Bovine luteinizing hormone iodinated at alpha Tyr21, alpha Tyr92, or alpha Tyr93 retains specific binding activity.

It has previously been shown that as the extent of iodination or nitration of LH is increased, receptor-binding activity is lost. To determine whether this loss is attributable to modification of a specific tyrosine, we located iodotyrosines in only those iodinated molecules that retained specific binding activity. Iodinated bovine LH (*bLH) with intact binding activity was separated from *bLH lacking activity by binding to and elution from receptors. Gel exclusion chromatography of tryptic peptides and microsequenator analysis of tryptic glycopeptides showed that iodotyrosine was present at each of the only readily accessible residues in intact hormone: alpha Tyr21, alpha Tyr92, and alpha Tyr93. Loss of activity with increased modification could not be explained by subunit dissociation, hormone aggregation, or degradative release of radioactive residues. These results together with the previous finding that those molecules of *bLH that can bind specifically to receptors do so with an apparent Ka indistinguishable from that of unmodified hormone show that any one of the residues, alpha Tyr21, alpha Tyr92, or alpha Tyr93, can be iodinated without an effect on binding and suggest that none of these residues interacts directly with receptor. They further suggest that it is modification of more than one tyrosine in the same molecule which negatively affects binding. We discuss how modification of two tyrosines might decrease binding activity when modification of any one has no observable effect.