Remodeling of the Carbohydrate Chains of hCG by Use of Sialyltransferases: Effects on the Biological Activity

Human chorionic gonadotropin (hCG) is a glycoprotein hormone which contains both N- and O-linked carbohydrate chains. It consists of two subunits, a and 8. The a-subunit contains two N-linked carbohydrate chains: a mono-antenna and a non-fucosylated bi-antenna. The $-subunit contains both (two) N and (four) O-linked carbohydrate structures. Both of the N-linked carbohydrate chainsare biantennary, one of which is fucosylated

SIALYLTRANSFERASES; THEIR SPECIFICITY AND THEIR USE IN CARBOHYDRATE REMODELLING.

In order to apply sialyltransferases in the remodelling of the carbohydrate chains on biologically active glycoproteins, it is a prerequisite to know the fine specificity of these enzymes. In this report the specificity of several sialyltransferses involved in the sialylation of O- and N-linked oligosaccharide chains is reviewed. Also novel results on the branch specificity of a3- and a6-sialyltransferase are reported. The potential application of these enzymes in carbohydrate remodelling was studied using human chorionic gonadotropin (hCG) as a model glycoprotein. Differently sialylated preparations of this hormone were obtained and tested for their stimulatory effect on steroidogenesis in Leydig cells in vitro. Asialo-hCcG appeared to be only 45% as effective as native hCG. a3-Resialylation of the O-linked chains on the ß-subunit of this hormone did not restore the biological activity to a higher level. By contrast, 55% a6- resialylation of the N-linked chains yielded a preparation which was almost as active as native hCG. Interestingly, further sialylation by the a6-sialyltransferase resulted in a decrease of the bio-activity to levels lower than obtained with asialo-hcG. It is concluded that the lectin-carbohydrate binding, which is part of the process that triggers the biological respons of the target cell can be mimicked by N-linked chains carrying a6-linked sialic acid. However, too high a density of such residues interferes with this interaction