Analogue Inhibition of the Active HCO−3Transport Site in the Characean Plasma Membrane

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Abstract
Competitive inhibition of the HCO−3 transport site, at the plasmalemma of Chara coraUina, by the CO2−3 ion is demonstrated. This CO2−3 inhibition was used to demonstrate that HCO−3 ions enter the cell by facilitated ‘diffusion’ when the HCO−3 transport system has been inactivated by treatment with 10 mM K+. Use of CO2−3 as a HCO−3 analogue is limited, however, because of the necessity to employ solutions of high pH. Inhibition was not observed in the presence of a range of organic and inorganic acid anions. These results demonstrate the stereo-specific nature of the HCO−3 binding site. A variety of amino compounds were found to inhibit H14CO−3 influx. Inhibition appeared to be competitive, being completely relieved at higher substrate (HCO−3) concentrations. A simple correlation was not found between the degree of inhibition and the concentration of neutral base. A combination of the presence of neutral base and experimental pH values of at least 8·0 was required to produce the reactive species that inhibited HCO−3 transport. This species is considered to be the amino carbamate. These results are discussed with respect to further HCO−3 analogue experiments.

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