Functional properties of the asialo-fifth component of human complement

Removal of exposed, terminal sialic acid (SA) from carbohydrate chains N-glycosidically linked to asparagine residues of highly pure human C5 with bacterial sialidase increased C-mediated hemolysis of antibody-sensitized sheep E maximally 2.77-fold. Sialidase-treated C5 used as a reagent for the titration of C6, C7, C8, and C9 resulted in increased titers of all these components compared to buffer-treated C5. As determined by a fluorometric method, ca. 65% of the SA was enzymically hydrolyzed under optimal conditions. Endoglycosidase F incubated with C5 followed by monosaccharide analyses by anion exchange chromatography with pulsed amperometric detection revealed both high mannose and complex (terminate in SA) oligosaccharides were hydrolyzed; no effect was found on the functional activity of C5. Approximately 4% of the complex oligosaccharides were hydrolyzed from C5. Comparison of sialidase- and buffer-treated C5 decay rates from EAC1gp(4b,oxy2a,3b)hu resulted in two linear components of the decay curve with sialidase-treated C5, but one linear component with buffer-treated C5. Of the sialidase-treated 125I-C5 15% was bound to EAC1gp(4b,oxy2a,3b)hu compared to 9.3% of buffer-treated 125I-C5. Furthermore, 27% of sialidase-treated 125I-C5 was bound to EAC1gp,4bhu compared to 16.6% of buffer-treated 125I-C5, but no lysis occurred after the addition of C6-C9. The mechanism of increased hemolytic activity after removal of SA from C5 is: the Tmax is prolonged at 30 degrees C (ca. 15 min vs 9 min), and a higher percentage of C5 binds to cellular intermediates compared to buffer-treated C5.