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Purified human peripheral blood neutrophils were disrupted by nitrogen cavitation or sonication and fractionated on sucrose density gradients in order to separate the plasma membranes and granule fractions. Quantitatively, the fractions containing the specific granules by marker enzyme/protein enrichment contained the most tritiated N-formyl-methionyl-leucyl-phenylalanine (fmet-leu-3Hphe)-binding activity. Competitive binding experiments using unlabeled formyl peptide analogues indicated that the intracellular binding sites display the same structure-function specificity as formyl peptide receptors on intact polymorphonuclear leukocytes (PMN) or isolated plasma membranes. Analysis of the fractions for membrane, primary, and secondary granule markers, as well as the distribution of 125I-labeled plasma membranes in sucrose density gradients, indicated that the specific fmet-leu-3Hphe binding to granule-containing fractions was not due to contamination by plasma membranes. In addition, membranes isolated from PMN previously stimulated with phorbol myristate acetate (PMA) demonstrated increased binding sites, while isolated membranes exposed to PMA under the same conditions failed to show such increases. The data lend direct support to the concept that there is an intracellular pool of fmet-leu-phe receptors that serves as a source of new surface membrane constituents and receptor material that may allow PMN to maintain functional responsiveness during chemotaxis.
Fletcher et al. (Sat,) studied this question.