Abstract
Phosphate transport across the membrane of human erythrocytes, at pH 6.42, is a saturable process with an external Km of 80 mM and a Vmax of 2.8 mmol per liter of red cells per min. Transport is inhibited in a reversible manner by the sulfanilate anion and in an irreversible manner by the isothiocyanate derivative of the sulfanilate anion. This reagent reacts with only one membrane protein. When 300,000 molecules of reagent per cell are covalently attached to protein, transport is completely inhibited. This transport protein is a glycoprotein, it belongs to the general class designated as component a by Bretscher, and it therefore spans the bilayer. Transport does not involve movement of the protein around an axis parallel to the plane of the membrane.
Highlights
Xssuming that the transport system, which has a bindin g step which precedes the transport step, has a turnover number that is as fast as the one for catalase, there would have to be 10: transport components per cell and this number would represent approximately 10% of the membrane protein
Materials &Sodium dodecyl sulfate was purchased from Sigma; [%]sulfanilic acid
Phosphate Transport-Phosphate entry into red cells was measured by dct.ermining the amount of radioactivity which remained in the extracellular fluid as a function of time
Summary
Phosphate transport across the membrane of human erythrocytes, at pH 6.42, is a saturable process with an external K, of 80 mM and a V,,, of 2.8 mmol per liter of red cells per min. When 300,000 molecules of reagent per cell are covalently attached to protein, transport is completely inhibited This transport protein is a glycoprotein, it belongs to the general class designated as component a by Bretscher, and it spans the bilayer. Several groups [12] have shown that a component with an apparent molecular weight around 100,000 and which represents 25y0 of the membrane protein does span the bilayer. It stems likely, that this protein might be the one involved in the process of anion transport. The results presented here indicate that the polypeptide with an apparent mass of 100,000 daltons is a glycoprotein and is responsible for anion transport
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