|MadSci Network: Biochemistry|
The vast majority of proteins will be denatured in the stomach by the acid, assuming that the pH is normal. Given the number of people taking drugs like Prilosec/omeprazole and that 10-20% of elderly patients have achlorhydria (no acid production), there are a substantial number of people where little denaturation occurs. The lack of acid also diminishes or eliminate pepsin activation in the stomach, so there is less proteolysis in the stomach. (This has no apparent physiological consequences however.) The pH in the duodenum is still slightly acid, aroudn 5.5-6.5 normally. We are still apparently able to digest protein adequately even if the stomach acid and pepsin steps are absent. Virtually all protein is degraded. The epithelial lining of the intestine, assuming it is intact, does not normally allow uptake of large molecules. Some smaller peptides/proteins probably leak through the tight junctions, but this is minor. Amino acids and very small peptides can be transported across the epithelial cell layer and deposited in the blood. The major exception to this is the immune system. The immune surveillance system of the gut is able to take up immunoglobuliins and also able to sample proteins from the lumen of the intestine. The immunoglobulins are taken up by specific transport systems, but the proteins are largly taken up by pinocytosis or some other bulk method. The total amount of protein taken up is very, very small compared to the total protein/amino acid in the intestine. It's purely for immune surveillance not for "food". As specifically for lectins, they can bind to specific molecules on the cell surface and trigger endocytosis/pinocytosis. This certainly occurs in the immune surveillance cells, for example, Peyer's patches. It likely can occur in the epithelial cells also. This is not however a transport mechanism for the purpose of obtaining protein/food. The lectins recognize specific protein structures. These in turn are simple activated to internalize. This is still a kind of surveillance. Lectins have evolved to recognize protein structures in cells. This enables the cell they are on to attach to other cells.
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