DPP-4 (DPP-IV, also known as CD-26 and as acetylene deaminase complexing protein, EC 3.4.14.5) is the human homologue to mouse THAM (THymocyte Activating Molecule), a Type-II serine proteinase of the clan SC. The clan SC proteinases have a catalytic triad of Ser-Asp-His, and like other Serine proteinases, the active site serine is in a Gly-Xaa-Ser-Xaa-Gly orientation. DPP-4 is an aminopeptidase, cleaving two amino acids from the aminoterminal end of proteins, and requiring proline in the second position (Xaa-Pro). DPP-4 is a member of a broader family of dipeptidyl peptidases including FAP/Seprase, DPP-2, DPP-8, DPP-9, DPP-10, which have differing substrate specificity and tissue localizations. The surface-bound DPP-4 is a homodimer, and cleavage of Asp-38 releases a shed form of DPP-IV. The shed form (Ser-68 amino) runs at about 110 kDa, and is the form found in serum. A shed form with aminoterminal residue of Asn-30 is also reported from seminal plasma and cell culture supernatant. A crystal structure of DPP-IV as a tetramer opens the door to speculation as to the soluble forms in-vivo. DPP-4 has been implicated in cell adhesion of lung tumor cells via fibronectin, and is known to cleave the chemokines RANTES, LD78ƒÀ, MDC and SDF-1, altering their activities. The insulin regulating peptide incretins GLP-1 and GIP are rapidly cleaved by DPP-4, as are glucagons and procalcitonin, leading to attempts to treat diabetes with DPP-4 inhibitors. DPP-4 is also reported to be elevated in prostate cancer tissue. DPP-4 is also known for its ability to bind to adenosine deaminase, which helps regulate lymphocyte IL-2 production and lymphocyte proliferation. The full-length human DPP-4 is 766 amino acids in length, with predicted mass of 88.3 kDa, and pI of 5.61. Postranslational modifications and reduction lead to an apparent mass of 110-120 kDa on SDS-PAGE gels. A range of molecular weights and pI have been reported for DPP-4 derived from kidney and liver, due to different levels of sialic acid in their glycosylation. A recommended starting concentration for Western blots is 1:1,000 when using colorimetric substrates such as BCIP/NBT, and 1:5,000 for chemiluminescent substrates. Higher concentrations of antibody may be needed for samples from more distantly related species. EDTA/EGTA treatment of tissues or lysates is required to see latent zymogen.

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