Centrifugal and chromatographic analyses of tryptophan and tyrosine uptake by red blood cells and GLUT1 proteoliposomes with permeability estimates and observations on dihydrocytochalasin B

• We analyzed transport into liposomes and proteoliposomes, separated the free and internalized radioactively labeled substrates by size-exclusion chromatography (SEC) and observed a net influx owing to nonfacilitated diffusion across the lipid bilayers during the separation. The permeabilities (10(-9) cm/s) of glucose transporter (GLUT1) proteoliposomes were estimated to be 4.6, 1.0, 1.4 and 2.1 for D-glucose, L-glucose, L-Tyr and L-Trp, respectively; 15, 3.3, 5.1 and 2.1 times higher than the corresponding permeabilities of liposomes. These values indicated that GLUT1 did not transport Tyr or Trp, or transported Tyr, and only Tyr, slowly. This interpretation was supported by further analyses. Dihydrocytochalasin B inhibited the transport of Tyr and, partially, Trp into human red blood cells (centrifugal analyses). It did not inhibit Tyr and Trp influx into GLUT1 proteoliposomes, but partitioned strongly into the bilayers and seemed to make them fragile. The GLUT1 inhibitor cytochalasin B and the GLUT1 substrate 2-deoxy-D-glucose did not inhibit Tyr transport into the cells. Upon immobilized biomembrane affinity chromatography, Trp decreased the cytochalasin B retardation by GLUT1 only at levels far above the physiological Trp concentration. Ethanol (commonly added to aqueous solutions for enhancing a compound's solubility) halved the retardation at 4% (v/v) concentration. Drastic modification of the SEC method is required to allow permeability measurements with nonlabeled and highly permeable substrates.

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