Proteins absorption, mechanism

Although the detailed mechanism of the longer circulation time of branched PEGylated protein is unclear, the architecture of PEG affects the release profile, the pharmacokinetics of the chug [29], and the behavior of the protein at the interface (e.g., protein absorption on hydrophobic surfaces [30]). 

Burton, P.S., Conradi, R.A. and Hilgers, A.R. (1991) Mechanisms of peptide and protein absorption. (2). Transcellular mechanism of peptide and protein, absorption passive aspects. Advanced Drug Delivery Reviews, 7, 365-386. 

In foods vitamin B2 occurs free or combined both as FAD and FMN and complexed with proteins. Riboflavin is widely distributed in foodstnffs, but there are very few rich sources. Only yeast and liver contain more than 2mg/100g. Other good sources are milk, the white of eggs, fish roe, kidney, and leafy vegetables. Since riboflavin is continuously excreted in the urine, deficiency is qnite common when dietary intake is insufficient. The symptoms of deficiency are cracked and red lips, inflammation of the lining of the month and tongue, mouth ulcers, cracks at the comer of the mouth, and sore throat. Overdose of oral intake present low toxicity, probably explained by the limited capacity of the intestinal absorption mechanism [417]. 

Polybrominated Biphenyls. The mechanism by which PBBs enter the blood stream from the lungs, skin, or gastrointestinal tract is not known and little information is available on how PBBs are distributed in the body. The available data indicate that the absorption mechanism is likely passive diffusion. Results from studies of Michigan subjects showed that in the blood stream, 80% of the PBBs was bound to protein and 20% was associated with lipids (Greaves et al. 1984). Of the fraction bound to protein, 73% was bound to apolipoprotein B and the remaining percent was bound to apolipoprotein A. In an in vitro model, shown to be representative of environmentally contaminated blood, the distribution of PBBs among plasma, erythrocytes, mononucleocytes, and polymorphonucleocytes was 89 9 <1 <1, respectively (Roboz et al. 1985). 

It is well established today that drug absorption through the alimentary canal walls is a complex event, which involves, in many cases, parallel or sequent microprocesses at the apical membrane of the absorptive cell (enterocyte) or between them (paracellular absorption). In addition to the various types of diffusion processes across the enterocyte membrane, numerous specific proteins—transporters and efflux pumps—are involved in the intricate drug absorption process. In the following sections the various epithelial tissues of the different organs of the GI tract will be looked at briefly. A review of major drug absorption mechanisms across epithelial cells, as they are customary today will follow. 

Nellans, H.N. 1991. Mechanism of peptide and protein absorption, paracellular intestinal transport Modification of absorption. Adv Drug Deliv Rev 7 339. 

Mechanisms of Protein Absorption after Pulmonary Delivery... 

We have recently reported that the action of protease inhibitors as dietary anticarcinogens may work via two mechanisms 1) An indirect effect on protein absorption and 2) A direct effect on cell transformation (Cancer Res,... 

Thirty years of research with bacteriorhodopsin has provided answers to many questions about how protons are transported by transmembrane pumps. In this small seven-transmembrane protein, absorption of light by the retinal chromophore Initiates a reaction cycle in which the initial state recovers through multiple conformational changes of the retinal and the protein, and a proton Is translocated stepwise from one side of the membrane to the other. Spectroscopy, extensive use of site-specific mutations, and crystallography have defined the photocycle reactions in atomic detail and provide a step-by-step description of the proton transfers, the transient local and global perturbations in the protein and how they arise, and the energy flow through the system, which add up to the mechanism of the pump. 

Fig. 1 Natural absorptive mechanism. The body absorbs peptides and proteins into the bloodstream by a natural process known as transcytosis which occurs deep in the lung. Transcytosis is the process by which large molecules move across an impermeable cell membrane without creating holes in the cells and destroying the barrier. It is performed by tiny membrane bubbles, or transcytotic vesicles, which form invaginations of the cell membrane on one side of the cell and dissolve back into the membrane on the other side of the cell. The result is that small volumes of alveolar fluid, including dissolved proteins, are carried by a bucket brigade from one side of a cell to the other.

Corradino RA. 1972. Strontium inhibition of the vitamin D-induces calcium-binding protein and the intestinal calcium absorptive mechanism. In International Conference on Strontium Metabolism, ed. Second international conference on strontium metabolism, Glasgow and Strontian, 16-19 August, 1972. 

Another approach is to proceed from the chemical composition. This may work for some fairly dry foods, although it implicitly assumes an absorption mechanism, which is that certain chemical groups bind certain amounts of water, and by determining the concentration of these groups, the water sorption can be calculated. It concerns especially ionized groups (a few water molecules per group) and dipoles, such as a peptide bond (<1 water molecule per group). This method works reasonably well for proteins around aw = 0.5. 

Whatever the coupling agent is, the control of non-specific protein absorption is important to the use of nanomaterials in specific protein binding. There are plenty of molecules used for protection of various surfaces from proteins with mechanisms as steric repulsion, hydration and solvent structuring. For example, the modification of CNTs with the absorption of biotinylated Tween 20 allowed streptavidin recognition by the specific biotin-streptavidin interaction, but provided resistance towards other protein absorption [133]. 

The mechanisms whereby peptides and protein absorption was improved by absorption enhancers were examined from various aspects. These mechanisms involve an increase in membrane fluidity, expansion of the dimension of the intercellular space, solubiHzation of the mucosal membrane, increase in water flux, and reduction of the viscosity of the mucus layer adhering to all mucosal surfaces [20]. Furthermore, for peptides and proteins, inhibition of peptidase activity is an important factor to improve absorption [21]. 

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