Peptidic hormone interactions at the molecular level‐preparation of highly labelled 3H oxytocin

The tritium labelling of oxytocin has been attained by a two step procedure. First, di‐iodo oxytocin is prepared. After a survey of various iodination reagents, ICI has been selected as it reacts much more rapidly with the tyrosyl ring than with the disulfide bridge, which under mild conditions remains unaffected. The di‐iodo derivative, nevertheless, has lost most or all of its biological activity. Catalytic substitution of the peptide bound iodine by tritium results in labelling the hormone and in restoration of the biological activity. The 3H oxytocin obtained retains 90 % of its hydrosmotic activity and about 65 % of its avian depressor activity. Its specific radioactivity reaches 36 Ci/mMole, a value allowing further studies of the hormone at a molecular level.