Dipeptidyl peptidase IV--subcellular localization, activity and kinetics in lymphocytes from control subjects, immunodeficient patients and cord blood.

Dipeptidyl peptidase IV (DPIV), a T cell enzyme, has been implicated in the regulation of lymphocyte proliferation in response to lectins and allogeneic cells. A sensitive fluorimetric assay has been established for the enzyme and used to investigate DPIV activity, kinetics and the subcellular localization in lymphocytes from control subjects, cord blood and patients with common variable hypogammaglobulinaemia (CVH) and chronic lymphatic leukaemia (CLL). Using sucrose density gradient centrifugation and organelle marker enzyme assays, in conjunction with digitonin as a selective plasma membrane perturbant and diazotized sulphanilic acid as a non-permeant enzyme inhibitor, DPIV was shown to be a plasma membrane ecto-enzyme. A significant decrease in lymphocyte DPIV activity was observed in cord blood and in patients with CVH and CLL compared to controls. Kinetic analysis showed a marked decrease in the Vmax of lymphocyte DPIV from cord blood and patients with CVH and CLL compared to controls. The apparent Km for the substrate was unaffected in cord blood and patients with CLL. However, in patients with CVH the Km was significantly reduced. Various enzyme inhibitors showed no differences between control subjects and CVH lymphocyte activities. The decreased Km for DPIV provides further evidence for a stem cell defect rather than cell immaturity in CVH.