The binding of ricin B-chain to Sepharose, a galactose-based adsorbent, was reversibly inactivated by acetylation of tyrosine residues in the absence of lactose. In the presence of lactose, two tyrosine residues were protected against modification and the B-chain retained its binding ability. Analyses of tryptic peptides from B-chain modified with N-[14C]acetylimidazole in the presence and absence of lactose showed that Tyr-248 is present in one of the galactose-binding sites.
