Epithelium acid neutralization

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. 

Resistance to acid may therefore depend more on the ability of the esophageal epithelium to neutralize an add load than on restriction of entry of acid between the cells. Neutralization of acid occurs at the surface due to net HCOf secretion and also diffusion across the paracellular pathway, because... 

Vitamin E, like neutral lipids, requires apoB lipoproteins at every stage of its transport (Fig. 27-2). Dietary vitamin E becomes emulsified in micelles produced during the digestive phase of lipid absorption and permeates the intestinal epithelium, similar to fatty acids and cholesterol. Uptake of vitamin E by enterocytes appears to be concentration dependent. Within intestinal cells, vitamin E is packaged into chylomicrons and secreted into lymph. During blood circulation of chylomicrons, some vitamin E may be released to the tissues as a consequence of partial lipolysis of these particles by endothelial cell-anchored lipoprotein lipase. The rest remains associated with chylomicron remnants. Remnant particles are mainly endocy-tosed by the liver and degraded, resulting in the release of fat-soluble vitamins. 

Hartnup disorder is an autosomal recessive impairment of neutral amino acid transport affecting the kidney tubules and small intestine. It is believed that the defect is in a specific system responsible for neutral amino acid transport across the brush-border membrane of renal and intestinal epithelium, but the defect has not yet been characterised. 

Maternal Immunoglobulins (antibodies) contained in ingested breast milk are transported across the intestinal epithelial cells of the newborn mouse and human by transcytosis (Figure 17-34). The F receptor that mediates this movement binds antibodies at the acidic pH of 6 found in the intestinal lumen but not at the neutral pH of the extracellular fluid on the basal side of the Intestinal epithelium. This difference in the pH of the extracellular media on the two sides of intes-... 

The secretion of HCOs by gastric mucosa is at most 10% of acid output. Assuming that the stomach is secreting 100 mM HCl, 10 mM HCOs can be secreted. Hence, this base is not able to neutralize a gastric solution at a pH of 1.0. However, if the stomach is only secreting 10 mM HCl, this can be neutralized by 10 mM HCOj. Secretion of the base could, in principle, then neutralize an unbuffered solution at a pH of 2.0. However, below a pH of 2.0, the neutralizing capadty of the epithelium is exceeded, and gastric surface pH must become addic, since there is no real barrier to proton back diffusion. 

An integrated model of acid resistance of the gastric epithelium, where the surface cell provides neutralization and generally a proton impermeable barrier. The parietal cell extrudes bicarbonate into the serosal side to supply this anion to the surface cell, and acid also appears to stimulate surface cell bicarbonate secretion. 

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