Hormonotoxins. Effects of modifying the gonadotropin alpha-subunit on the generation of lutropin-toxin conjugates.

On the basis of the observation that in the hormonotoxin oLH-gelonin conjugation of the toxin occurs via the alpha-subunit of lutropin, an attempt was made to develop a general method for generation of similar hybrid proteins involving other glycoprotein hormones. In this approach sites suitable for conjugation would be first introduced into the alpha-subunit, hybridized with any native hormone specific beta-subunit of choice (LH-beta, FSH-beta, TSH-beta) following which the toxic component gelonin would be added on in the form of gelonin-S-S-alpha---beta complex. Thus, thiolated lutropin alpha-subunit recombined well with free lutropin beta-subunit, yielding a hybrid which was active in terms of receptor binding, immunoreactivity, and steroidogenic properties. However, subsequent conjugation with thiolated gelonin, a ribosome inactivating protein, resulted in dissociation of beta-subunit from its non-covalent union with the thiolated alpha-subunit. It is concluded that the addition of positively charged gelonin at particular sites on the alpha-subunit led to the destabilization of the lutropin quaternary structure.