The term "neuropeptidases" refers to those enzymes that participate in the inactivation of synaptically released neuropeptides and therefore serve to turn off the generated peptide signal. In general, these enzymes are integral plasma membrane proteinases located as ectoenzymes. Some of the enzymes listed in the accompanying Tables are able to hydrolyze neuropeptides, but are present as cytosolic or secreted enzymes, or not present as ectoenzymes. Their physiological roles are therefore not fully established.
Only a relatively small group of CNS peptidases have been characterized as neuropeptide-inactivating ectoenzymes. The majority are zinc metalloenzymes for which specific and potent inhibitors have generally been developed. Some of these are found in abundance elsewhere, especially the renal and intestinal brush border membrane, but others appear to be more specific to the nervous system, e.g. pyroglutamyl aminopeptidase II. Neprilysin (NEP) is the prototype neuropeptidase originally discovered in the CNS as an enkephalin-degrading activity and subsequently as a substance P-hydrolyzing enzyme. It is appropriately located on neuronal membranes especially in the striatonigral pathway. Thus, it can act in an analogous manner to that of acetylcholinesterase at cholinergic terminals. Like several other neuropeptidases, it is also present in cells of the immune system where it may hydrolyze immunoregulatory peptides. In vivo, NEP is a broadly specific enzyme hydrolyzing a wide range of susceptible peptide substrates (enkephalins, substance P, atrial natriuretic peptide) as is its close homolog NEP II or secreted endopeptidase (SEP). Some other neuropeptidases appear to be much more substrate-specific. For example, pyroglutamyl amino-peptidase II appears to hydrolyze thyrotropin releasing hormone (TRH) exclusively. The unusual peptidase, glutamate carboxypeptidase II (GCP II) specifically inactivates the peptide neurotransmitter, N-acetylaspartylglutamate (NAAG) and GCP II inhibition can protect against some forms of neuronal death and may have applications in treatment of neuropathy.
Studies of changes in levels of neuropeptides in neurological disease have been limited in extent and no consistent pattern yet emerges. Likewise, factors regulating the expression of neuropeptidases both in the normal and the pathological state in the nervous system are little explored. Inhibitors of neuropeptidases are useful both as pharmacological tools in studies of neuropeptide physiology and also as potential therapeutic agents. Selective inhibitors of these enzymes have, to date, been obtained from natural products (e.g. phosphoramidon as NEP and endothelin converting enzyme 2 inhibitor) or designed (e.g. thiorphan selective for NEP) by analogy with similar enzymes from bacterial or other sources (e.g. thermolysin). Dual peptidase inhibitors (e.g. of NEP and angiotensin converting enzyme (ACE)) are increasingly finding favor as potential new therapeutics). Design of ACE, NEP and neurolysin inhibitors, in particular, will be aided by their recently solved three-dimensional structures.
A particular role for several neuropeptidases (especially NEP) has recently emerged in the turnover of the amyloidogenic Ab-peptide in Alzheimer's disease and age-dependent loss of NEP in the brain may contribute to the pathology. Strategies to up-regulate these CNS peptidases may therefore prove beneficial but raises some concerns with regard to potential side effects of their chronic inhibition.
Genome sequencing studies reveal that there are probably 8-10 NEP-like enzymes in the human genome and 24 in Drosophila. The Tables below describe the best characterized neuropeptidases. This includes a novel homolog of angiotensin converting enzyme (ACE2), not inhibited by classical ACE inhibitors, which appears to counterbalance the actions of ACE through its ability to convert angiotensin II to angiotensin-(1-7). Like several ectopeptidases, ACE2 serves serendipitously as a viral receptor, in this case for the severe acute respiratory syndrome (SARS) virus.
The Tables below contain accepted modulators and additional information. For a list of additional products, see the Materials section below.
AI: Angiotensin I
AII: Angiotensin II
AIII: Angiotensin III
AMC: 7-Amido-4-methyl coumarin
ANP: Atrial natriuretic peptide
CALLA: Common acute lymphoblastic leukemia antigen
CD: Cluster differentiation antigen
CPE: Carboxy-phenyl ethyl
CPP: Carboxy-phenyl propyl
EDTA: Ethylenediaminetetraacetic acid
EC33: (S)-3-Amino-4-mercaptobutyl sulfonic acid
GEMSA: Guanidinoethylmercaptosuccinic acid
LHRH: Luteinizing hormone-releasing hormone
MGTA: 2-Mercaptomethyl-3-guanidinoethylthiopropranoic acid
MLN: (S,S)-2-[1-Carboxy-2-[3-(3,5-dichlorobenzyl)-3H-imidazol4-yl]-ethylamino]-4-methylpentanoic acid)
NAALA: N-Acetylated a-linked acidic dipeptidase
NKA: Neurokinin A
NKB: Neurokinin B
NPY: Neuropeptide Y
PC18: 2-Amino-4-methylsulfonyl butane thiol
PYY: Peptide YY
SARS: Severe acute respiratory syndrome
SP: Substance P
TRH: Thyrotropin releasing hormone