Dipeptidases and

Gener ally, a family of peptidases contains either exopeptidases or endopeptidases, but there are exceptions. Family Cl contains not only endopeptidases such as cathepsin L, but also the aminopeptidase bleomycin hydrolase. Some members of this family can act as exopeptidases as well as endopeptidases. For example, cathepsin B also acts as a peptidyl-dipeptidase, and... 

Several different types of proteases hydrolyze intact storage proteins first into large fragments and then into smaller peptides and amino acids within the protein body. The peptides are transported to the cytosol where other enzymes, e. g. amino-peptidases, carboxypeptidases, dipeptidases and tripeptidases, cleave them and eventually form a pool of free amino acids [11]. 

This enzyme [EC 3.4.13.3] (also referred to as Xaa-His dipeptidase, X-His dipeptidase, aminoacylhistidine dipeptidase, and homocarnosinase), is a zinc-dependent dipeptidase that catalyzes the hydrolysis of Xaa-His dipeptides. Carnosine, homocarnosine, and anserine are preferred substrates for this mammalian cytosolic enzyme. Other aminoacylhistidine dipeptides are weaker substrates (including homoanserine). The enzyme is activated by thiols and inhibited by metal-chelating agents. O. W. Griffith (1986) Ann. Rev. Biochem. 55, 855. 

There are many dipeptidases [EC 3.4.13.x]. Cytosol nonspecific dipeptidase [EC 3.4.13.18] (also referred to as peptidase A, glycylglycine dipeptidase, glycylleucine dipeptidase, and A -)3-alanylarginine dipeptidase) catalyzes the hydrolysis of dipeptides. Membrane dipeptidase [EC 3.1.13.19] (also known as microsomal dipeptidase, renal dipeptidase, and dehydropeptidase I) is a zinc-dependent enzyme (a member of the peptidase family M19) that also catalyzes the hydrolysis of dipeptides. 

Radhakrishnan, A. N. 1977. Intestinal dipeptidases and dipeptide transport in the monkey and in man. In Peptide Transport and Hydroly i iba Foundation Symposium, -359. New York Elsevier Science Publishers. 

Gobbetti, M., Smacchi, E., Corsetti, A. 1996b. The proteolytic system of Lactobacillus sanfrancisco CB1 Purification and characterization of a proteinase, a dipeptidase, and an aminopeptidase. Appl Environ Microbiol 62 3220-3226. 

Luminal and Membrane Metabolism of Peptides and Proteins. In meaningful studies on peptide and protein drug absorption in the small intestine, it is prerequisite to distinguish among cavital, membrane contact, and intracellular drug metabolism.Cavital metabolism takes place in the lumen of the small intestine by enzymes such as trypsin, chymotrypsin, carboxypepti-dase, and elastase, which are secreted by the pancreas. Membrane contact metabolism is carried out by aminopeptidases lo-calized on the brush border membrane. Intracellular metabolism occurs inside of the cells. The known intra-celluar enzymes are cytoplasmic peptidases, prolidase, dipeptidase, and tripeptidase.A more detailed dis-cussion of this topic is presented in section Intestinal Absorption Barriers, later. 

Aminopeptidases, dipeptidases, and tripeptidases are associated with the intestinal epithelial cells. 

Digestive enzymes produced by the intestinal epithelial cells (aminopep-tidases, dipeptidases, and tripeptidases) cleave the small peptides to amino acids. 

In most cases, the initial product of GSH conjugation is a GSH thioether adduct but this often undergoes a series of subsequent reactions involving a y-glutamyl transferase, a dipeptidase and an acetyl-coenzyme A linked acetylase to yield the corresponding mercapturlc acids (43) (Figure 6). 

Glutathione, y-glutamyl derivatives, and (3-lyase substrates have been investigated as renal-specific prodrugs based on the presence of certain transporters such as plasma membrane transport systems for glutathione [28] and cysteine [29] conjugates and/or enzymes such as y-glutamyltrans-ferase (EC 2.3.2.2), dipeptidase, and p-lyase (EC4.4.1.13). 

Most of the evidence suggests that the enzymes are newly synthesized, though in the majority of cases rigorous proof is unavailable. Enzyme development in a number of seeds is prevented by inhibitors of protein and/or RNA synthesis. For example, dipeptidase and isocitrate lyase development in Cucurbita maxima [92, 105] are suppressed by protein synthesis inhibitors, while actinomycin D, which inhibits some DNA-dependent RNA synthesis, prevents the increase in lipase and isocitrate lyase of castor beans [14, 77]. Studies with inhibitors can be criticized on several grounds especially since these chemicals may have previously unsuspected side effects. But more satisfactory evidence of the kind known for barley aleurone layers has been found in some seeds. De novo synthesis of isocitrate lyase in cotyledons of Citrullus vulgaris (watermelon) [52] and of endopeptidase in mung bean [21 a] have been shown to occur by means of density-labelling experiments with D2O. 

Numerous peptidases with varying specificities have been described. Peptidases may be defined as enzymes that split peptide bonds of only terminal amino acids and thus are exopeptidases. Aminopeptidases hydrolyze peptide bonds involving acids with a free ai-amino group carboxypep-tidase requires that the a-carboxyl group be free. Dipeptidases and tripeptidases will split substrates in which there is only one peptide bond or only two peptide bonds, respectively. Peptidases such as prolinase, prolidase, leucineaminopeptidase, etc., exhibit side-chain specificity requirements as well as backbone requirements. Comprehensive reviews of peptidase activity have recently been written by Smith. 

Enzymes that hydrolyze amide and ester bonds may be divided into three classes (1) those requiring a thiol group for activity, such as papain, ficin, and other plant enzymes (2) those inhibited by diisopropylphosphorofluo-ridate (DFP), such as a-chymotrypin, trypsin, subtilisin, cholinesterase, and thrombin (3) those that require a metal ion for activity. This last class includes dipeptidases, and exopeptidases such as carboxypeptidase and leucine aminopeptidase. The metal ion is involved in the stabilization of the tetrahedral intermediate (refer to Section 4.4.1). 

Analysis of synthetic substrates hydrolyzed by the supernatant one sees in these cells, a good deal of dipeptidase and perhaps even tripeptidase activity, indicating a varied group of peptidases in these cells. In contrast to the blood enzyme (carboxypeptidase-N), enzymes in the white cells do not hydrolyze hippuryl-L-arginine or hippuryl-L-lysine. Therefore, from this evidence and other evidence we have, we have concluded that the type of attack by the kininase in the white cell is not the same as that of the plasma carboxypeptidase. The latter attacks at the carboxyl-terminal arginine of bradykinin. The attack by leucocyte kininases is probably somewhere in the middle of the molecule. 

The chemical nature of the effector groups depends on the substrate attacked. Dipeptidases and amino-peptidases eombine with a free amino group in their substrates, possibly by means of an aldehyde group. The activily of papain, cathepsin, urease, and perhaps arginase, depends on their state of oxidation-reduction as determined by the E—S—S—E + 2H E—SH + HS—E equili-... 

---