Тhe enzymes adenosine deaminase (ADA), dipeptidyl peptidase 4 (DPP4) and glutaminase (GL) are involved in the development of both the type 1 (T1DM) and type 2 diabetes mellitus (T2DM). They can be considered as targets in the prevention and/or treatment of these diseases. The autoimmune response of the body is caused by the post-translational protein modifications, one of which is protein citrullination, when arginine residue transforms to citrulline. This work compares the ADA, DPP4, GL enzymes and protein citrullination in blood plasma from T1DM, T2DM patients and relatively healthy individuals. The enzymes were isolated and purified from the blood samples. We found that DPP4 activity was significantly higher in the plasma of patients compared to controls, particularly in T1DM. Currently the medicines based on DPP4 inhibitors are used in the treatment of T2DM. Our results indicate that this treatment strategy can be recommended in the T1DM treatment as well.