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P. Kumar, U. Kelly, L. Yu, H. Jiang, M.M. Frank, M.A. Hauser, C. Bowes Rickman; The Effects of Heparin and Heparan Sulfate on the Complement Regulatory Activity of Factor H . Invest. Ophthalmol. Vis. Sci. 2006;47(13):4166.
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The purpose of this study is to determine the effects of heparin and heparan sulfate (HS) on the complement regulatory activity of the wild type and common variant forms of the complement regulatory protein, factor H. Factor H is responsible for regulating the alternative pathway of complement by binding C3b, accelerating decay of the alternative pathway C3–convertase, and acting as a cofactor for cleavage of C3b by factor I. The common variant under investigation is a T1277C substitution in exon IX determining a Tyr402His (Y402H) change in a region that binds polyanions such as heparin and HS. These molecules, often located on cell surfaces, are thought to influence the activity of factor H by modulating its binding to C3b. This common variant of factor H is associated with an increased risk of developing age–related macular degeneration.
The effect of heparin and HS on factor H activity was measured using a functional C3c release assay, which quantifies the inactivation of cell–bound radioactive C3b by human factor H and I. Different concentrations of heparin and HS were assayed with purified factor H and normal human plasma to obtain a standard curve. Factor H in plasma from 4 groups of patients, normal Grade 1 AMD homozygous for Y402H or Y402 and Grade 5 exudative AMD Y402H or Y402 homozygotes, was then used in the same assay to determine differential activity of normal factor H versus the common variant, Y402H, in the presence of heparin or HS.
In contrast to heparin, where increasing amounts of heparin decrease factor H activity, increasing concentrations of HS (bovine kidney) increases the activity of factor H. Interestingly, HS isolated from porcine intestinal mucosa does not demonstrate the same ability to increase the activity of factor H. There does not appear to be a significant difference in cofactor activity of factor H between wild type and the common variant in the presence of heparin.
The common variant Y402H does not appear to impact interactions with heparin, and in our assay did not demonstrate a difference in the cofactor activity of factor H. The increase in factor H activity in the presence of heparan sulfate is noteworthy, and may indicate the importance of this molecule in enhancing factor H activity in proteoglycan and heparan sulfate–rich regions like Bruch’s membrane in the retina.
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