A cation carrier in the yeast cell wall

Abstract

At about pH 6-7, the physiological carrier of K can transport large amounts of other inorganic cations when these are present as the only cation species (apart from the minute H+ ion concentration) and in sufficient concentration. The relative affinities for the carrier of K, Ru, Ce, Na, Li and Mg, under the conditions investigated (determined by competition with the K+ ion), were approximately 100:42:7:3.8:0.5:0.5, yet the maximum rates of uptake differed relatively little. Such maximum rates were reached at very different external concentrations; that for K was at about 1.6 m[image] whereas that for Li was about 300 m[image]. The double reciprocal graphs of uptake against external concentration (1/v against 1/S) show that the ions compete for the same carrier, this being illustrated by the competition data for Na and K. The relative affinities of various inorganic cations for the carrier were determined with fermenting yeast and non-fermenting Na-rich yeast. The same order of affinities was found under both sets of conditions, indicating that the same carrier was operative. Measurement of the competitive effects of some organic cations on uptake of K showed that ethylamine had about 9% and ethanolamine 1.4% affinity of K for the carrier. Alanine showed no appreciable affinity. Twenty-four amino acids in 5 m[image]-concentration were investigated for their effect on Na excretion from Na-rich yeast. The most effective were ornithine, arginine, lysine, citruline and histidine, causing increases ranging from 94 to 55%. Alanine and glutamic acid caused no increase. The binding power of the carrier expressed as meq/kg of yeast was determined by a special displacement of K+ ions from it with Ru, and it was found to be 0.13 [plus or minus] 0.009 meq/kg in air, increased by anoxia and hydrogenation to 0.24 [plus or minus] 0.03 meq/kg of centrifuged yeast.