Dipeptidyl peptidase IV from porcine seminal plasma: purification, characterization, and N-terminal amino acid sequence

Activatable Fluorescence and Bio/Chemiluminescence Probes for Aminopeptidases: From Design to Biomedical Applications

Aminopeptidases are exopeptidases that catalyze the cleavage of amino acid residues from the N-terminal fragment of protein or peptide substrates. Owing to their function, they play important roles in protein maturation, signal transduction, cell-cycle control, and various disease mechanisms, notably in cancer pathology. To gain better insights into their function, molecular imaging assisted by fluorescence and bio/chemiluminescence probes has become an indispensable method to their superiorities, including excellent sensitivity, selectivity, and real-time and noninvasive imaging. Numerous efforts are made to develop activatable probes that can effectively enhance efficiency and accuracy as well as minimize the side effects. This review is classified according to the type of aminopeptidases, summarizing some recent works on the design, work mechanism, and sensing, imaging, and theranostic performance of their activatable probe. Finally, the current challenges are outlined in developing activatable probes for aminopeptidases and provide possible solutions for future advancements.

Purification and biochemical characterization of a novel secretory dipeptidyl peptidase IV from porcine serum

Purification of DPP-IV enzyme from porcine serum, is presented in this study for the first time. The high molecular weight DPP-IV from porcine serum was fractioned using Sephadex G-75 gel filtration followed by DEAE Sephadex anion exchange and Sephadex G-100 gel filtration chromatography columns with a final yield of 11.25%. The SDS-PAGE of the purified sample showed a single band of molecular mass nearing 160 kDa. Distinct single band was observed after PAS staining confirmed it to be a glycoprotein. The purified enzyme showed an optimum pH and temperature of 8 and 37 °C, respectively. The enzyme effectively cleaved fluorogenic substrate Gly-Pro-AMC with Km and Vmax of 4.578 µM and 90.84 nmoles/min, respectively. Purified DPP-IV activity was inhibited by Diprotin A with an IC₅₀ value of 8.473 µM. Among the three plant extracts used to study DPP-IV inhibition, the aqueous hot extract of Terminalia chebula showed the highest inhibition of 87.19%, followed by the aqueous cold extract of Momordica carantia, ( 31.6%) and Azadirachta indica (34.16%) at the concentration of 25 µg.

Rapid detection of DPP-IV activity in porcine serum: A fluorospectrometric assay

Dipeptidyl peptidase IV (DPP-IV) is an aminopeptidase that cleaves the N-terminal dipeptide from peptides bearing proline or alanine residues. Currently, DPP-IV activity is quantified by spectrophotometric or fluorometric methods, which employ Gly-Pro-pNA and Gly-Pro-AMC respectively, as substrate. However, these methods require high enzyme and substrate concentrations. In this study, we adapted the DPP-IV fluorospectrometric assay using NanoDrop 3300, which requires only nanogram levels of the enzyme (30 ng crude DPP-IV) and considerably low substrate concentrations (100 μM). Fluorescence measurement required a reaction mixture of only 2 μL, thus eliminating the need for microtiter plates or cuvettes.We employed this assay to demonstrate DPP-IV activity in porcine serum for the first time. The enzymatic activity peaked at pH 8.0 in porcine (84 nM/min), human (87 nM/min) and bovine (89.1 nM/min) sera, with the optimum temperature of 37 °C. The enzyme showed maximum activity upon incubation for 40 min at 37 °C. In contrast, activity in the porcine serum was the highest after incubation for 30 min at the same optimized parameters. The IC₅₀ values of diprotin A against DPP-IV from human, porcine, and bovine sera were 7.83, 8.62, 9.17 μM, respectively. The present assay procedure is a convenient, sensitive, accurate and high-throughput method suitable for primary screening of DPP-IV inhibitors.

Improvement of blood glucose levels and obesity in mice given aronia juice by inhibition of dipeptidyl peptidase IV and α-glucosidase

Aronia berries have many potential effects on health. Previous human studies have shown that aronia juice may be useful for treatment of obesity disorders. Recently, we have reported that aronia juice has an inhibitory effect on dipeptidyl peptidase (DPP IV) activity and that the DPP IV inhibitor in aronia juice was identified as cyanidin 3,5-diglucoside. In this study, we found that body weights and blood glucose levels were reduced in diabetes model KK-Ay mice given aronia juice. We also found that weights of white adipose tissues were reduced in KK-Ay mice given aronia juice. Furthermore, levels of DPP IV activity in the serum and liver from KK-Ay mice were lower than those in the serum and liver from C57BL/6JmsSlc mice. Interestingly, although levels of DPP IV activity were not changed in the serum and liver from aronia-juice-administered KK-Ay mice, levels of DPP IV activity were increased in those from aronia-juice-administered C57BL/6JmsSlc mice. Furthermore, α-glucosidase activity was inhibited in the upper region of the small intestine from aronia-juice-administered KK-Ay mice but not in the lower region. Inhibition of α-glucosidase activity in the upper portion of the small intestine induced a reduction of glucose-dependent insulinotropic polypeptide (GIP) level. The results suggest that DPP IV activity in diabetic mice is inhibited by aronia juice, that the GIP level in the upper region of the small intestine is reduced by inhibition of α-glucosidase activity and that weights of adipose tissues are reduced by aronia juice.

Identification and characterization of a dipeptidyl peptidase IV inhibitor from aronia juice

Aronia berries have many potential effects on health, including an antioxidant effect, effect for antimutagenesis, hepatoprotection and cardioprotection, an antidiabetic effect and inhibition of cancer cell proliferation. Previous human studies have shown that aronia juice may be useful for treatment of obesity disorders. In this study, we found that aronia juice has an inhibitory effect against dipeptidyl peptidase IV (DPP IV) (EC 3.4.14.5). DPP IV is a peptidase that cleaves the N-terminal region of incretins such as glucagon-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). Inactivation of incretins by DPP IV induces reduction of insulin secretion. Furthermore, we identified that cyanidin 3, 5-diglucoside as the DPP IV inhibitor in aronia juice. DPP IV was inhibited more strongly by cyanidin 3, 5-diglucoside than by cyanidin and cyanidin 3-glucoside. The results suggest that DPP IV is inhibited by cyanidin 3, 5-diglucoside present in aronia juice. The antidiabetic effect of aronia juice may be mediated through DPP IV inhibition by cyanidin 3, 5-diglucoside.

Effect of divalent cations on the porcine kidney cortex membrane-bound form of dipeptidyl peptidase IV

Dipeptidyl peptidase IV is an ectopeptidase with multiple physiological roles including the degradation of incretins, and a target of therapies for type 2 diabetes mellitus. Divalent cations can inhibit its activity, but there has been little effort to understand how they act. The intact membrane-bound form of porcine kidney dipeptidyl peptidase IV was purified by a simple and fast procedure. The purified enzyme hydrolyzed Gly-Pro-p-nitroanilide with an average V(max) of 1.397±0.003 μmol min(-1) mL(-1), k(cat) of 145.0±1.2 s(-1), K(M) of 0.138±0.005 mM and k(cat)/K(M) of 1050 mM(-1) s(-1). The enzyme was inhibited by bacitracin, tosyl-L-lysine chloromethyl ketone, and by the dipeptidyl peptidase IV family inhibitor L-threo-Ile-thiazolidide (K(i) 70 nM). The enzyme was inhibited by the divalent ions Ca(2+), Co(2+), Cd(2+), Hg(2+) and Zn(2+), following kinetic mechanisms of mixed inhibition, with K(i) values of 2.04×10(-1), 2.28×10(-2), 4.21×10(-4), 8.00×10(-5) and 2.95×10(-5) M, respectively. According to bioinformatic tools, Ca(2+) ions preferentially bound to the β-propeller domain of the porcine enzyme, while Zn(2+) ions to the α-β hydrolase domain; the binding sites were strikingly conserved in the human enzyme and other homologues. The functional characterization indicates that porcine and human homologues have very similar functional properties. Knowledge about the mechanisms of action of divalent cations may facilitate the design of new inhibitors.

Peptidase inhibitors in the MEROPS database

The MEROPS website (http://merops.sanger.ac.uk) includes information on peptidase inhibitors as well as on peptidases and their substrates. Displays have been put in place to link peptidases and inhibitors together. The classification of protein peptidase inhibitors is continually being revised, and currently inhibitors are grouped into 67 families based on comparisons of protein sequences. These families can be further grouped into 38 clans based on comparisons of tertiary structure. Small molecule inhibitors are important reagents for peptidase characterization and, with the increasing importance of peptidases as drug targets, they are also important to the pharmaceutical industry. Small molecule inhibitors are now included in MEROPS and over 160 summaries have been written.

Mechanisms underlying the rapid degradation and elimination of the incretin hormones GLP-1 and GIP

The incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP, gastric inhibitory peptide) are secreted from intestinal L and K cells and stimulate insulin secretion from pancreatic beta cells. However, they are immediately inactivated mainly via N-terminal degradation by dipeptidyl peptidase IV (DPP IV, CD26), a specialised enzyme located on the cell surface enzyme of endothelial, epithelial and some other cell types. Cleavage by neprilysin (neutral endopeptidase) is a minor degradation route, and renal clearance eliminates incretin/fragments, but appears of less importance for regulating incretin bioactivities. Based on these observations two novel types of drugs for the treatment of type 2 diabetes have been developed: DPP IV inhibitors and DPP IV-resistant incretin analogues. Both have distinct advantages and disadvantages. Potential side effects of DPP IV inhibitors may result from affecting the bioactivity of other hormones, neuropeptides or chemokines and also by their cross-reactivity with DPP IV-related enzymes.

Sulphur and nitrogen regulation of the protease-encoding ACP1 gene in the fungus Botrytis cinerea: correlation with a phospholipase D activity

Sulphur and nitrogen catabolic repressions are regulations that have long been recognized in fungi, but whose molecular bases remain largely elusive. This paper shows that catabolic repression of a protease-encoding gene correlates with the modulation of a phosphatidylethanolamine (PE)-specific phospholipase D (PLD) activity in the pathogenic fungus Botrytis cinerea. Our results first demonstrate that the ACP1 gene is subject to sulphur catabolic repression, with sulphate and cysteine inhibiting its expression. Sulphate and cysteine also cause a decrease of the total cellular PLD activity and, reciprocally, the two PLD inhibitors AEBSF [4-(2-aminoethyl)benzenesulphonyl fluoride] and curcumin negatively affect ACP1 expression in vivo. Cysteine moreover inhibits the PE-specific PLD activity in cell extracts. ACP1 is regulated by nitrogen, but here we show that this regulation does not rely on the proximal AREA binding site in its promoter, and that glutamine does not play a particular role in the process. A decrease in the total cellular PLD activity is also observed when the cells are fed ammonia, but this effect is smaller than that produced by sulphur. RNA-interference experiments finally suggest that the enzyme responsible for the PE-specific PLD activity is encoded by a gene that does not belong to the known HKD gene family of PLDs.

Characterization and cDNA cloning of dipeptidyl peptidase IV from the venom of Gloydius blomhoffi brevicaudus

Dipeptidyl peptidase activity was investigated in snake venoms from Gloydius blomhoffi brevicaudus, Gloydius halys blomhoffii, Trimeresurus flavoviridis and Crotalus atrox. The strongest dipeptidyl peptidase IV (DPP IV) activity was found in venom from G. blomhoffi brevicaudus. The substrate specificity, susceptibility to inhibitors, and pH optimum of the partially purified enzyme were similar to those of known DPP IVs from bacteria and eukaryotes. The G. blomhoffi brevicaudus venom gland cDNA library was screened to isolate cDNA clones using probes based on amino acid sequences highly conserved in known DPP IVs. Two cDNA species encoding DPP IV were obtained, and designated as DPP IVa and DPP IVb. This is the first study to report the primary structure of DPP IV from a reptile. The deduced amino acid sequences for DPP IVa and DPP IVb both consist of 751amino acid residues and are highly homologous to each other. A putative catalytic triad for serine proteases, Ser-616, Asp-694, and His-726, is present. It is of particular interest that the deduced NH(2)-terminal sequence associated with the characteristic signal peptide is identical to that determined from the purified DPP IV. This indicates that the signal peptide of snake venom DPP IV is not cleaved off during biosynthesis, unlike those of other snake venom proteins.

The Interaction of Human Tryptase-β with Small Molecule Inhibitors Provides New Insights into the Unusual Functional Instability and Quaternary Structure of the Protease

Human tryptase-beta (HTbeta) is a serine protease with an atypical tetrameric structure and an unusual dependence on heparin binding or high salt for functional and structural stability. In the absence of heparin and at physiological salt, pH, and temperature, HTbeta rapidly loses activity by a reversible process that we have called spontaneous inactivation. The role of tetramer dissociation in this process is controversial. Using small irreversible or competitive inhibitors of HTbeta as stabilizing ligands, we were able to examine tetramer stability under inactivating (decay) conditions in the absence of heparin and to define further the process of spontaneous inactivation. Size exclusion chromatography showed that interaction with inhibitors stabilized the tetramer. Using sedimentation equilibrium, spontaneously inactivated HTbeta (si-HTbeta) was shown to be a destabilized tetramer that dissociates upon dilution and which in the presence of a competitive inhibitor re-formed a stable tetramer. Addition of inhibitors to si-HTbeta rescued catalytic activity as was shown after inhibitor displacement. At high concentrations of si-HTbeta (4-5 microM), the binding of inhibitor alone provided sufficient free energy for complete reactivation and tetramer stabilization, whereas at low si-HTbeta concentration (0.1 microM) where the destabilized tetramer would be mostly dissociated, reactivation required more free energy which was provided by the binding of both an inhibitor and heparin. The results demonstrate that HTbeta is a tetramer in the absence of heparin and that tetramer dissociation is a consequence of and not a prerequisite for inactivation. Heparin binding likely stabilizes the tetramer by favoring a functionally active conformation with stable intersubunit contacts, rather than by simply cross-linking active monomers.

The purification and characterisation of novel dipeptidyl peptidase IV-like activity from bovine serum

The discovery of a potentially novel proline-specific peptidase from bovine serum is presented which is capable of cleaving the dipeptidyl peptidase IV (DPIV) substrate Gly-Pro-MCA. The enzyme was isolated and purified with the use of Phenyl Sepharose Hydrophobic Interaction, Sephacryl S-300 Gel Filtration, and Q-Sephacryl Anion Exchange, producing an overall purification factor of 257. SDS PAGE resulted in a monomeric molecular mass of 158kDa while size exclusion chromatography generated a native molecular mass of 328kDa. The enzyme remained active over a broad pH range with a distinct preference for a neutral pH range of 7-8.5. Chromatofocusing and isoelectric focusing (IEF) revealed the enzyme's isoelectric point to be 4.74. DPIV-like activity was not inhibited by serine protease inhibitors but was by the metallo-protease inhibitors, the phenanthrolines. The enzyme was also partially inhibited by bestatin. Substrate specificity studies proved that the enzyme is capable of sequential cleavage of bovine beta-Casomorphin and Substance P. The peptidase cleaved the standard DPIV substrate, Gly-Pro-MCA with a K(M) of 38.4 microM, while Lys-Pro-MCA was hydrolysed with a K(M) of 103 microM. The DPIV-like activity was specifically inhibited by both Diprotin A and B, non-competitively, generating a K(i) of 1.4 x 10(-4) M for both inhibitors. Ile-Thiazolidide and Ile-Pyrrolidide both inhibited competitively with an inhibition constant of 3.7 x 10(-7) and 7.5 x 10(-7) M, respectively. It is concluded that bovine serum DPIV-like activity share many biochemical properties with DPIV and DPIV-like enzymes but not exclusively, suggesting that the purified peptidase may play an important novel role in bioactive oligopeptide degradation.

Cell-surface peptidases

The cell surface has various functions: communicating with other cells, integrating into the tissue, and interacting with the extracellular matrix. Proteases play a key role in these processes. This review focuses on cell-surface peptidases (ectopeptidases, oligopeptidases) that are involved in the inactivation or activation of extracellular regulatory peptides, hormones, paracrine peptides, cytokines, and neuropeptides. The nomenclature of cell-surface peptidases is explained in relation to other proteases, and information is provided on membrane anchoring, catalytic sites, regulation, and, in particular, on their physiological and pharmacological importance. Furthermore, nonenzymatic (binding) functions and participation in intracellular signal transduction of cell surfaces peptidases are described. An overview on the different cell-surface peptidases is given, and their divergent functions are explained in detail. An example of actual pharmacological importance, dipeptidyl-peptidase IV (CD26), is discussed.

Transcellular proteolysis demonstrated by novel cell surface-associated substrates of dipeptidyl peptidase IV (CD26)

Proteolytic enzymes contribute to the regulation of cellular functions such as cell proliferation and death, cytokine production, and matrix remodeling. Dipeptidyl peptidase IV (DP IV) catalyzes the cleavage of several cytokines and thereby contributes to the regulation of cytokine production and the proliferation of immune cells. Here we show for the first time that cell surface-bound DP IV catalyzes the cleavage of specific substrates that are associated with the cellular surface of neighboring cells. Rhodamine 110 (R110), a highly fluorescent xanthene dye, was used to synthesize dipeptidyl peptidase IV (DP IV/CD26) substrates Gly(Ala)-Pro-R110-R, thus facilitating a stable binding of the fluorescent moiety on the cell surface. The fixation resulted from the interaction with the reactive anchor rhodamine and allowed the quantification of cellular DP IV activity on single cells. The reactivity, length, and hydrophobicity of rhodamine was characterized as the decisive factor that facilitated the determination of cellular DP IV activity. Using fluorescence microscopy, it was possible to differentiate between different DP IV activities. The hydrolysis of cell-bound substrates Xaa-Pro-R110-R by DP IV of neighboring cells and by soluble DP IV was shown using flow cytometry. These data demonstrate that ectopeptidases such as DP IV may be involved in communication between blood cells via proteolysis of cell-associated substrates.

Peptidase activities in human semen

Enkephalins are one of the opioids present in human semen and to date their function in this tissue remains unknown. The present work studies enkephalin-degrading enzyme activities, puromycin-sensitive alanyl aminopeptidase (AAP-S), puromycin-insensitive alanyl aminopeptidase N (Ap N) and neprilysin (NEP) in human seminal fractions. AAP-S activity was not detected in any fractions, whereas Ap N appeared in soluble and particulate sperm fractions in seminal fluid and in prostasome fraction. With regard to NEP activity, this was exclusively located in prostasome membranes. The high activity values observed in the prostasome fraction suggested that these peptidases and their substrates could be involved in seminal physiology.

Proline specific peptidases

Proline is unique among the 20 amino acids due to its cyclic structure. This specific conformation imposes many restrictions on the structural aspects of peptides and proteins and confers particular biological properties upon a wide range of physiologically important biomolecules. In order to adequately deal with such peptides, nature has developed a group of enzymes that recognise this residue specifically. These peptidases cover practically all situations where a proline residue might occur in a potential substrate. In this paper we endeavour to discuss these enzymes, particularly those responsible for peptide or protein hydrolysis at proline sites. We have detailed their discovery, biochemical attributes and substrate specificities and have provided information as to the methodology used to detect and manipulate their activities. We have also described the roles, or potential roles that these enzymes may play physiologically and the consequences of their dysfunction in varied disease states.

A prediction of DPP IV/CD26 domain structure from a physico-chemical investigation of dipeptidyl peptidase IV (CD26) from human seminal plasma

Human DPP IV, isolated from seminal plasma by means of immobilised adenosine deaminase, occurs in different forms which are distinguishable by net charge and native molecular weight. Charge differences arise primarily from different degrees of glycosylation containing various amounts of sialic acid. The majority of DPP IV isolated from total seminal plasma consists of the extracellular part of the protein starting at Gly-31. It is a very stable protein resisting high concentrations of denaturant. Unfolding experiments under reducing conditions are indicative of the existence of at least two domains which function independently. One of these domains is highly stabilised by disulfide bonds. Disruption of the disulfide bonds does not affect the activity, the dimeric state nor the adenosine deaminase binding properties of the protein but renders it more susceptible to proteolysis. The low-angle X-ray scattering spectrum is consistent with a model for a protein containing two subunits, each composed of three domains linked by flexible regions with low average mass. The secondary structure composition, determined by FTIR spectrometry, indicates that 45% of the protein consists of beta-sheets, which is higher than expected from computed secondary structure predictions. Our results provide compelling experimental evidence for the three-domain structure of the extracellular part of DPP IV.

Dipeptidyl peptidase II from porcine seminal plasma: purification, characterization, and its homology to granzymes, cytotoxic cell proteinases (CCP 1-4)

Dipeptidyl peptidase II (DPP II) was purified to homogeneity from porcine seminal plasma by polyacrylamide gel electrophoresis (PAGE). The molecular weight of the purified enzyme was calculated to be approx. 185,000 and 200,000 on Superdex 200 column chromatography and non-denatured PAGE, respectively, and to be 58,000 and 61,000 on SDS-PAGE in the absence and presence of beta-mercaptoethanol (beta-ME), respectively. These findings suggested that the enzyme is composed of three identical subunits. The enzyme rapidly hydrolyzed the substrates Lys-Ala-MCA and Gly-Pro-MCA at acidic pH. The Km and V(max) values of DPP II at optimal pH (pH 6.0) were 1330 microM and 2.9 mumol/mg per min for Gly-Pro-MCA, and 360 microM and 1.43 mumol/mg per min for Lys-Ala-MCA, respectively. It was strongly inhibited by diisopropylphosphofluoride (DFP), and moderately by 4-(2-aminoethyl)benzenesulfonyl fluoride (AEBSF). These findings suggest that DPP II is a serine peptidase. Furthermore, the enzyme activity was also strongly inhibited by copper ions. The amino-acid sequence of the first 41 residues of the enzyme was determined as Ala1-Ser-Pro-Pro-Glu-Pro-Gly-Phe-Arg- Glu10-Val-Tyr-Phe-Glu-Gln-Leu-Leu-Asp-His-Phe20-Asn-Phe-Glu- Arg-Phe- Gly-Lys-Lys-Thr-Phe30-Arg-Gln-Arg-Phe-Leu-Val-Ser-Asp-Lys-Phe40 -Trp. This sequence showed homology (11.6-30.2%) to the N-terminal amino-acid sequences of cytotoxic cell proteinases (CCP 1-4), granzymes. Other properties of DPP II including pH optimum, pH stability, and heat stability were characterized.

Use of immobilized adenosine deaminase (EC 3.5.4.4) for the rapid purification of native human CD26/dipeptidyl peptidase IV (EC 3.4.14.5)

The leukocyte differentiation antigen CD26 identified as dipeptidyl peptidase IV.(EC 3.4.14.5), cleaves off N-terminal dipeptides from peptides when a proline or alanine is located at the penultimate position. Seminal plasma and especially prostasomes, prostate-derived organelles which occur freely in seminal plasma, contain high amounts of CD26/dipeptidyl peptidase IV and therefore are suitable sources for the purification of the protein. The use of adenosine deaminase (EC 3.5.4.4) affinity chromatography for its purification is described. CD26/dipeptidyl peptidase IV was purified from human seminal plasma and prostasomes by a two step procedure. Ion exchange chromatography on DEAE-Sepharose, followed by affinity chromatography on adenosine deaminase-Sepharose resulted in the pure, native protein with an overall yield ranging from 35 to 55%. The N-terminal sequence of the amphiphilic enzyme purified from human prostasomes was determined to be Met-Lys-Thr-Pro-Trp-Lys-Val-Leu. The preparation obtained was free of contaminating aminopeptidase activity and proved to be very stable (up to 1 month at 37 degrees C). The calf intestinal adenosine deaminase we used is commercially available and can be employed for the purification of human, bovine and rabbit CD26/dipeptidyl peptidase IV. High affinity binding of porcine dipeptidyl peptidase IV was not observed. The availability of a source with high specific activity and the introduction of adenosine deaminase affinity chromatography permits the rapid purification of milligram quantities of natural mammalian CD26/dipeptidyl peptidase IV.

Human serum dipeptidyl peptidase IV (DPPIV) and its unique properties

Dipeptidyl peptidase IV (DPPIV, EC 3.4.14.5) has been purified 18,000-fold in a yield of 2.2% from human serum. Serum DPPIV, a serine enzyme with an apparent mass of 250 kDa, consists of two identical subunits with an apparent mass of 100 kDa and is inhibited by DPPIV-specific inhibitor Diprotin A and also by p-chloromercuribenzoate (p-CMB), 2-mercaptoethanol, HgCl2, CdCl2, SrCl2, and ZnCl2. One of the remarkable properties of DPPIV is that its activity is greatly enhanced by Gly-X (X: especially, Gly, Gln, Glu and Ser) dipeptides. Gly-X dipeptides increase not only an apparent Km of serum DPPIV for glycyl-L-proline 3,5-dibromo-4-hydroxyanilide nearly 10-fold, but also an apparent kcat nearly 4-fold. This mechanism is unclear, but one possibility is that Gly-Pro from substrate might bind amino acids or dipeptides instead of water molecules as DPPIV transpeptidyl activity reported previously. Another remarkable property of DPPIV is the ability to bind adenosine deaminase-I and -II, as is the case with recombinant soluble CD26 (rsCD26). This probably indicates that DPPIV purified from human serum by our method originates from T-lymphocytes.