Proteolysis absorption

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. 

It is probable that proteases such as trypsin and bromelain, which are somewhat stable to self-proteolysis, would be absorbed to a greater extent than otho1 proteins of comparable size. Coadimiustraiion of soybean trypsin inhibitors or pancreatic duct ligation increases the absorption of intact protein [39]. 

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

In the diet, vitamin B12 is bound to proteins. Although some release of protein-bound vitamin B12 begins in the mouth, most of the release occurs in the stomach on exposure of food to gastric acid (HC1) and the proteolytic enzyme pepsin. For this reason, either hypo-chlorhydria (abnormally low concentration of HC1 in gastric fluid) or achlorhydria (the absence of HC1 in gastric fluid) may decrease the availability of dietary vitamin B12 for absorption by preventing the activation of pepsinogen to pepsin, the principal enzyme responsible for proteolysis in the stomach. Achlorhydric patients with adequate production of IF may have low normal or subnormal serum B12 concentrations because of failure to liberate B12 bound to food. 

Degradation in the gut is caused by endogenous proteases such as trypsin and a-chymotrypsin. Entrapment of macromolecules in liposomes results in better protection against proteolysis [30], Since lipid-soluble molecules are absorbed quickly in the gut, it could be expected that liposome delivery will give enhanced absorption by diffusion. However, this problem is not simple, as illustrated by the lack of uptake of liposomes by enterocytes [30,31]. Insulin entrapped in liposomes is absorbed in this way [32],... 

Recently Puski (20) determined the effects of proteolysis on the water absorption, viscosity, and gelation properties of a soy protein isolate. The water absorption of enzymically hydrolyzed soy protein isolate increased in direct relation to the enzymic treatment. Apparently as the number of polar amino and carboxyl groups increased, the uptake of water increased proportionately. Limited proteolysis of the soy protein isolate resulted in a decided decrease in the viscosity and gelling properties. 

Phosphopeptides. It is claimed that phosphopeptides prepared from casein hydrolysates stimulate the absorption of Ca in the intestine, but views on this are not unanimous. Such peptides are resistant to proteolysis due to the high density of negative charges they have been detected in the small intestine of the rat and may pass intact through the intestinal wall. Since... 

PROPERTIES OF SPECIAL INTEREST Natural fibers with high strength and comphance, high energy absorption before failure, durable fibers with high luster, resistant to proteolysis. 

Protein, as well as nonprotein energy (i.e., carbohydrates and fats), must be provided in sufficient amounts to drive protein synthesis and prevent protein-energy deficiency [18]. However, too many nonprotein calories will inCTease weight but not lean body mass, which may be the case in certain patients with inherited metabohc disorders on low protein, high caloric intakes. The type of nonprotein energy can make a difference on protein status. Studies have indicated that carbohydrate (CHO), and not fat, can rednce postprandial protein degradation [19,20]. Net protein utilization improved by 5 % and nitrogen retention by 14 % when carbohydrate was offered. Excess CHO withont protein stimulates post-absorptive proteolysis and protein synthesis [21],... 

The subcutaneous route for administration of insulin has many serious drawbacks, and alternative routes continue to attract considerable research interest. Nearly all available orifices of the human body seem to have gained attention as presenting possible noninvasive sites for insulin absorption. However, even by using modem enhancer techniques, only a small or minor fraction of the hormone becomes bioavailable when provided by most of these routes, except perhaps the pulmonary route. Key barriers to insulin absorption via the alternative routes are the resistance of those membranes to insulin penetration, the tendency of insulin to exist in associated form, and insulin proteolysis. Protection from proteolysis through some sort of encapsulation, the use of complex emulsion systems, and/or the use of protease inhibitors—association of the hormone with polymeric particles, and addition of permeation enhancers have been utilized to overcome those barriers. The absorption and enzymatic barriers to nonparenterally administered protein dmgs and the use of enhancers to modify absorption have been discussed in recent reviews (Lee, 1986 Lee e/a/., 1991a Zhou, 1994). The present review of alternative administration of insulin mainly covers investigations published since 1970. 

Riboflavin is delivered in form of free vitamin, or as its coenzymes, i.e. flavin mononucleotide (FMN) and adenine dinucleotide (FAD), which occurs mainly as a prosthetic group of flavoproteins. Release of coenzymes from flavoproteins by acidification in stomach and proteolysis, both gastric and intestinal, must precede the absorption. This hydrolysis also releases several percentages of covalently bound FAD from 8a-(peptidyl)riboflavins (Chia et al. 1978). Free riboflavin is physiologically preferred form of absorbed vitamin B2 (Daniel et al. 1983). The upper small intestine enzymes which catalyse reversible reactions of conversion nucleotides into riboflavin are located in the brush-border membrane of enterocytes (Figure 36.1). 

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