Dipeptidyl peptidase IV activity in the blood of the American alligator (Alligator mississippiensis)

Dipeptidyl peptidase IV (DPP4) enzyme activity, associated with cell surface and soluble CD26, was measured in alligator plasma and whole blood. DPP4 activity was higher in whole blood than in the plasma, presumably due to the inclusion of both membrane-bound CD26 on the surface of T-cells and sCD26 in the plasma. The plasma DPP4 activity was measured within 5 min after the addition of the substrate, and maximal accumulation of product was reached at 120 min. Alligator whole blood DPP4 activity peaked at 120 min. In addition, the DPP4 activity in both plasma and whole blood was temperature-dependent, with moderate activities at lower temperatures (5–20 °C) and peak activity at 40 °C. Alligator plasma DPP4 displayed classical Michaelis–Menten kinetics, with a Vmax value of 267 ± 9 nmol/min, and a Km of 21.1 ± 1.9 μM, while these values were 286 ± 14 nmol/min and 57.1 ± 3.7 μM, respectively, for whole blood. The proteolytic activity was inhibited in a concentration-dependent manner by a specific DPP4 inhibitor, diprotin A, which indicates that the activity is probably due to the presence of DPP4. The significance of the presence of this activity in alligator blood is still unknown.