Possible existence of hybrid glycosyltransferase in heterozygous blood group AB subjects

The human blood group glycosyltransferases A and B have a dimeric structure, i.e., the A enzyme is an aa dimer and the B enzyme is a bb dimer. Considering the fact that the ABO blood group determinant are not x-linked, i.e. both A and/or B genes are expressed in a given cell, a hybrid enzyme (ab dimer) may exist in heterozygous A1B subjects. Because the A enzyme, but not the B enzyme, adsorbs with Sepharose 4-B, the adsorption characteristics of the A and B enzymes from plasma fo various phenotypes were examined to look for this hybrid enzyme. The A enzyme activity from A1 plasma and from a mixture of AU and B plasma was completely adsorbed to Sepharose 4-B, while 25-50% of A enzyme activity from heterozygous A1B plasma was not adsorbed. The results indicated that heterozygous A1B plasma contains an additional enzyme component which does not exist in a mixture of A1 and B plasma, suggesting the existence of a hybrid heterodimer (ab) in heterozygous A1B subjects.