Purification and characterization of a sperm-binding glycoprotein from human endometrium

A sialic-acid-binding protein (SABP) was purified to apparent homogeneity from human endometrial scrapings taken at various stages of the menstrual cycle from normal cycling females. The 54 kDa monomer was found to be an O-linked glycoprotein with a total carbohydrate content of 34%. This protein agglutinated washed 2% v/v rabbit red blood cells (RBC) in the presence of calcium. Amongst sialic acids and sialoglycoproteins tested for haemagglutination inhibitory activities, N-glycolyl neuraminic acids and human alpha 1-acid glycoprotein were found to be the most potent, the agglutination activity being totally abolished on desialylation of the RBC in the presence of neuraminidase. Western blot studies showed it to be present in the uterine fluid but absent in normal female serum and in full-term placenta. It was also absent in endometrial homogenates of some cases of unexplained primary infertility. Specific binding studies and Scatchard analysis revealed that 125I-labelled human SABP ligand can bind to human spermatozoa with a Ka = 2.6 x 10(9) M-1, their receptors probably being glycoconjugates having a terminal sialic acid moiety, since the sperm-protein interaction could also be abolished when spermatozoa were desialylated with neuraminidase. The binding occurred specifically on the sperm head plasma membrane and decreased markedly when spermatozoa were previously capacitated in vitro using human serum albumin, implicating the possible loss of a sialoglycoprotein receptor to which the ligand binds during capacitation. The biological importance of this sperm-binding secretory glycoprotein and its functional significance in human reproduction have been discussed.