Glycosylation is the structural basis for changes in polymorphism and immunoreactivity of pituitary glycoprotein hormones

Abstract

Glycoprotein hormones have long been known to display extensive polymorphism and changes in bioactivity according to the endocrine status of the patient. Structural analysis has shown that pituitary gonadotropins (lutropin and follitropin) and thyrotropin are synthesized and secreted as a panel of isoforms which differ in glycosylation, bioactivity and circulatory half-life. Ultrasensitive immunoassays could reveal that glycosylation of plasma hormones is structurally different from the pituitary stock so that the ratio of circulating glycoforms may vary according to the physiopathology of the pituitary axis. However, contradictory results between immunoassays have been often reported, suggesting that some plasma forms can escape recognition by monoclonal antibodies which have been raised to the pituitary or urinary antigen. When hormone levels do not correlate with clinical features, one can also suspect that inactive or hyperactive forms are being measured. At the molecular level, very limited information has been gained toward the expression of hormone epitopes as a function of carbohydrate structure. To address this issue, we have compared the recognition of pituitary and recombinant human thyrotropin by various polyclonal and monoclonal antibodies before and after neuraminidase treatment. Both, pituitary and recombinant thyrotropin bound to anti-alpha and anti-beta antibodies, demonstrating thereby that recombinant thyrotropin can be used to calibrate immunoassays. While removal of sialic acid did not alter the recognition of the recombinant hormone in various immunoassays, this treatment specifically abolished the binding of pituitary thyrotropin to anti-beta monoclonal antibodies. These findings show that immunoreactivity of circulating hormone glycoforms, which are often more sialylated than their pituitary counterparts, may very well account for variation depending on the antibodies used in the immunoassays.