Effects of histidine modification on the biological and immunological activities of equine chorionic gonadotropin

The histidine residues of equine chorionic gonadotropin have been modified and the resultant effects on the biological and immunological activities of the hormone examined. The modification of histidine is carried out by rose bengal-sensitized photooxidation. The kinetics of histidine destruction is biphasic, a rapid loss followed by a slower decrease. An average loss of 15–20% of the total histidines in the molecule resulted in 70–80% disappearance of the lutropin activity with no significant loss in the immunological activity. The loss of lutropin activity paralleled the decline of follitropin activity. Further destruction of histidine of up to 70–90% of the total resulted in over a 99% loss of biological activity with no significant effect on the immunological activity. Modified equine chorionic gonadotropin was unable to compete with the native molecule in the rat Leydig cell assay. These observations suggest that the immunorecognition site in equine chorionic gonadotropin is different from the region responsible for the biological activity and does not involve a histidine residue. Furthermore, it also appears that the histidine residues are important for the manifestation of both lutropin and follitropin activities.