title: Additional forms of human decay-accelerating factor (DAF).
abstract: Decay-accelerating factor (DAF) of human erythrocytes is a glycoprotein with a Mr of 65,000 that is anchored in the membrane via a glycolipid tail. During the purification of DAF, two lower m.w. forms were noted. DAF-A had an Mr of 63,000, and DAF-B had an Mr of 55,000. In a fluid phase assay, both forms accelerated the decay of the classical and the alternative C3 convertases with a specific activity similar to that of DAF. However, the decay-accelerating activity for the cell-bound C3 convertases was abolished, suggesting that neither could insert into E membranes and therefore that the glycolipid tail is altered. Analysis by molecular sieve high-pressure liquid chromatography demonstrated that DAF-A eluted with a Mr of approximately 450,000, similar to native DAF, and was thus in an aggregated form. In contrast, DAF-B eluted as a monomer with a Mr of approximately 60,000. DAF-A, but not DAF-B, bound to a hydrophobic column. To further characterize these two forms, surface-labeled human erythrocytes were incubated with phosphatidyl inositol-specific phospholipase C or papain. The phospholipase inefficiently released a form of DAF that was slightly larger (Mr of 64,000) than DAF-A. Papain efficiently released a 55,000 fragment that had the same Mr as DAF-B. To determine if DAF was cleaved by endogenous enzymes, surface-labeled erythrocytes were incubated with leukocytes. The kinetics of the leukocyte-induced degradation was similar to those observed with papain, and the released fragment aligned on seizing gels with the papain-derived fragment. We hypothesize that endogenous phospholipases and proteases cleave DAF to produce fragments similar to DAF-A and DAF-B, respectively.
mesh_terms: D000440:Butanols; D018958:CD55 Antigens; D002462:Cell Membrane / Q000378:metabolism; D003167:Complement Activation; D050577:Complement C3-C5 Convertases / Q000378:metabolism; D004912:Erythrocytes / Q000502:physiology*; D006801:Humans; D007962:Leukocytes / Q000378:metabolism; D008565:Membrane Proteins / Q000032:analysis* / Q000378:metabolism; D008970:Molecular Weight; D010206:Papain / Q000378:metabolism; D010446:Peptide Fragments / Q000378:metabolism; D013329:Structure-Activity Relationship
chemical_list: D000440:Butanols; D018958:CD55 Antigens; D008565:Membrane Proteins; D010446:Peptide Fragments; D050577:Complement C3-C5 Convertases; D010206:Papain