Photochemical labeling of human erythrocyte membrane proteins with radioiodinated 4-azidosalicylic acid derivatives of G(M3), G(D3), G(M1), and FucG(M1) gangliosides

Photoreactive gangliosides of high specific radioactivity may prove useful for studies on glycosphingolipid functions. We prepared 4-azidosalicylic acid (ASA) acylated derivatives of GM3, GD3, GM1, and FucGM1 gangliosides (gangliosides-ASA). Gangliosides-ASA were characterized by their TLC mobility, UV spectra, carbohydrate composition, and digestion with leech endoceramidase. After radioiodination to about 200 Ci/mmole gangliosides-ASA were used for photochemical labeling of human erythrocytes. Radioiodinated gangliosides-ASA were incorporated into erythrocytes in a time and concentration dependent manner, the kinetics and extent of incorporation being similar for all the gangliosides-ASA used. Radioiodinated gangliosides-ASA incorporated into erythrocytes were resistant to trypsin digestion while treatment with 1% BSA removed about 90% of the label. Incubation with cholera toxin protected radioiodinated GM1-ASA and, to a lesser extent, FucGM1-ASA but not GM3-ASA and GD3-ASA, against removal with BSA. After photolysis about 40-50% of radioactivity was firmly bound to erythrocyte lipids and proteins. The ratio of lipid- to protein-bound radioactivity ranged from 2.2:1 to 3.2:1. Photolabeled proteins were analyzed by SDS/PAGE followed by autoradiography. Band 3 was the most extensively photolabeled protein with all the radioiodinated gangliosides-ASA used. DIDS, an inhibitor of band 3 protein activity, caused reduction in photolabeling of this protein by about 20%.