The beta-subunit of human chorionic gonadotrophin exists as a homodimer

The free beta-subunit of human chorionic gonadotrophin (hCG beta) is well r ecognised as a product of many epithelial tumours. Recently, it has been sh own that this ectopic production may have a functional relationship to tumo ur growth. The growth-promoting activity of hCG beta may be explained bq it s structural similarity to a family of growth factors which all contain the same distinct topological fold known as the cystine-knot motif. Since the other members of this family all exhibit their activities as homo- and hete rodimers, it is possible that the same may be true for hCG beta. Using size-exclusion chromatography, low stringency SDS-PAGE and matrix ass isted laser desorption/ionisation (MALDI) time-of-flight (TOF) mass spectro metry (MS) we have shown that pure preparations of hCG beta contain hCG bet a beta homodimers. Size-exclusion chromatography revealed asymmetric elutio n profiles with a forward peak corresponding to the size-exclusion characte ristic of a globular protein with an approximate mass of 44-54 kDa and a la te shoulder centered around an elution position expected for a globular pro tein of approximately 29 kDa. Two immunoreactive hCG beta. species, of appr oximately 32 and 64 kDa, were clearly resolved by SDS-PAGE and Western blot ting. When analysed by MALDI-TOF MS a similar to 23 kDa monomer and a simil ar to 46 kDa dimer were identified. Formation of hCG beta beta homodimers is consistent with the behaviour of o ther cystine-knot growth factors and strengthens the inclusion of the glyco protein hormones within this superfamily. It has yet to be determined wheth er it is this dimeric molecular species that is responsible for growth-prom oting activity of hCG beta preparations in tumours.