Human blood-group MN and precursor specificities: structural and biological aspects.

The human blood-group MM and NN antigens carry 2 to 4 immunodominant groupings per repeating subunit and differ only by one sialic acid residue per immunodominant group. This residue covers in the MM antigen the beta-D-galactopyranosyl group that is terminal in the N immunodominant structure and that, together with a terminal alpha-linked N-acetylneuraminic acid residue, is responsible for N specificity. M specificity was readily converted into N specificity by mild acid treatment. N structure is the immediate biochemical precursor of M structure, and M and N antigenic specificities are not determined by two allelic genes as believed hitherto. The NN antigen was inactivated by beta-D-galactosidase as well as by removal of N-acetylneuraminic acid. Some of the reactivities of the NN antigen, lost upon beta-D-galactosidase treatment, reappeared on subsequent partial N-acetylneuraminic acid removal. The structure uncovered by complete sialic acid depletion of MN antigens is the Thomsen-Friedenreich T antigen, the specificity of which is determined by beta-D-galactopyranosyl groups. Beta-D-Galactosidase treatment transformed the T antigen into one possessing Tnactivity. The significance of blood-group MN active substances extends to human breast cancer, where MN antigens were found in benign and malignant glands, but some of their precursors in cancerous tissue only.