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1. Proton uptake in the presence of various amino acids was studied in washed yeast suspensions containing deoxyglucose and antimycin to inhibit energy metabolism. A series of mutant strains of Saccharomyces cerevisiae with defective amino acid permeases was used. The fast absorption of glycine, l-citrulline and l-methionine through the general amino acid permease was associated with the uptake of about 2 extra equivalents of protons per mol of amino acid absorbed, whereas the slower absorption of l-methionine, l-proline and, possibly, l-arginine through their specific permeases was associated with about 1 proton equivalent. l-Canavanine and l-lysine were also absorbed with 1-2 equivalents of protons. 2. A strain of Saccharomyces carlsbergensis behaved similarly with these amino acids. 3. Preparations of the latter yeast grown with maltose subsequently absorbed it with 2-3 equivalents of protons. The accelerated rate of proton uptake increased up to a maximum value with the maltose concentration (K(m)=1.6mm). The uptake of protons was also faster in the presence of alpha-methylglucoside and sucrose, but not in the presence of glucose, galactose or 2-deoxyglucose. All of these compounds except the last could cause acid formation. The uptake of protons induced by maltose, alpha-methylglucoside and sucrose was not observed when the yeast was grown with glucose, although acid was then formed both from sucrose and glucose. 4. A strain of Saccharomyces fragilis that both fermented and formed acid from lactose absorbed extra protons in the presence of lactose. 5. The observations show that protons were co-substrates in the systems transporting the amino acids and certain of the carbohydrates.

The absorption of protons with specific amino acids and carbohydrates by yeast