Cells free amino acids

There are two main classes of proteolytic digestive enzymes (proteases), with different specificities for the amino acids forming the peptide bond to be hydrolyzed. Endopeptidases hydrolyze peptide bonds between specific amino acids throughout the molecule. They are the first enzymes to act, yielding a larger number of smaller fragments, eg, pepsin in the gastric juice and trypsin, chymotrypsin, and elastase secreted into the small intestine by the pancreas. Exopeptidases catalyze the hydrolysis of peptide bonds, one at a time, fi"om the ends of polypeptides. Carboxypeptidases, secreted in the pancreatic juice, release amino acids from rhe free carboxyl terminal, and aminopeptidases, secreted by the intestinal mucosal cells, release amino acids from the amino terminal. Dipeptides, which are not substrates for exopeptidases, are hydrolyzed in the brush border of intestinal mucosal cells by dipeptidases. 

The series of molecular events responsible for the uptake process constitutes the endocytic pathway, which enables cells to internalize macromolecules from the cell exterior, forming an endosome. The endosome is an intermediate organelle that serves as an essential component for many receptor-mediated signaling pathways and as a transport vector for eventual delivery to a specialized organelle known as the lysosome. Once in the lysosomal lumen, digestive enzymes provide essential metabolites from these macromolecules (i.e. free amino acids and lipids) directly to the cytosol for their use. 

Additionally, amino acids may be reclaimed as dipeptides. The transport mechanisms for dipeptides are less specific than those for individual amino acids but require the dipeptide to carry a net positive charge so there is cotransport of protons, rather than of Na+ as for free amino acids. A potential advantage of dipeptide transport process is the favourable cell-lumen concentration gradient, which exists for peptides compared with free amino acids. 

There are at least two answers to question (i). First, abnormal proteins can arise in cells due to spontaneous denaturation, errors in protein synthesis, errors in post-translational processing, failure of the correct folding of the protein or damage by free radicals. They are then degraded and replaced by newly synthesised proteins. Secondly, turnover helps to maintain concentrations of free amino acids both within cells and in the blood. This is important to satisfy the requirements for synthesis of essential proteins and peptides (e.g. hormones) and some small nitrogen-containing compounds that play key roles in metabolism (see Table 8.4). 

Although free amino acids and those in proteins in eukaryotes are entirely of the L-form (except glycine, which is not optically active), D-amino acids do occur in nature, for example in bacterial cell walls (D-alanine and D-glutamate). Consequently, they enter the body from bacteria in food and from the digestion of bacteria in the... 

Combinations of several enzymes with different specificities are required for complete degradation of proteins into free amino acids. Proteinases and peptidases are found not only in the gastrointestinal tract (see p. 268), but also inside the cell (see below). 

The most superficial layer of skin is the stratum comeum (SC), which consists of terminally differentiated keratinocytes (comeocytes) that originate from actively proliferating keratinocytes in lower epidermis (basale, spinosum, and granulosum cells), and contain a lamellar lipid layer secreted from lamellar bodies (Fig. 7a). Flydration of the SC is an important determinant of skin appearance and physical properties, and depends on a number of factors including the external humidity, and its structure, lipid/protein composition, barrier properties, and concentration of water-retaining osmolytes (natural moisturizing factors, NMFs) including free amino acids, ions, and other small solutes. 

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... 

Free amino acids and dipeptides are taken up by the intestinal epithelial cells. There, the dipeptides are hydrolyzed in the cytosol to amino acids before being released into the portal system. Thus, only free amino acids are found in the portal vein after a meal containing protein. These amino acids are either metabolized by Ihe liver or released into the general circulation. 

The amino acid pool is defined as all the free amino acids in cells and extracellular fluid. 

In the stomach, hydrochloric acid denatures dietary proteins, making them more susceptible to proteases. Pepsin, an enzyme secreted in zymogen form by the serous cells of the stomach, releases peptides and a few free amino acids from dietary proteins. 

In the small intestine, proteases released by the pancreas as zymogens become active. Each has a different specificity for the amino acid R-groups adjacent to the susceptible peptide bond. Examples of these enzymes are trypsin, chymotrypsin, elastase, and car-boxypeptidase A and B. The resulting oligopeptides are cleaved by aminopeptidase found on the luminal surface of the intestine. Free amino acids and dipeptides are then absorbed by the intestinal epithelial cells. 

A quantitatively important pathway of cysteine catabolism in animals is oxidation to cysteine sulfinate (Fig. 24-25, reaction z),450 a two-step hydroxyl-ation requiring 02, NADPH or NADH, and Fe2+. Cysteine sulfinic acid can be further oxidized to cyste-ic acid (cysteine sulfonate),454 which can be decarbox-ylated to taurine. The latter is a component of bile salts (Fig. 22-16) and is one of the most abundant free amino acids in human tissues 455-457 Its concentration is high in excitable tissues, and it may be a neurotransmitter (Chapter 30). Taurine may have a special function in retinal photoreceptor cells. It is an essential dietary amino acid for cats, who may die of heart failure in its absence,458 and under some conditions for humans.459 In many marine invertebrates, teleosts, and amphibians taurine serves as a regulator of osmotic pressure, its concentration decreasing in fresh water and increasing in salt water. A similar role has been suggested for taurine in mammalian hearts. A chronically low concentration of Na+ leads to increased taurine.460 Taurine can be reduced to isethionic acid... 

In addition to the 20 amino acids most frequently found in proteins a large group of amino acids occur in plants, bacteria, and animals that are not found in proteins. Some are found in peptide linkages in compounds that are important as cell wall or capsular structures in bacteria or as antibiotic substances produced by bacteria and fungi. Others are found as free amino acids in seeds and other plant structures. Some amino acids are never found in proteins. These nonprotein amino acids, numbering in the hundreds, include precursors of normal amino acids, such as homoserine and diaminopimelate intermediates in catabolic pathways, such as pipecolic acid d enantiomers of normal amino acids and amino acid analogs, such as azetidine-2-carboxylic acid and canavanine, that might be formed by unique pathways or by modification of normal amino acid biosynthetic pathways. 

Gly) formed in the reaction is transported by an uncharacterized transport system (3) and cleaved by an intracellular protease (4) or cleaved by a membrane-bound protease and the free amino acids transported. The y-glutamyl cycle enzymes are found in those tissues for which the transport of glutathione into cells is an important function. This includes liver and kidney cells in animals. 

Dally, J. E., Gomiak, J., Bowie, R., and Bentzley, C. M. (2003). Quantitation of underivatized free amino acids in mammalian cell culture media using matrix assisted laser desorption ionization time-of-flight mass spectrometry. Anal. Chem. 75 5046-5053. 

The action of the gastric and pancreatic enzymes causes the release of small peptides as well as free amino acids, the peptide fraction being the quantitatively dominant one (9). Thus further hydrolysis is crucial, if the dietary protein is to be completely utilized by the organism. The final stages of hydrolysis is associated with the intestinal mucosal cells. Larger peptides are probably hydrolyzed by enzymes at the brush border membrane. Di- and tripeptides may be absorbed as such and hydrolyzed intracellularly (9, 10). 

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