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Evidence for a multisite carrier at high pH

Carolyn W. Slayman ¹ and Clifford L. Slayman ¹

¹ From the Departments of Microbiology and Physiology, Yale University School of Medicine, New Haven, Connecticut 06510

At low extracellular pH (4–6), net uptake of potassium by Neurospora is a simple exponential process which obeys Michaelis kinetics as a function of [K]_o. At high pH, however, potassium uptake becomes considerably more complex, and can be resolved into two distinct exponential components. The fast component (time constant = 1.2 min) is matched quantitatively by a rapid loss of sodium; it is attributed to ion exchange within the cell wall, since it is comparatively insensitive to low temperature and metabolic inhibitors. By contrast, the slower component (time constant = 10.9 min) is inhibited markedly at 0°C and by CN and deoxycorticosterone, and is thought to represent carrier-mediated transport of potassium across the cell membrane. This transport process exhibits sigmoid kinetics as a function of [K]_o; the data can be fitted satisfactorily by two different two-site models (one involving a carrier site and a modifier site, the other an allosteric model). Either of these models could also accommodate the simple Michaelis kinetics at low pH.

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