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  • Title: Stoichiometry of H+/amino acid cotransport in Neurospora crassa revealed by current-voltage analysis.
    Author: Sanders D, Slayman CL, Pall ML.
    Journal: Biochim Biophys Acta; 1983 Oct 26; 735(1):67-76. PubMed ID: 6226314.
    Abstract:
    Coupling of ions to the uptake of neutral and basic amino acids via a general amino acid transport system (System II), was studied in a mutant of Neurospora crassa (bat mtr) which lacks other transport systems for these solutes. All amino acids tested--including ones bearing no net charge--elicited rapid membrane depolarization, as expected for ion-coupled transport. (Since amino acid transport in Neurospora is not dependent on extracellular Na+ or K+, the associated ion is presumed to be H+.) Although the 14C-labeled amino acid fluxes through System II are largely independent of the identity of the amino acid, the depolarization caused by basic amino acids (L-lysine and L-ornithine) is 60-70% greater than that for neutral amino acids (e.g. L-leucine). This difference is consistent with a constant H+/amino acid stoichiometry of 2, the extra charge for lysine and ornithine being that on the amino acid itself, so that the charge ratio basic:neutral amino acids is 3:2. When actual membrane charge flow associated with amino acid uptake was compared with measured 14C-labeled amino acid influx, ratios of 2.07 charges/mol L-leucine and 3.40 charges/mol L-lysine were obtained, again in accord with a constant translocation stoichiometry of 2H+/amino acid. The advantages of this electrical method for estimating H+/solute stoichiometry in cotransport are discussed in relation to more familiar methods.
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