Intestine important features

Nonrepetitive but well-defined structures of this type form many important features of enzyme active sites. In some cases, a particular arrangement of coil structure providing a specific type of functional site recurs in several functionally related proteins. The peptide loop that binds iron-sulfur clusters in both ferredoxin and high potential iron protein is one example. Another is the central loop portion of the E—F hand structure that binds a calcium ion in several calcium-binding proteins, including calmodulin, carp parvalbumin, troponin C, and the intestinal calcium-binding protein. This loop, shown in Figure 6.26, connects two short a-helices. The calcium ion nestles into the pocket formed by this structure. 

Figure 3.12 The mammalian gastrointestinal tract showing important features of the small intestine, the major site of absorption for orally administered compounds (A) liver (B) stomach (C) duodenum (D) ileum (E) colon (F) longitudinal section of the ileum showing folding, which increases surface area (G) detail of fold showing villi with circular and longitudinal muscles, (H) and (I) respectively, bounded by (J) the serosal membrane (K) detail of villi showing network of (L) epithelial cells, (M) capillaries, and (N) lacteals (O) detail of epithelial cells showing brush border or (P) microvilli. The folding, vascularization, and microvilli all facilitate absorption of substances from the lumen. Source From Ref. 1.

These external neural influences on intestinal motility are common targets for prokinetic drugs, but events within the bowel can have important effects on intestinal motility and cause the bowel to be refractory to traditional prokinetic therapy. Release of cytokines from activated inflammatory cells is probably an important feature of ileus in many cases. Ileus secondary to reperfusion injury is an anticipated response in horses with small intestinal obstruction. However, even apparently mild intestinal injury can initiate cellular responses that lead to impaired motility. Mild intestinal insult by gentle surgical manipulation activated adhesion molecules on leukocytes and increased the expression of P-selectin and intercellular adhesion molecule 1 on endothelial cells within the vasculature of the muscularis layer of the intestine (Kalff et al 1999). Surgical manipulation of the rodent small intestine resulted in substantial extravasation of leukocytes into the intestinal muscularis, consisting mainly of polymorphonuclear neutrophils, monocytes and mast cells and lasting for days. This cellular inflammatory response within the intestinal muscularis externa was associated with a marked decrease in jejunal circular muscle activity (Kalff et al 1998). 

In this section, we describe the factors that influence the movement of water in and out of cells, an important feature of the life of both plants and animals. The following section discusses other transport phenomena that are critical to essential physiological processes, focusing on the asymmetrical distribution of certain transport proteins in epithelial cells. We will see how this permits absorption of nutrients from the intestinal lumen and acidification of the stomach lumen. 

The way in which treatments modulate the intestinal microbial community is an important feature but is seldom investigated. Even when this is the case, the observations have often remained limited to increases or decreases... 

The metabolic roles and regulation of glucose-6-P phosphohydrolase activity have been considered in detail in reviews by Cahill et al. (6) and Ashmore and Weber (7). More recently, possible metabolically important roles for phosphotransferase activities of this enzyme in liver, kidney, and intestine have been described in reviews by the present author (9, 10)—who also considered a variety of regulatory features based on interaction of substrates, inhibitors, and activators with the multifunctional enzyme—and by Cohn et al. (11). 

Features of the test substance aminopyrine (dimethyl-aminopyrine) are (i.) rapid and complete intestinal resorption, (2.) low protein binding, and (i.) almost exclusive metabolization by the liver. The aminopyrine test is known to be non-invasive, methodologically simple and quick to perform. Of importance is a resting period for the patient of at least 30 minutes prior to the intravenous administration of C-aminopyrine in a dosage of 1.5 fCi. Depending on the liver function, radiation exposure of the patient corresponds to 0.6-2.5 mrem (= 1-2 thorax x-rays). The test is also influenced by numerous medicaments and alcohol as well as any alteration in the basal metabolic rate. The procedure finishes with a triple breath test at ten-minute intervals. 

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