Relationship of large hepatitis B surface antigen polypeptide to human serum albumin.

A significant proportion (20 to 40%) of highly purified 22-nm hepatitis B surface antigen (HBsAg) particles contain human serum albumin (HSA) as demonstrated by specific precipitation of radioiodinated particles by anti-HSA. Preparations of the isolated major HBsAg polypeptides (P-1, P-2, and P-6) were iodinated and analyzed by radiommunoprecipitation for reactivity with rabbit antisera to human plasma proteins. Only the P-6 fraction (molecular weight, 68,000) was precipitated and only by anti-HSA; specific precipitation was observed with guinea pig antisera to P-6 and native HBsAg and goat or rabbit antisera to HSA. Coprecipitation of P-6 with antiserum to HBsAg and with anti-HSA, compared to precipitation with each antiserum alone, indicated that the HBsAg and HSA determinants were on separate molecules. The P-6 polypeptide may represent a precursor protein of the hepatitis B virion.