An elusive role for glycosylation in the structure and function of reproductive hormones

The crescendo of events leading first to ovulation and subsequently to birth is orchestrated by a broad repertoire of hormones. The major hormones of the ovulatory cycle are representatives of four hormone classes: neurotransmitters, releasing factors, trophic hormones acting on target tissues, and steroid-like molecules released by the target tissues. The punctuate and staccato rhythm of the neurotransmitters and releasing hormones relentlessly drive the swelling and protracted wave of activity by the luteotrophic and steroid hormones. Carbohydrates alone are notably absent as hormones and the predominant role for glycosylation appears to be the conferment of increased solubility to endocrine molecules, either during their manufacture or by modulating circulatory half-life. Rarely considered examples of the importance of glycosylation in reproductive hormones include adenosine, important for spermatozoan activity, and the hormone-binding globulins, which ensure the aqueous transport of hydrophobic steroids. The archetype glycoprotein hormones, especially human chorionic gonadotrophin (HCG), are discussed more extensively, as the structural and functional roles of carbohydrate in these hormones have been studied exhaustively. Conversely, the direct involvement of HCG and the importance of its carbohydrate for autonomous growth, in both placental invasion and tumorigenesis, has received little attention in the literature.