15] Radioiodination of cell surface lipids and proteins for use in immunological studies

Abstract

This chapter describes the use of radioiodinated diazotized iodosulfanilic acid (ISA) and a lactoperoxidase-catalyzed iodination method for the labeling and examination of plasma membrane proteins and lipids in cells under humoral immune attack. There are currently available several reagents, including ISA and a NaI-lactoperoxidase (LPO) complex, that have been used to label the cell membranes radioisotopically, presumably by labeling exposed tyrosine and/or histidine residues on the plasma membrane proteins. The lactoperoxidase-catalyzed NaI reaction has been shown to label a variety of cell surface lipids, including phospholipids, triacylglyceddes, free fatty acids, and lysophosphatides. The labeling reaction is enzyme-dependent in its initial steps, in which an oxidizing agent (H2O2) is generated, with the subsequent oxidation of I– (from NaI) to I2. The final interaction of I2, with lipid molecules, does not involve a simple addition reaction of I to the unsaturated fatty acid constituents of lipids, because fully saturated fatty acids or saturated fatty acid-containing neutral lipids and phospholipids can also be labeled by this reaction. This suggests that I2 substitution or exchange reactions can also occur in this system.