Role of sialic acid residues in the in vitro superactivity of human choriogonadotropin (hCG) in rat Leydig cells.

The binding activity (B) of porcine Luteinizing Hormone (pLH) to rat LH receptor as well as its stimulating activity (S) of testosterone secretion by rat Leydig cells in vitro are similar to those of the homologous hormone rat LH (S/B = 1). By contrast, the human Chorionic Gonadotropin (CG) and hLH exhibit stimulating activities relative to rat LH that are considerably higher than their relative binding activities (S/B > 100) indicating that they have an abnormally high transducing efficiency (superactivity) after receptor binding. The heterologous hybrid alpha pLH x beta hCG is as superactive as native hCG and recombined alpha hCG x beta hCG whereas alpha hCG x beta pLH exhibits no superactivity, like native pLH and alpha pLH x beta pLH demonstrating that hCG superactivity is due to its beta-subunit. The removal of sialic acid residues with neuraminidase dramatically diminished hCG stimulating activity without impairing its receptor binding activity but the S/B ratio for asialo-hCG never reached values lower than 1. Similar treatments had no effect on the S/B ratios of non-superactive gonadotropins, pLH and equine CG. Sialic acid residues in the Asn beta 30 carbohydrate chains of hLH and hCG appear to be responsible for their superactivity in the in vitro stimulation of testosterone secretion by rat Leydig cells.