Dipeptidyl peptidase IV (CD26) in human leukemia cells: immunological and biochemical data

To study dipeptidyl peptidase IV (DDP-IV), CD26, in various subpopulations of lymphoid cells, a comparison has been made of the immunofluorescence findings on the expression of the antigen CD26 on the surface of some leukemia cells and the enzyme activity in lysates of these cells. It was found that monoclonal antibodies (MAb) BA-5b against activation antigen CD26 of human T-lymphocytes react with purified DDP-IV preparations from human kidneys and leukemic T-cells. Binding of MAb BA-5b with DDP-IV molecule does not influence the enzyme activity. This indicates that they bind outside the zone of active center. MAb BA-Sb react with cells from patients with B-cell chronic lymphoid leukemia identifying 20-50% of CD26+ cells. This confirms previous enzymologic data on the presence of DDP-IV in leukemia B-cells but contradicts low DDP-IV activity in the above cells. These findings and those on the cells of T-lymphoblastoid lines Jurkat, GEM, YT show that the percentage of CD26+ cells estimated by binding of MAb BA-5b on cell surface does not reflect DDP-IV activity. Thus, immunological and biochemical findings do not coinside which may be explained by the presence of DDP-IV intracellular pool, of inactive form of this enzyme or existance of several molecular forms of DDP-IV.