Homology between ABH-carrier α2-seminoglycoprotein and Mac-2 binding protein

α2-seminoglycoprotein (α2-SGP), purified from human seminal plasma, is a carrier of glycoprotein for the ABO blood grouping. The α2-SGP exists in the secretions of the seminal vesicle and various glands. However, the function of α2-SGP is, as yet, unknown. In this study, we determined that two internal amino acid sequences of 8 and 12 residues of α2-SGP were Ala-Val-Asp-Thr-Trp-Ser-Trp-Gly and Thr-Leu-Gln-Ala-Leu-Glu-Phe-His-Thr-Val-Pro-Phe. These sequences were completely coincident with the domain 3 of human Mac-2 binding protein (M2BP), which was identified as a tumor-associated antigen. In addition, we also confirmed an α2-SGP binding activity to galectin-3 that was one of a ligand for M2BP, and the immunological cross-reactivity between α2-SGP and M2BP. These findings strongly suggested that α2-SGP was identical with M2BP.