Galactose intestinal absorption

Play, B. et al. (2003). Glucose and galactose regulate intestinal absorption of cholesterol. Biochem. Biophys. Res. Commun. 310(2) 446 151. 

Studies on intestinal absorption were carried out in vivo (102). It was found that in contrast to the situation in kidney the galactose transport mechanism was well developed in the small intestine of newborn... 

Interactions of cephradine and cefaclor with the intestinal absorption of D-galactose. J Pharm Pharmacol... 

Glucose and galactose are absorbed by a sodium-dependent active process (section 3.2.2.3). The sodium pump and the sodium/potassium ATPase create a sodium gradient across the membrane the sodium ions then re-enter the cell together with glucose or galactose. These two monosaccharides are carried by the same transport protein, and compete with each other for intestinal absorption. 

Glucose and galactose enter the absorptive cells by way of secondary active transport. Cotransport carrier molecules associated with the disaccharidases in the brush border transport the monosaccharide and a Na+ ion from the lumen of the small intestine into the absorptive cell. This process is referred to as "secondary" because the cotransport carriers operate passively and do not require energy. However, they do require a concentration gradient for the transport of Na+ ions into the cell. This gradient is established by the active transport of Na+ ions out of the absorptive cell at the basolateral surface. Fructose enters the absorptive cells by way of facilitated diffusion. All monosaccharide molecules exit the absorptive cells by way of facilitated diffusion and enter the blood capillaries. 

Source and kinds of disaccharidases The final digestive processes occur at the mucosal lining of the small intestine. Several disaccharidases [for example, lactase (p-galactosidase), sucrase, maltase, and isomal-tase] produce monosaccharides (glucose, galactose, and fructose). These enzymes are secreted by and remain associated with the luminal side of the brush border membranes of intestinal mucosal cells. Absorption of the monosaccharides requires specific trans porters. 

Other cotransporters facilitate the transport of other sugars, osmolytes, and amino acids. In humans, a disorder of intestinal glucose and galactose absorption is due to a defective sodium-glucose transporter. 

Glucose, galactose, and fructose formed by the digestive enzymes are transported into the absorptive epithelial cells of the small intestine by protein-mediated Na -dependent active transport and facilitative diffusion. Monosaccharides are transported from these cells into the blood and circulate to the liver and peripheral tissues, where they are taken up by facilitative transporters. Facilitative transport of glucose across epithelial cells and other cell membranes is mediated by a family of tissue-specific glucose transport proteins (GLUT I-V). The type of transporter found in each cell reflects the role of glucose metabolism in that cell. 

Beans, peas, soybeans, and other leguminous plants contain oligosaccharides with (1,6)-linked galactose residues that cannot be hydrolyzed for absorption, including sucrose with 1, 2, or 3 galactose residues attached (see Fig. 27.10). What is the fate of these polysaccharides in the intestine ... 

Fig. 27.12. Na -dependent and facilitative transporters in the intestinal epithelial cells. Both glucose and fructose are transported by the facilitated glucose transpxrrters on the luminal and serosal sides of the absorptive cells. Glucose and galactose are transported by the Na -glucose cotransporters on the luminal (mucosal) side of the absorptive cells.

The host enzymes that cleave various dietary substances to allow absorption by the host are not uniformly distributed along the Intestinal tract. Some are located In the proximal portion of the small Intestine (e.g., the lactase enzymes) others are located In the distal portion of the small Intestine and In the colon Itself (e.g., the maltase enzymes). This localization alone allows one to make certain Inferences about the presence or absence of the substrates for these cleavage and absorption enzymes. Furthermore, the different absorption systems within the same general region often exhibit very different activities. For example, the host s transport system for the absorption of galactose Is very active and nearly Impossible to saturate under physiological conditions whereas that for the absorption of arablnose Is relatively Ineffective. Thus, from Indirect Information concerning the relative abundance of various substances In the diet, the localization of host enzyme systems that permit absorption of these, and the rates at which absorption... 

Non-starch polysaccharides (NSP) Plant cell-wall NSP Other NSP Many different types of polysaccharides Main constituents arabinose, xylose, mannose, galactose, glucose, uronic acids Many types of constituents Escape digestion in the small intestine and are fermented to different extents Encapsulate and slow absorption of nutrients. Good marker for naturally fiber-rich diets for which health benefits have been shown Food additives. Minor components of the human diet. The amounts added to foods are known and regulated... 

Sucrase (also called saccharase and invertase) hydrolyzes sucrose. In contrast to the abundant information available on the yeast invertase, little is known of the mammalian sucrases. The significance of invertase and lactase in absorption is illustrated by inborn errors of metabolism in which these two enzymes are absent in the intestinal secretion. (The fact that lactase deficiency does not interfere with growth indicates that the galactose needed for biosynthesis of brain lipids or lens proteins can be synthesized endogenously in amounts sufficient to fulfill the metabolic requirements.) In that case, there is an intolerance to lactose or sucrose with no increase in blood glucose levels or without an increase in the levels of disaccharides... 

Absorption of the major monosaccharides, n-glucose, n-fructose, and D-galactose, apparently involves phosphorylation in the intestinal mucosa and liberation of the sugar in the blood stream 2). The mechanism is believed to be analogous to the formation of urine in the renal tubules. Mannose and the pentoses seem to be absorbed only by diffusion (S). 

Hexose Absorption.—The relative speed of sugar absorption from the intestine is in the order galactose > glucose > fructose the concentration of sugar affects the duration but not the rate of absorption, showing that it is not a simple diffusion process. 

The hexoses on entering the cells of the intestinal wall are converted into the corresponding phosphoric esters, and by this means the diffusion gradient is kept favourable for sugar absorption. Pentoses, disaccharides and hexoses other than D-glucose, D-fruc-tose, D-galactose and D-mannose are not phosphorylated by the... 

Fisher, R. B., and Parsons, D. S., 1953b, Galactose absorption from the surviving small intestine of the rat, /, Physiol. (London) 119 224. 

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