Peptidases intracellular

The action of a peptidase can be neutralized by an inhibitor. Some inhibitors are very broad in their action and are capable of inhibiting many different peptidases, including peptidases of different catalytic types. Some inhibitors are assumed to be specific for a particular catalytic type, but can inhibit peptidases of different types. Leupeptin, for example, is widely used as an inhibitor of serine peptidases from family SI, but it is also known to inhibit cysteine peptidases from family Cl. Cysteine pqrtidase inhibitors such as iodoacetic acid interact with the thiol of the catalytic cysteine. However, this reduction can occur on any thiol group and can affect other, predominantly intracellular, peptidases with a thiol dependency. One example is thimet oligopepti-dase. Metal chelators such as EDTA can inhibit meta-llopeptidases, but can also affect peptidases that have a requirement for metal ions that is indq>endent of their catalytic activity, such as the calcium-dependent cysteine endopqrtidase calpain 1. 

The isolation and investigation of intracellular tissue peptidases is more difficult than that of extracellular peptidases, and, as a result, less is known about such peptidases. Illustrative examples of peptidases with their intracellular localization are presented in Table 2.3 [7a],... 

Table 2.3. Subcellular Location of Some Intracellular Peptidases ...

During the last ten years, it has become apparent that calcium-dependent papain-like peptidases called calpains (EC 3.4.22.17) represent an important intracellular nonlysosomal enzyme system [35][36], These enzymes show limited proteolytic activity at neutral pH and are present in virtually every eukaryotic cell type. They have been found to function in specific proteolytic events that alter intracellular metabolism and structure, rather than in general turnover of intracellular proteins. Calpains are composed of two nonidentical subunits, each of which contains functional calcium-binding sites. Two types of calpains, i.e., /i-calpain and m-calpain (formerly calpain I and calpain II, respectively), have been identified that differ in their Ca2+ requirement for activation. The activity of calpains is regulated by intracellular Ca2+ levels. At elevated cytoplasmic calcium concentrations, the precursor procal-pain associates with the inner surface of the cell membrane. This interaction seems to trigger autoproteolysis of procalpain, and active calpain is released into the cytoplasm [37]. 

The long tail of myosin contains a high proportion of the amino acids leucine, isoleucine, aspartate and glutamate. These are released upon the degradation of myosin by intracellular proteases and peptidases and they provide nitrogen for the synthesis of glutamine. It is then stored in muscle and is a very important fuel for immune cells (Chapter 17). 

This enzyme [EC 3.4.22.17] is an intracellular, nonlyso-somal member of the peptidase family C2. The enzyme catalyzes the calcium ion-dependent hydrolysis of peptide bonds with preference for Tyr-Xaa, Met-Xaa, or Arg-Xaa with a leucyl or valyl residue at the P2 position. There are two main types of calpain. One has a high calcium sensitivity in the micromolar range and is called (,-calpain or calpain I. The other calpain has a low calcium sensitivity in the millimolar range and is called m-calpain or calpain II. Forms of calpain exhibiting intermediate calcium sensitivity also exist. 

A modeT that accounts for the selective degradation of proteins based on the amino acid that is present on the amino- or N-end of nascent proteins. Intracellular processing of nascent, noncompartmentalized proteins generates the mature protein via the action of amino-terminal peptidases. In model studies using /3-galactosidase... 

D Silva PD, Schilke B, Walter W, Andrew A, Craig EA (2003) J protein cochaperone of the mitochondrial inner membrane required for protein import into the mitochondrial matrix. Proc Natl Acad Sci USA 100 13839-13844 Dalbey RE, Lively MO, Bran S, van Dijl JM (1997) The chemistry and enzymology of the type I signal peptidases. Protein Sci 6 1129-1138 Daley DO, Clifton R, Whelan J (2002) Intracellular gene transfer reduced hydrophobicity facilitates gene transfer for subunit 2 of cytochrome c oxidase. Proc Natl Acad Sci USA 99 10510-10515... 

The starter cells begin to die off at the end of curd manufacture (Figure 10.21) the dead cells may lyse and release their intracellular endopeptidases (Pep O, Pep F), aminopeptidases (including Pep N, Pep A, Pep C, Pep X), tripeptidases and dipeptidases (including proline-specific peptidases) which produce a range of free amino acids (Figure 10.22). About 150 peptides have... 

Lactic streptococci initiate casein degradation through the action of cell wall-associated and cell membrane-associated proteinases and peptidases. Small peptides are taken into the cell and hydrolyzed to their constituent amino acids by intracellular peptidases (Law and Sharpe 1978). Peptides containing four to seven residues can be transported into the cell by S. cremoris (Law et al. 1976B). S. lactis and S. cremoris have surface-bound peptidases and thus are not totally dependent on peptide uptake for protein use (Law 1979B). Some surface peptidases of S. cremoris are located in the cell membrane, whereas others are located at the cell wall-cell membrane interface (Exterkate 1984). Lactic streptococci have at least six different aminopeptidase activities, and can be divided into three groups based on their aminopeptidase profiles (Kaminogawa et al 1984). 

Proteolytic enzymes such as proteases and peptidases are ubiquitous throughout the body. Sites capable of extensive peptide and protein metabolism are not only limited to the liver, kidneys, and gastrointestinal tissue, but also include the blood and vascular endothelium as well as other organs and tissues. As proteases and peptidases are also located within cells, intracellular uptake is per se more an elimination rather than a distribution process [13]. While peptidases and proteases in the gastrointestinal tract and in lysosomes are relatively unspecific, soluble peptidases in the interstitial space and exopeptidases on the cell surface have a higher selectivity and determine the specific metabolism pattern of an organ. The proteolytic activity of subcutaneous tissue, for example, results in a partial loss of activity of SC compared to IV administered interferon-y. 

To summarize, pre-pro-apoA-I undergoes intracellular cotranslational cleavage to pro-apoA-I (G25). Pro-apoA-I is secreted from the cell and undergoes posttranslational proteolytic cleavage to mature apoA-I in plasma by a pro-apoA-I peptidase (Gl, G25). Pro-apoA-I is isoprotein 2 in plasma and apoA-I is isoprotein 4. Isoprotein 3 seems likely to be a variant of pro-apoA-I, and isoprotein 5 a variant of isoprotein 4. A postulated converting enzyme is responsible for the conversion of pro-apoA-I to mature apoA-I in the plasma compartment. 

Figure 1.1 shows the major sites of enzyme activity in the GI tract, and we will consider each of these in turn. While most of the enzymes that hydrolyse macromolecules enter the gut in the pancreatic fluid and hence are found in the lumen of the gut, there is significant peptidase activity located on the membranes of the intestinal cells, the so-called brush border. Consideration should also be given to the enzymes that are located inside the cells of the intestinal mucosa, namely, the epithelial cells or enterocytes. This is for two reasons first, the intestinal mucosa has a turnover of 3-6 days in humans and this means that the enterocytes are constantly being sloughed-off into the lumen of the gut. Thus intracellular enzymes and brush border enzymes will be found in the lumen of the gut, though the precise quantity is difficult to assess (see later in Section 1.6). 

Hydrolysis of peptides and proteins in the GI tract can occur luminally, at the brash border and intracellularly. Luminal activity from the pancreatic proteases trypsin, chymotrypsin, elastase and carboxypeptidase A is mainly directed against large dietary proteins. The main enzymatic activity against small bioactive peptides is derived from the bmsh border of the enterocyte. Brash border proteases, such as aminopeptidase A and N, diaminopeptidease IV and Zn-stable Asp-Lys peptidase, preferentially cleave oligopeptides of up to 10 ammo acid residues and are particularly effective in the cleavage of tri- and tetra-peptides. 

Concentration of intracellular bacterial peptidases in the vicinity of the protein-fat interface following bacterial lysis... 

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