Characterization of Complement C3 as a Glycyrrhizin (GL)-Binding Protein and the Phosphorylation of C3α by CK-2, Which Is Potently Inhibited by GL and Glycyrrhetinic Acid In Vitro

Abstract

The physiological interaction between glycyrrhizin (GL) and serum complement C3, and the inhibitory effects of GL, glycyrrhetinic acid (GA), and a GA derivative (oGA) on the phosphorylation of C3 by casein kinase 2 (CK-2), were investigated in vitro. C3 was found to be a GL-binding protein (gbP), because (i) of its high affinity for a GL-affinity HPLC column; and (ii) both GL and GA induce conformational changes in C3. At least four trypsin-resistant fragments (p30, p25, p18, and p15) were detected when the ³²P-labeled C3α was digested with trypsin in the presence of 100 μM GA. Two of these (p25 and p15) were immuno-precipitated with anti-C3a serum. Furthermore, it was found that C3a contains GL-binding domains, because (i) C3a (anaphylatoxin) could be selectively purified from the synovial fluids of patients with rheumatoid arthritis by GL-affinity column chromatography (HPLC); and (ii) purified human C3a has a high affinity for a GL-affinity column. In addition, C3α (p115) of C3 was effectively phosphorylated by CK-2 in the presence of poly-Arg (a CK-2 activator) in vitro. This phosphorylation was completely inhibited by 10 μM oGA, 30 μM GA, or 100 μM GL. Taken together, these results suggest that the GL-induced inhibition of the physiological activities of C3a and C3a may be involved in the anti-inflammatory effect of GL in vivo.