Intestinal proteolysis

Vitamin B12 is special in as far as its absorption depends on the availability of several secretory proteins, the most important being the so-called intrinsic factor (IF). IF is produced by the parietal cells of the fundic mucosa in man and is secreted simultaneously with HC1. In the small intestine, vitamin B12 (extrinsic factor) binds to the alkali-stable gastric glycoprotein IF. The molecules form a complex that resists intestinal proteolysis. In the ileum, the IF-vitamin B 12-complex attaches to specific mucosal receptors of the microvilli as soon as the chymus reaches a neutral pH. Then either cobalamin alone or the complex as a whole enters the mucosal cell. 

The increased GI absorption of peptides observed in neonates correlates with the decreased intestinal proteolysis that exists in the neonatal state. 

Alumot, E., and Nitsan, Z., 1961, The influence of soybean antitrypsin on the intestinal proteolysis of the chick,... 

Numerous studies have been published on the in vivo metabolism of peptides. However, these studies are concerned mainly with assessment of pharmacokinetic parameters such as half-life and clearance. Only seldom is the in vivo biotransformation of peptides that contain only common amino acids investigated in any detail, due to the difficulty of monitoring products of proteolysis that are identical to endogenous peptides and amino acids. More importantly, such studies fail to yield mechanistic and biochemical insights. For this reason, we begin here with a discussion of the metabolism of just a few peptides in some selected tissues, namely portals of entry (mouth, gastro-intestinal tract, nose, and skin), plasma, organs of elimination (liver, kidney), and pharmacodynamic sites (brain and cerebrospinal fluid). These examples serve as introduction for the presentation in Sect. 6.4.2 of the involvement of individual peptidases in peptide metabolism. 

Redox reaetions and hydrolysis are the predominant metabolie eonversions triggered by the intestinal microflora. The main reductive enzymes produeed by the intestinal mieroflora are nitroreductase, deaminase, urea dehydroxylase, and azoreduetase the hydrolytic enzymes are P-glucoronidase, P-xylosidase, P-galaetosidase, and a-L-arabinosidase. Studies conducted by Macfarlane and co-workers have shown that proteolysis ean also happen in the colon [31]. More recent findings by this group indieate that bacterial fermentation of proteins in humans could account for 17% of... 

Glutamine is present in peptide bonds in the protein in the food but it is also present in the free form in meat and some root vegetables. It is released by proteolysis in the small intestine and absorbed into the enterocytes. Here some of it is metabolised so that only about 50% or less of the absorbed glutamine enters the blood. However, if the food is supplemented with glutamine or if it is ingested as a bolus, a significant proportion enters the blood. To maintain the physiological blood level under normal conditions, it must be synthesised de novo in the body. 

B. Degradation of dietary proteins (proteolysis) is catalyzed by proteases in both the stomach and small intestine. 

Proteolysis. Proteolysis is the cleavage of amide bonds that comprise the backbone of proteins and peptides. The reaction can occur spontaneously in aqueous medium under acidic, neutral, or basic conditions. This process is accelerated by proteases, ubiquitous enzymes that catalyze peptide-bond hydrolysis at rates much higher than occur spontaneously. In humans, these enzymes only recognize sequences of L-amino acids but not d-amino acids. They are found in barrier tissues (nasal membranes, stomach and intestinal linings, vaginal and respiratory mucosa, ocular epithelium), blood, all internal solid organs, connective tissue, and fat. The same protease may be present in multiple sites in the body. 

The liver eliminates proteins on first pass after oral administration and on each pass of hepatic blood flow. Hepatocytes, Kupffer cells, adipocytes, and endothelial cells can all be involved in proteolysis (Figure 5.6). Proteolysis can occur in lysosomes after endocytosis of a protein and lysosomal fusion. Endocytosis of a protein may be a nonspecific or receptor-mediated process. Proteolytic products are eliminated from the liver through biliary excretion, and subsequently digested further in the intestinal tract. 

Hydrophilic polymers are currently undergoing investigation for improving the transport of biomacromolecules across the intestinal walls. Hydrophilic polymers have been shown to protect proteins and peptides from proteolysis. Multiple methods utilize the properties of polymers to protect biomacromolecules without removing them from the aqueous environment of the intestines. 

Peptides, when administered orally, are susceptible to degradation in the stomach by gastric enzymes and the proteinases of the pancreas and brush border of the small intestine. Their lifetimes in the plasma are often short due to rapid proteolysis and other metabolic processes. Early efforts were made to improve the resistance of renin inhibitors to hydrolysis in vivo by the use of blocking groups at the Eland C-terminii [39] and replacement of susceptible peptide bonds other than the renin cleavage site. Studies of SAR have shown that various N- and C-terminal groups, some based on the morpholine nucleus and derivatives of it, have a favorable effect on the duration of inhibition in the... 

Protein catabolism begins with hydrolysis of the covalent peptide bonds that link successive amino acid residues in a polypeptide chain (fig. 22.3). This process is termed proteolysis, and the enzymes responsible for the action are called proteases. In humans and many other animals, proteolysis occurs in the gastrointestinal tract this type of proteolysis results from proteases secreted by the stomach, pancreas, and small intestine. 

Okagawa, T., et al. 1994. Susceptibility of ebiratide to proteolysis in rat intestinal fluid and homogenates and its protection by various protease inhibitors. Life Sci 55 677. 

Proteolysis of peptides and proteins commences in the stomach when pepsin is present. As a result, protein or peptide drugs will be hydrolyzed into smaller fragments like amino acids or oligopeptides, which are absorbed through the mucosa either by diffusion or by a carrier-mediated transport [18], In an average individual, 94-98% of the total protein is completely digested and absorbed [19], Proteolysis continues in the intestine with pancreatic enzymes like trypsine and brush-border enzymes. 

Drags that structurally resemble nutrients such as polypeptides, nucleotides, or fatty acids may be especially susceptible to enzymatic degradation. For example, the proteolytic enzymes chymotrypsin and trypsin can degrade insulin and other peptide drags. In the case of insulin, proteolysis was shown to be reduced by the coadmmistration of carbopol polymers at 1% and 4% (w/v%), which presumably shifted the intestinal pH away from the optimal pH for proteolytic degradation. 

Most biotin in foods is present as biocytin, incorporated into enzymes, which is released on proteolysis, then hydrolyzed by biotinidase in the pancreatic juice and intestinal mucosal secretions to yield free biotin. Biocytin is not absorbed to any significant extent. 

Biocytin is hydrolyzed by biotinidase, which acts on free or peptide-incorporated biocytin to release biotin, but has no general peptidase or esterase activity. Biotinidase is most active toward free biocytin, but it will also release biotin from biocytin-containing peptides. The activity decreases as the size of the peptide increases, so it is likely that in vivo the catabolism of biotin-containing enzymes is by proteolysis, followed by biotinidase action, rather than the release of biotin, leaving the apoenzyme as a substrate for proteolysis. Biotinidase is found in all tissues, including the pancreatic juice and intestinal mucosa. 

Proteolysis of the crl Protein Regulates Viral Growth in the Intestine and... 

One of the earliest suggestions that total enzymatic hydrolysis was possible came from the studies of Frankel (1916), who showed that over 90 % of the bonds in several proteins could be broken when proteolysis with pepsin, trypsin, and chymotrypsin was followed by prolonged hydrolysis with the erepsin preparation of Cohnheim (1901). The recognition in later years of several peptidases in intestinal exti acts which will specifically act upon bonds that are not susceptible to the endopoptidases (Bcrg-mann, 1942) probably accounts for these obseiwations. The specific peptidases such as prolidase, iminodipeptidase (prolinase), glycylglycine dipeptidase, tripeptidase, and leucine aminopeptidase, whi( h are present in mucosa, attack many of the bonds that resist the action of endopoptidases. 

Proteolysis also provides carbon skeletons for gluconeogenesis. During starvation, degraded proteins are not replenished and serve as carbon sources for glucose synthesis. Initial sources of protein are those that turn over rapidly, such as proteins of the intestinal epithelium and the secretions of the pancreas. Proteolysis of muscle protein provides some of three-carbon precursors of glucose. However, survival for most animals depends on being able to move rapidly, which requires a large muscle mass, and so muscle loss must be minimized. 

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