Differential effects of alkylation of methionine residues on the activities of pituitary thyrotropin and lutropin

Methionine residues of the alpha and beta subunits of bovine lutropin (LH) and bovine thyrotropin (TSH) have been specifically alkylated with iodoacetic acid. The alpha subunit has been modified so that two of the four methionines are quantitatively alkylated (residues 8 and 33, in agreement with studies by Cheng, K.-W. (1976) Biochem. J. 159, 71-77). Reassociation of the modified alpha subunit with unmodified LH-beta or thyrotropin (TSH)-beta resulted in reconstituted hormones which differed markedly in their respective biological activities. The alpha-modified TSH was fully active in both radioligand receptor and in vivo assays, while the alpha-modified LH, because of lowered affinity for receptor, lost approximately 70% of its activity in its radioligand receptor assay. This observation is the first to show that modification of the alpha subunit leads to a differential loss of activity in one glycoprotein hormone versus another. Circular dichorism studies revealed no changes in conformation; thus, the data strongly support, for LH, a direct interaction of the common subunit with receptor. Methionine 32 in TSH-beta can be modified with retention of full activity under conditions where methionines 8, 9, and 58 are not modified. In contrast, previous work on the modification of lysine 42 in LH-beta which lies in an analogous domain implicates that residue in receptor interaction (e.g. Liu, W.-K., Yang, K.-P., Nakagawa, Y., and Ward, D. N. (1974) J. Biol. Chem. 249, 5544-5550; Sairam, M. R., and Li, C.-H., (1975) ARch. Biochem. Biophys. 167, 534-539). These results further emphasize the probable importance of this domain in hormone specificity.