Immunologic homology of human blood group glycosyltransferases and genetic background of blood group (ABO) determination

The human blood groups (ABO) are known to be determined by the terminal carbohydrate residues attached to common carbohydrate chains of the red cell surface. Specific transferases, i.e., N-acetylgalacto-saminyltransferase (A enzyme) in blood group A persons and galactosyltransferase (B enzyme) in blood group B persons, are responsible for transferring the carbohydrates to the terminal galactose of the H-substance of the O red cell surface, with both enzymes absent in blood group O persons. It has not been clear whether the expression of these transferases is related to allelic structural genes or is under regulatory control. The activities of A and B enzymes were completely neutralized by the anti-A-enzyme antibody, which was obtained from a rabbit immunized with purified A enzyme. When pretreated with a protein component obtained from blood group O plasma, the antibody had no capacity for neutralization of the two enzymes. If the transferases are under regulatory control, no immunologically cross-reactive protein should be produced in blood type O persons. The existence of an immunologically active component in blood group O plasma proves that the genes for A enzyme, B enzyme, and the immunologically cross-reactive protein in O plasma are allelic and excludes the possibility of involvement of regulatory control in blood group expression.