Fluorocytosine causes uncoupled dissipation of the proton gradient and behaves as an imperfect substrate of the yeast cytosine permease.
At pH 5-6 ATP-depleted washed cell preparations of strain NC233-10b[pII4-9], in which the cytosine permease was overexpressed, absorbed cytosine, hypoxanthine or fluorocytosine stoichiometrically with, respectively, about 1, 1.4 and 5 proton equivalents. The cellular pH fell proportionately. The membrane depolarization caused by each compound was assayed in the presence of glucose with a voltage-sensitive dye and increased in the same order. Fluorocytosine significantly lowered the growth yield that a 'petite' strain of the yeast formed at limiting glucose concentrations. At pH 5.6 with extracellular [K+] below 1 mM, each of the three substrates was accumulated about 200-fold from a dilute solution at the expense of the proton gradient. This concentration ratio corresponds to a solute gradient (delta mu(s)) of 13 kJ mol-1. Raising [K+]o systematically lowered the substrate accumulation ratio and delta muH. The mean ratio delta mu(s)/delta muH was 0.82 for all three substrates. It was concluded that whereas the behaviour of cytosine approximated to that expected for a symport of unit proton stoichiometry, the absorption of protons with fluorocytosine and, to a lesser extent, hypoxanthine, was only partly conserved as useful work. A possible mechanism of this novel phenomenon is outlined.
Hopkins
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P
Department of Biochemistry & Applied Molecular Biology, University of Manchester, U.K.
Shaw
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Acik
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Oliver
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Eddy
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D000255:Adenosine Triphosphate / Q000378:metabolism; D001693:Biological Transport, Active / Q000187:drug effects; D002352:Carrier Proteins / Q000378:metabolism*; D003596:Cytosine / Q000378:metabolism; D005437:Flucytosine / Q000378:metabolism*; D005947:Glucose / Q000494:pharmacology; D019271:Hypoxanthine; D007042:Hypoxanthines / Q000378:metabolism; D007700:Kinetics; D008564:Membrane Potentials; D026901:Membrane Transport Proteins / Q000378:metabolism*; D008954:Models, Biological; D033702:Nucleobase Transport Proteins; D011188:Potassium / Q000494:pharmacology; D017494:Proton Pumps / Q000502:physiology*; D011994:Recombinant Proteins / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins
D002352:Carrier Proteins; C043181:FCY2 protein, S cerevisiae; D007042:Hypoxanthines; D026901:Membrane Transport Proteins; D033702:Nucleobase Transport Proteins; D017494:Proton Pumps; D011994:Recombinant Proteins; D029701:Saccharomyces cerevisiae Proteins; D019271:Hypoxanthine; D003596:Cytosine; D000255:Adenosine Triphosphate; D005437:Flucytosine; D005947:Glucose; D011188:Potassium