Contact area of bovine somatotropin dimer: involvement of tyrosine 142.

The presence of tyrosine residues in the contact area between protomers of bovine somatotropin dimers (Fernandez & Delfino, Biochem. J.209, 107-115, 1983) was investigated taking advantage of the impaired self-associating ability of molecules iodinated at such residues. Reaction of bovine somatotropin dissolved in 8 m urea with the NaI-Chloramine T couple (2.1 × 10−4m) rendered a preparation with 3.1 iodine atoms per molecule which, by stepwise elimination of the denaturant and gel filtration through Sephadex G-100, originated two distinguishable populations: one able (iododerivatives I), the other unable (iododerivatives II) to self-associate. After frontal analysis, iododerivatives II were found to be unable to interact even with native molecules. Identification of the reacting tyrosine residues indicated that iodination of tyrosine 142 was responsible for the loss of the ability to form dimers in iododerivatives II. Iodohormones retained the ability to bind to somatogenic mouse hepatocyte receptors - the relative potency for iododerivatives I and II being 0.60 (0.34–1.03) and 0.71 (0.41–1.22) respectively.