The yeast Succharomyces cererisiae takes up adenine, guanine, hypoxanthine, and cytosine via a common energy-dependent transport system. The apparent affinity of the transport system to these and other purines and pyrimidines is correlated with their capability to be protonated to the positively charged form. Further organic molecules are competitive inhibitors when they are cationic, e.g. guanidine and octylguanidine in contrast to urea. or hexadecyltrimethylammonium in contrast to dodecylsulfate and Triton X-1 00.The influence of the pH on the kinetic constants of hypoxanthine transport points to a stoichiometry of one proton being associated to the transport system together with one substrate molecule.The pK, values of two ionizable groups that are involved in substrate binding are revealed; one of which (pKa= 1.8) may be attributed to the substrate, the other (pKa= 5.1) to an amino acid residue in the recognition site of the transport system. Studies with group-specific inhibitors indicate that this amino acid residue contains a carboxyl group.The results are in accordance with the assumption that a carboxyl group of the transport system, a proton and a substrate molecule arrange to an uncharged ternary complex.The yeast Succharomyces cerevisiue takes up with high affinity the purine bases adenine, guanine, hypoxanthine, and the pyrimidine base cytosine via a common transport system [l -41. The uptake activity of the cells depends strongly on the culture conditions and has its maximum in the exponential phase. Cells of the early stationary phase can be stimulated for optimum uptake by pretreatment with a suitable energy source, e.g. a buffered glucose solution [3,5].The system is able to accumulate at least three of the substrate bases against a steep concentration gradient In a recent publication [6] we proposed a model of energy coupling with the purine transport system working as a proton symporter and a cation antiporter. According to this model the transmcmbrane gradient of protons (outside concentration high) and of predominantly potassium ions (inside concentration high) may be used for the accumulation of purine bases and cytosine in metabolically active yeast.The following experimental results support the assumption of such a mechanism. (a) A decrease in and Na' act ascompetitive inhibitors, the inhibitory effect of which increases with the pH. (e) In the yeast Candida utilis, purine transport appears to be coupled with the K + pump also The H + symport/K+ antiport model (Fig. l), which has also been supposed for the absorption of certain amino acids by Succharomyces carlsbergensis [9], corresponds formally to the classic ping-pong 181.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.