Evidence for a novel thioredoxin-like catalytic property of gonadotropic hormones

It has been proposed that dithiol-disulfide interchange and oxidation-reduction reactions may play a role in hormone-induced receptor activation. Inspection of the sequences of the gonadotropic hormones revealed a homologous tetrapeptide (Cys-Gly-Pro-Cys) between the beta subunit of lutropin (LH) and the active site of thioredoxin (TD). The beta subunit of follitropin (FSH) has a similar sequence (Cys-Gly-Lys-Cys). Thioredoxin is a ubiquitous protein serving as an electron donor for ribonucleotide reductase, but it also exhibits disulfide isomerase activity. The catalytic activity of TD was assayed by its ability to reactivate reduced and denatured ribonuclease. In this assay, the purified ovine FSH and bovine LH preparations tested were approximately 60 and approximately 300 times, respectively, as active as TD on a molar basis. This heretofore unsuspected catalytic property of FSH and LH may be important in understanding their mechanism of receptor activation and signal transduction.