Glycosylation abolishes an in vitro insulin-like action of prolactin.

Ovine prolactin stimulated 14C-CO2 production from labeled glucose in adipose tissue of hypophysectomized rats in vitro, an insulin-like activity. Glycosylation of the hormone by attachment of a carbohydrate unit at asparagine31 abolished this in vitro insulin-like action. However, neither nonglycosylated nor glycosylated prolactin exhibited in vivo insulin-like action, as they did not lower serum glucose or non-esterified fatty acids in fasted hypophysectomized rats. Hindrance of receptor binding by the carbohydrate unit may account for the absence of in vitro insulin-like activity in glycosylated prolactin, but the dichotomy between in vivo and in vitro insulin-like actions for prolactin remains obscure.