Immunochemical studies on blood groups LXVI. Competitive binding assays of A1 and A2 blood group substances with insolubilized anti-A serum and insolubilized A agglutinin from Dolichos biflorus.

Abstract

There are conflicting views on the nature of subgroups A, and A2 of blood group A, One holds that the same determinants are present on either, but that there are fewer determinants on A2 than on A, erythrocytes. Soluble A2 and A, substances would thus have the same kinds of determinants, but in different numbers. The A1 and A2 transferases are different enzymes, but the A2 enzyme is less efficient than the A, transferase, but it has the same specificity (1) in adding terminal N-acetyl-D-galactosaminyl residues to precursor blood group H oligosaccharide side chains. Such a difference in enzymatic activity has been proposed to be responsible for the H activity of A2 cells (2). A population of those anti-A, antibodies which do not agglutinate A2 erythrocytes is known and has been prepared from the purified IgM fraction of anti-A sera by absorption with A2 erythrocytes, but not from the IgG fractions of the same sera (3). Such anti-A antibodies have been assumed (3) to have a low affinity so that if they use only one of their ten valences per erythrocyte, they would be unable to hold two erythmcytes together. The anti-A, antibodies are hypothesized to agglutinate A, erythrocytes which have receptors that are more numerous and closer together, so that each erythrocyte could be bound by two or more of the combining sites of each antibody molecule.The other concept, based predominantly on immunochemical studies with A1 and A2 glycoproteins, favors a qualitative difference (4). Absorption with insolubilized polyleucyl A2 substance did not remove all of the anti-A, but left anti-A~; had all determinants been present on A2 as well as on A~ substances, no anti-A~ should have remained.Since several different determinants on soluble blood group A substances have been found (5, 6, 7), a basis for a structural difference between A~ and A2 exists. All A determinants have the structure ~ucal 2 DGalNAcal--*3DGal but differ in that this trisaccharide may be linked /~1--.3 or B1~4 to vGlcNAc to give

A-Active Trisaccharides Isolated from A1 and A2 Blood-Group-Specific Glycoproteins

“…The present study has revealed no differences between the types of A-determinant present in A1 and AZ glycoproteins and does not support the hypothesis of Kabat et al [2,4]. Studies on erythrocytes [9,35,37,38] have not shown any differences between the types of A-determinant on A1 and A1 cells and indicate that there are fewer A-determinants and more H-determinants in A2 cells than in A1.…”

“…Kabat and co-workers proposed [2,4] that the A-determinants in A2 materials are present only on type 2 chains, while A1 materials have A-determinants on both type 1 and type 2 chains. The work described in this paper was undertaken primarily to test this hypothesis by the isolation and characterisation of A-active oligosaccharides from A1 and A2 glycoproteins.…”

“…The present study was undertaken to test the hypothesis of Kabat and co-workers [2,4] by establishing whether or not both type 1 and type 2 A-determinants are present in blood-group-specific glycoproteins from A1 and A:! persons.…”

“…Kabat and others [2,4] cite the findings of quantitative precipitation experiments, absorption studies, and competitive binding assays as evidence of structural differences between the A-determinants present in A1 and A2 materials. Two different A-determinants have so far been identified in Aactive ovarian cyst glycoproteins, the type 1 and type 2 [5-81.…”

Repetitive A epitope (type 3 chain A) defined by blood group A1-specific monoclonal antibody TH-1: chemical basis of qualitative A1 and A2 distinction.

“…Although the serology and genetics of Al and A2 have been well-established (2), the chemical basis of the distinction between A1 and A2 has been an unresolved problem. Some investigators have concluded that Al erythrocytes contain greater quantities of A determinants than do A2 erythrocytes (3); others have suggested a qualitative difference between A1 and A2 based on the production of specific antibodies reacting with A1 but not A2 erythrocytes (4), and on binding kinetics of A1 and A2 blood group glycoproteins with insolubilized anti-A antibodies (5). The glycolipid fraction with low TLC mobility separated from blood group A2 erythrocytes was weakly A active, in contrast to the same slow-migrating fraction separated from A1 erythrocytes (6 …”