Simple N-linked Sugar Chains Are Bound to the Lutropin of the Bullfrog Rana catesbeiana

Abstract

The structures of Asn-linked sugar chains of the bullfrog ( Rana catesbeiana) lutropin (LH) with different isoelectric points (p I 9.3 and p I 8.8) have been analyzed. The LH of p I 9.3, which is three times more active in Xenopus ovulation assays and radioreceptor assays with Xenopus testis than LH of p I 8.8, has Man-GlcNAc-GlcNAc and Man-Man-GlcNAc-GlcNAc as the major sugar chain components both at Asn-57 and 83 of the α-subunit. On the other hand, the α-subunit of the LH of p I 8.8 contained more mannose and phosphate bound at the nonreducing end of the sugar chains. This is the first study to identify the presence of a phosphate group bound to the mannose in LH. The structure of the major sugar chain bound to the β-subunit was Man-Man-GlcNAc-(Fuc)GlcNAc at Asn-8 of the LH from both p Is. These sugar chains bound to the bullfrog LH are more simple than those of mammalian LH, which have N-acetyl-galactosaminylated biantennary oligosaccharides and more complicated sugar chains. The microheterogeneous p I of bullfrog LH is thus due to the different phosphate contents in the sugar chains bound to the α-subunit instead of sulfate groups which are found in mammalian lutropins.