The choline transport system of erythrocytes. Distribution of the free carrier in the membrane |
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Authors: | R.M. Krupka R. Devés |
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Affiliation: | 1. Research Institute, Canada Agriculture, London, Ontario N6A 5B7 Canada;2. Department of Biochemistry, University of Western Ontario, London, Ontario N6A 5C1 Canada |
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Abstract: | A method is described, based on the kinetics of transport, for determining the equilibrium distribution of the carrier site on the inner and outer surfaces of the cell membrane, and this method is applied to the choline carrier of human erythrocytes. This method depends on measurement of flux ratios for both entry and exit, i.e., the transport rates of a low concentration of labeled substrate into a solution which contains either no substrate or a saturating concentration of unlabeled substrate. The concentrations of inward-facing and outward-facing carrier are found to be nearly equal, and therefore the 5-fold difference in choline affinity on the inner and outer surfaces of the membrane cannot be explained by an unequal carrier distribution. It is also shown that both reorientation and dissociation of the carrier-substrate complex are far more rapid than reorientation of the free carrier. |
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Keywords: | Carrier distribution Asymmetric carrier Choline transport Affinity constant |
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