Modulation of sialic acid-binding proteins of rat uterus in response to changing hormonal milieu

A group of sialic acid binding (SAS) agglutinins has been isolated from the rat uteri at different stages Proestrus (P), estrus (E) and diestrus (D)] of estrous cycle. Studies of biochemical properties indicate that SAS agglutinins are glycoprotein in nature having molecular weights between 28–31 Kd and microheterogenous pI. Function-based characterization revealed that inspite of the fact that all three proteins exhibit sialic acid binding property, the sialic acid binding affinities, calculated from Scatchard analysis, using 4-methylumbelliferyl sialic acid as a ligand, varied in stage specific manner (Ka:D-SAS-9.03×10⁵ M^–1, P-SAS-2.33×10⁵ M^–1, E-SAS-2.13×10⁵ M^–1). Circular dichroism spectra of these three agglutinins suggested that differences exist in the secondary structures of the proteins isolated from different stages. Removal of carbohydrate moiety by trifluoromethane sulfonic acid treatment and CNBr cleavage studies showed some homology between these proteins, however, the variation in the carbohydrate moiety was apparent from the sugar analysis data. Functionally and immunologically these proteins can be grouped as estrogenic and progestogenic SAS agglutinins.