Receptor‐Mediated Gonadotropin Action in Ovary

The role of cell‐surface sialic acid in the interaction of 125 I‐labeled choriogonadotropin with gonadotropin receptors from plasma membranes of bovine corpus luteum was investigated. Pretreatment of plasma membranes with neuraminidases purified from Clostridium perfringens or Vibrio cholera enhanced the binding activity of 125 I‐choriogonadotropin in a concentration‐dependent manner. C. perfringens neuraminidase (100 mU/ml) and V. cholerae neuraminidase (1.0 I.U./ml) maximally stimulated gonadotropin‐binding activity. The neuraminidase stimulation of binding was due to unmasking of gonadotropin‐binding sites and not due to a change in affinity of the receptor for 125 I‐choriogonadotropin. The activity of plasma‐ membrane 5′‐nucleotidase, a glycoprotein enzyme, however, was not affected by neuraminidase treatment. The stimulatory effect of C. perfringens enzyme was abolished by co‐incubation of plasma membrane with glycoproteins, fetuin and mucin. Furthermore, the stimulatory effect of neuraminidase was due to an intrinsic property of the enzyme and was accompanied by a parallel dose‐dependent loss of plasma‐membrane‐associated sialic acid residues. Other neuraminidase preparations including that purified from Arthrobacter ureafaciens also enhanced 125 I‐choriogonadotropin binding with concomitant hydrolysis of plasma‐membrane sialic acid. By contrast, enzyme purified from influenza virus did not release plasma‐membrane sialic acid and consequently failed to modulate receptor activity. Treatment of plasma membrane with neuraminidase and subsequent fractionation demonstrated that the enzyme hydrolyzed 50–60% sialic acid from glycoprotein and ganglioside components. Preincubation of plasma membrane with wheat‐germ agglutinin partially blocked the hydrolysis of glycoprotein sialic acid as well as stimulation of 125 I‐choriogonadotropin‐binding activity. Incubation with other lectins, concanavalin A and peanut agglutinin, neither affected the hydrolysis of glycoprotein/ganglioside sialic acid by neuraminidase nor prevented the stimulatory action of enzyme on 125 I‐choriogonadotropin‐binding activity. These studies demonstrate a possible involvement of plasma‐membrane‐associated sialic acid (sialoglycoproteins) in the activity of gonadotropin receptors.