The distribution of surface proteins during phagocytosis by rabbit peritoneal polymorphonuclear leukocytes was studied to determine whether the proteins of the phagocytic vesicles of these differentiated cells were representative of the entire set of plasma membrane proteins. Phagocytosis of bovine serum albumin-diisodecylphthalate emulsion by lactoperoxidase-iodinated rabbit neutrophils was linear over 15-20 min at a rate of 96 microgram oil/min/mg cell protein. This rate was similar to that of unlabeled cells. Incorporation of cell-associated free iodine by endogenous myeloperoxidase during phagocytosis was inhibited by 1 mM cyanide, which had no effect on the rate of particle uptake. The surface of intact neutrophils contained at least 13 iodinated proteins distinguishable by polyacrylamide gel electrophoresis followed by autoradiography. Isolated phagosomes were deficient in six of these proteins. The plasma membrane fraction of these cells was missing five of these same proteins which, however, were enriched in a dense surface fraction (Willinger, M., and F. R. Frankel. J. Cell Biol. 82: 32-44). When experimental conditions were reversed, and the PMNs were labeled after phagocytosis, these five proteins remained on the cell surface, while at least three of the major proteins found on resting cells were depleted. Incubating the cells with colchicine, which has been shown to affect the distribution of some plasma membrane constituents during phagocytosis, had no effect on the distribution of surface proteins in our system. These results indicate that a nonrandom interiorization of lactoperoxidase-labeled surface proteins of polymorphonuclear leukocytes occurs during phagocytosis.

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