DPP4 and diabetes mellitus: Diabetes led to a 26% increase in plasma DPP4 activity ([N] 1.6 ± 0.5 x 105 AU vs [D] 2.0 ± 0.7 x 105 AU; P < 0.05), whereas linagliptin reduced DPP4 activity by 77% (0.5 ± 0.1 x 105 AU vs [D] P < 0.001) (Fig 1A).