Cytosolic leucine aminopeptidase

A novel concept of using bioadhesive polymers as enzyme inhibitors has been developed [97]. Included are derivatives of poly acrylic acid, polycarbophil, and car-bomer to protect therapeutically important proteins and peptides from proteolytic activity of enzymes, endopeptidases (trypsin and a-chymotrypsin), exopeptidases (carboxypeptidases A and B), and microsomal and cytosolic leucine aminopeptidase. However, cysteine protease (pyroglutamyl aminopeptidase) is not inhibited by polycarbophil and carbomer [97]. 

Many aminopeptidases are metalloenzymes.437 Most studied is the cytosolic leucine aminopeptidase which acts rapidly on N-terminal leucine and removes other amino acids more slowly. Each of the subunits of the hexameric enzyme contains two divalent metal ions, one of which must be Zn2+ or Co2+438/439 A methionine aminopeptidase from E. colt contains two Co2+ ions440/441 and a proline-specific aminopeptidase from the same bacterium two Mn2+.442 In all of these enzymes the metal ions are present as dimetal pairs similar to those observed in phosphatases and discussed in Section D,4 and to the Fe-Fe pairs of hemerythrin and other diiron proteins (Fig. 16-20). A hydroxide ion that bridges the metal ions may serve as the nucleophile in the aminopeptidases.438 A bound bicarbonate ion may assist.4383... 

Many aminopeptidases are metalloenzymes. Most studied is the cytosolic leucine aminopeptidase which acts rapidly on N-terminal leucine and removes other amino acids more slowly. Each of the subunits of the hexameric enzyme contains two divalent metal ions, one of which must be Zn + or A me-... 

These enzymes have been linked here because they have some common applications in diagnostic enzymology. Alanine aminopeptidase (AAP) and leucyl arylamidase (LAAP) hydrolyze the N-terminal amino acids and some amino amides the enzymes respectively hydrolyze leucyl- and alanyl-4-nitroanilide substrates. These enzymes occur in microsomes and are also membrane bound they have been used in studies of both hepatotoxicity and nephrotoxicity. They should not be confused with cytosolic leucine aminopeptidase (LAP) this enzyme is an aminopeptidase that hydrolyzes N-amino acid residues of proteins, in particular those with an N-terminal 1-leucine, where l-leucyl-(3-napthylamide is commonly used as substrate. Urinary alanine aminopeptidase is a useful marker of nephrotoxicity (Jung and Scholz 1980). 

Aminopeptidases that catalyze the hydrolysis of cysteinyl peptides are known. The membrane-bound aminopeptidases are glycoproteins, usually with molecular weights of about 100,000 daltons. They appear to be metalloproteins, one of the better known being a zinc-containing enzyme. Other enzymes, such as the leucine aminopeptidase, are cytosolic but, at least in this case, are also zinc-containing. The substrate specificity of these enzymes varies but most are relatively nonspecific. 

Proteasomes rather than cytosolic carboxy-peptidases act to trim the C-terminal amino acids to conform the peptide to the proper size for MHC class-I presentation. Presentation from N-extended precursors is inhibited by acetylation of the terminal a-amino group at the N-terminus [354], which prevents the peptide to be cleaved by aminopeptidases e.g. leucine aminopeptidase) but not by proteasomes or endopeptidases. The TAP system transports peptides to the ER including both mature epitopes and longer precursors. It seems then that the peptides to be presented by MHC class-I can arise from N-extended precursors both in the cytosol and in the endoplasmic reticulum (ER). This assertion has been experimentally confirmed [355,356]. 

Cytosol lactate dehydrogenase leucine aminopeptidase p-glucosidase fructose-1,6 biphosphatase (proximal tubule) pyruvate kinase (distal tubule)... 

Leucine aminopeptidase preferentially hydrolyses peptide bonds adjacent to an Al-terminal residue that carries a large hydrophilic side chain, in particular a leucyl residue. Commonly used synthetic substrates are leucinamide, leucine 4-nitroanilide and leucine hydrazide. This cytosolic zinc-metalloenzyme has been identified in virtually all animal tissues, and most studies have been performed on the enzyme (Mj 324,000) from bovine lens, which has been crystallized. It consists of 6 identical subunits (M, 54,000 487 amino acids 2 Zrf per subunit), and its catalytically active site is in the C-terminal domain. The enzyme is present in many other cells and tissues, e.g. lung, stomach, kidney intestine, serum and leukocytes. In clinical chemistry, this enzyme is a marker for hepatic cell lysis, and may even be a more sensitive marker for acute hepatitis than the aminotransferases. 

A -Protected a-aminoalkyl-//-phosphinic acids (857) were found to be novel practical building blocks in three-component Mannich-type A -phosphonomethylation condensations with formaldehyde (858) and secondary amine/amino acids (859). This reaction led to multifunctional phosphinic pseudodipeptides (860), designed to act as extended transition state analogue inhibitors of selected cytosolic leucine and microsomal alanyl aminopeptidases (Scheme 215). ... 

Aminopeptidase, cytosol Aminopeptidase, leucine Aminopropyltransferase, putrescine Aminotransferase Aminotransferase, alanine Aminotransferase, aspartate Aminotransferase, glutamate-glyoxylate Aminotransferase, ornithine-keto acid Aminotransferase, serine-glyoxylate Ammonia... 

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