Interactions between polymerized human albumin, hepatitis B surface antigen, and complement: II. involvement of Clq in or near the hepatitis B surface antigen receptor for polyalbumin

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Abstract
A species‐specific receptor for polymerized human albumin (PHALB) has been reported on hepatitis B surface antigen (HBsAg)‐carrying particles. Our previous observations that human Clq also binds PHALB in a species‐restricted manner led us to investigate the possibility that HBsAg‐associated Clq is involved in the PHALB receptor on HBsAg particles. The temperature, ionic strength, and pH requirements necessary for binding of PHALB to both Clq and HBsAg were compared and found to be similar. Normal human serum and purified Clq inhibited the PHABL‐HBsAg interaction; the inhibition was markedly reduced in heat‐inactivated and Clq‐depleted serum. Heat‐denatured or reduced and alky‐lated Clq failed to inhibit the PHALB‐HBsAg binding. Moreover, human Clq was found to be present in purified preparations of HBsAg and the quantity detected paralleled the degree of PHALB‐HBsAg binding. While anti‐Clq inhibited the PHALB‐HBsAg interaction, anti‐Clr, ‐Cls, ‐C3, and ‐Ig were not inhibitory. Collagenase treatment of purified HBsAg reduced both PHALB‐binding activity and the degree of HBsAg‐associated Clq. These observations provide evidence that HBsAg‐associated Clq is involved in or near the HBsAg‐binding site for PHALB.