Biomedical Engineering Reference
In-Depth Information
the processing of a wide range of substrates. Indeed, many in vitro kinetic
studies carried out on DPP-4 have characterized its truncation of several
bioactive peptides and resulted in the identification of numerous ecient
substrates. The general purpose of DPP-4-mediated peptide hydrolysis is
probably to overcome the diculty of cleaving the post-proline bond
when harvesting amino acids from food or returning amino acids to the
nutrient pool.
36,37
However, DPP-4-mediated cleavage of certain substrates
has more specialized physiological importance, particularly when hydrolysis
is rapid.
2
Among the
30 diverse substrates identified for DPP-4 are members of the
pancreatic polypeptide family such as neuropeptide Y (NPY) and peptide YY
(PYY),
54
the glucagon family such as GLP-1
5
and vasoactive intestinal peptide
(VIP),
55
the chemokine family such as SDF-1a
27,30
and eotaxin,
24
and other
neuropeptides such as substance P
56
and endomorphin.
57
DPP-4 substrates
range in size from four amino acids (endomorphin) to 103 amino acids
(monokine induced by IFN-gamma, Mig) with the average substrate size being
approximately 37 amino acids. The cleavage of these substrates is generally
rapid with SDF-1a and GLP-1 being among the most ecient substrates,
exhibiting the shortest half-lives of several minutes.
As described above, DPP-4 acts by cleaving two amino acids from the N-
terminus of its substrates. In some cases, a substrate can be cleaved sequentially
whereby further dipeptidyl cleavage occurs following removal of the first two
residues. This occurs with substance P
56
and VIP.
55
While DPP-4 is predominantly a post-prolyl cleaving enzyme, it is also
capable of cleaving substrates which have an alanine (GLP-1), glycine
(macrophage-derived chemokine, MDC) or serine (glucagon) in the penulti-
mate (P1) position. However, some peptides, within the potential substrate
size range and containing proline in the P1 position, are not cleaved by DPP-4
(e.g. interleukin-1b (IL-1b) and IL-2).
58
This indicates that the amino acid
sequence at positions P2, P1, P1
0
,andP2
0
, surrounding the cleavage site, is
not the only determinant for substrate specificity. In addition, structural
features, including size, can influence the catalytic activity. This is illustrated
by the more rapid cleavage of the long form of pituitary adenylate cyclase-
activating peptide (PACAP-38) compared to the shorter form, PACAP-27.
55
Certainly, a free and flexible N-terminus is required for ecient DPP-4
cleavage.
In vivo, such bioactive peptides act in the picomolar or nanomolar range. The
tight control of these peptides is crucially important in their diverse biological
functions. Thus, cleavage, and subsequent inactivation, of these peptides by
DPP-4 is evidence for the significance of this protease in a wide range of
physiological processes. New DPP-4 substrates are continually being identi-
fied,
37,59
and greater emphasis now lies in investigating the physiological rele-
vance of these substrates in vivo. To achieve this, a multi-disciplinary approach
is required, making use of both the DPP-4 gko mouse strain and specific DPP-4
inhibitors as well as high-resolution mass-spectrometry techniques, involving
the new field of degradomics/peptidomics.
37,60
B
