 4, H + competitively inhibits the transport of each cation. Between pH 4 and 6, H+ acts kinetically as a predominantly non-competitive inhibitor. Both effects can be reversed by increasing the concentrations of cations. However, the concentrations required to reverse the competitive effect are considerably lower than those required to reverse the apparently non-competitive effect. It is suggested that H+ and the alkali metal cations can combine with two sites, a transport or carrier site, and a second, non-transporting site that influences the maximal rate of transport. Because the non-competitive inhibitory effect of H + is considerably greater on the other cations than on K + , the discrimination in favor of K+ is increased severalfold at low pH, beyond that predicted on the basis of the relative affinities for the transport site.The yeast cell possesses a transport system by means of which monovalent cations can be absorbed in large amounts. This transport system requires the presence of an utilizable substrate and, in well starved cells, the cation absorbed is balanced by the excretion of an equivalent amount of hydrogen ion whose source is the metabolism of the cell (3, 9). Kinetically, cation uptake appears to follow a saturation curve that can be adequately described by the Michaelis-Menten equation (5,11). If pairs of ions of the alkali metal series are present, each interferes in a competitive manner with the uptake of the other (5).The uptake of the alkali metal cations is markedly influenced by pH. As the pH is reduced the net rate of potassium uptake not only decreases but also may even become negative at pH levels below 2. Similar effects of pH on the net uptake of other cations have been reported (4). The mechanism by which

Interactions of fungi with toxic metals

“…Most information is available for K"^ since this ion is preferentially taken up by the monovalent cation transport system (Conway & Duggan, 1958;Borst-Pauwels, 1981). Early work described this system as 'the physiological Kĉ arrier' although other monovalent ions including Rb"^, Na^, Cs^, Li^ and NH4^ may be transported with lower affinities (Jennings, Hooper & Rothstein, 1958;Armstrong, 1964). Ca^+ and Mg'+ can also be taken up via the K^ transport system though the affinity for these cations is very low (Borst-Pauwels, 1981).…”

“…As the pH is progressively decreased below pH 7-0, the selectivity for K"^ and Rb^ over all other ions is increased. In a 1 :1 mixture of K"^ and Na^ at pH 4-0, the combined effects of competitive inhibition of K"" against Na"^ and the effect of low pH against Na^ mean that Na"^ uptake becomes negligible (Armstrong & Rothstein, 1964). It should also be noted that the kinetics of Kt ransport can alter depending on growth and assay conditions.…”

Effects of Sodium Chloride on Steady-state Growth and Metabolism of Saccharomyces cerevisiae

“…The significance of increased glycerol production is also obscure. Its production could be a result of the activation of certain enzymes on the pathway leading to glycerol by NaCl (Schoeffeniels, 1969); alternatively, it might be a byproduct of increased organic acid production necessary for NAD/NADH, regulation (a fall in pH value would be advantageous as it decreases the velocity of Na+ uptake; Armstrong & Rothstein, 1964). It is, however, difficult to explain why the increased production of glycerol expressed itself primarily as a maintenance effect.…”

Activation of the potassium uptake system during fermentation in Saccharomyces cerevisiae

“…The K+ uptake system of S. cerevisiae also mediates the uptakes of Na+ and Li', although these cations show much higher Kms than K+ and Rb+, especially in K+-starved cells (1,12). Experiments with Na+ and Li+ similar to those carried out with Rb+ showed that their influxes were also stimulated by glucose (e.g., the Vm.s of Li+ influx in the absence and in the presence of glucose were 1 and 9 nmol mg1 min1, respectively).…”