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Immunochemistry of the hepatitis B virus: 125I HB Ag ligand.

Immunochemical studies of the Hepatitis B virus and its gene products require labeled virus or derivative gene products of high antigenic and structural integrity. Such ligands can be provided by 125I Hepatitis B antigen (HB Ag). Techniques for the iodination of microgram quantities of 22 nm HB Ag particles with either immobilized lactoperoxidase or chloramine-T are described. Specific activities of 2 to 10.0 µCi/µg can be achieved. The labeled products of both methods compare favorably with unlabeled antigen in terms of sedimentation velocity and density; however, the solid-phase lactoperoxidase method produces an 125I HB Ag ligand of superior antigenic integrity. 125I HB Ag prepared by lactoperoxidase has an in vivo clearance rate (T½) of 23 hr as compared to 19 hr for chloramine-T-labeled 125I HB Ag. Competitive inhibition radioimmunoassays, with 125I HB Ag, are influenced by the antigenic integrity of the ligand and only with the lactoperoxidase product can concentrations as low as 10 ng/ml (3 × 10-12 M) of purified unlabeled 22 nm HB Ag particles be detected. Deterioration of trichloroacetic acid precipitability and immunoprecipitability of 125I HB Ag is observed over a period of weeks, which imposes a requirement for fresh 125I HB Ag if maximum antigenic integrity of the ligand, sensitivity, and immunochemical precision are to be achieved.