Competition between glycoprotein hormones and horseradish peroxidase for mannose-specific binding sites in cells of endocrine organs.

Mannose-specific binding sites for horseradish peroxidase (HRP) were studied in paraformaldehyde-fixed, frozen sections of endocrine organs by a cytochemical method reported previously. In the testis, HRP was bound to interstitial cells, probably macrophages, and to sites extending along the surface of spermatozoa in the seminiferous tubules. In the epididymis, cells in the connective tissue, probably fibroblasts or macrophages, showed the specific reaction. In the ovaries, the reaction for lectin-bound HRP was observed in connective tissue cells of the theca externa, and in the mucosa of the uterus, binding of HRP occurred to many fibroblasts. The glycoprotein was also bound to cells in the connective tissue of the thyroid, probably mast cells, as well as to endothelial cells in the adrenal medulla and cortex. In all cases, the binding reaction required Ca2+ and was suppressed by mannose or mannan. Partially purified and highly purified preparations of glycoprotein hormones [ovine follicle-stimulating hormone, ovine luteinizing hormone, bovine thyroid-stimulating hormone, and human chorionic gonadotropin] as well as bovine thyroglobulin and yeast invertase competed with the binding of HRP to all the cells mentioned thus showing that the hormones were bound to the same sites as HRP. When 1 microM HRP was present in the incubation medium, the addition of 15-25 microM of highly purified hormones almost suppressed the reaction for lectin-bound HRP and competitive effects could be observed at even lower concentrations of the hormones.