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Hepatitis B surface antigen (HBs Ag): tryptophan content and biological activity.

Summary
The tryptophan content of hepatitis B surface antigen (HBsAg) was studied by spectrophotometric titration with N-bromosuccinimide (NBS) at pH 4.0 in 8.0 m-urea. A tryptophan value of 13.9 % or 22.4 residues/mol (mol. wt. = 30000) was obtained by this method. This value coupled with 2.3 mol per cent tyrosine content obtained by amino acid composition studies accounts for the unusually high extinction coefficient of HBsAg (at 280 nm E
1 %
1 cm = 37.26). The modification of tryptophan residues with NBS did not cause cleavage of the protein or loss of biologic activity as determined by radioimmunoassay.