Passive absorbers

In humans, vitamin B in phosphorylated forms is mostly unavaHable until hydrolyzed, then passively absorbed in the smaH intestine. The free vitamins are interconverted and suppHed to the ckculation by the Hver, then enter ceHs by simple diffusion and are phosphorylated. A typical adult pool of... 

In humans, thiamine is both actively and passively absorbed to a limited level in the intestines, is transported as the free vitamin, is then taken up in actively metabolizing tissues, and is converted to the phosphate esters via ubiquitous thiamine kinases. During thiamine deficiency all tissues stores are readily mobilhed. Because depletion of thiamine levels in erythrocytes parallels that of other tissues, erythrocyte thiamine levels ate used to quantitate severity of the deficiency. As deficiency progresses, thiamine becomes indetectable in the urine, the primary excretory route for this vitamin and its metaboHtes. Six major metaboHtes, of more than 20 total, have been characterized from human urine, including thiamine fragments (7,8), and the corresponding carboxyHc acids (1,37,38). 

We plan to try some experiments with oxygen instead of air on the anode side. Wick-like materials will be tried for passively absorbing and transporting water to the PEM and transporting water from the ELAT anode catalyst. 

H Lu, J Thomas, D Fleisher. Influence of D-glucose-induced water absorption on rat jejunal uptake of two passively absorbed drugs. J Pharm Sci 81 21-25, 1992. 

Pade, V., Stavchansky, S., Estimation of the relative contribution of the transcellular and paracellular pathway to the transport of passively absorbed drugs in the Caco-2 cell culture model, Pharm. Res. 1997, 34, 1210-1215. 

Some of the zinc taken up by the intestinal epithelial cells is rapidly transferred to the portal plasma where it associates with albumin, a2 macroglobulin, and amino acids. About 67% of the zinc in plasma is bound to albumin, and about 3% is stored in liver (Stemlieb 1988). Soluble organozinc complexes are passively absorbed across the plasma membrane of the mucosa of the intestinal villi the soluble, nondiffusable complexes are transported in the intestinal products and excreted in feces (NAS 1979). Zinc loss from urine and sweat is usually small (Casey and Hambidge... 

Several in silica models for prediction of oral absorption are available [133-136]. Simple models are based on only few descriptors like logP, logD, or polar surface area (PSA), while they are only applicable if the compounds are passively absorbed. In case of absorption via active transporters or if efflux is involved, prediction of absorption is still not successful. 

Unconjugated bile acids have pK values of around 6, which means they will be un-ionised in the intestinal lumen and may be passively absorbed. However, conjugated bile acids are ionised and require transporters to cross the enter-ocyte in much the same way as found in the hepatocyte. Although OATP2 is not expressed in small bowel a sodium-independent transporter is present in the... 

Larval PA sequestration, similar to that in U. omatrix, is common within the Arctiidae (Table 7.1). Arctiid larvae, as exemplified by Creatonotus transiens and Tyria jacobaeae, feed on plants with PAs predominantly in the A-oxide form. The A-oxides are reduced to the lipophilic free bases in the midgut and are passively absorbed (Hartmann and Ober, 2000 but see Wink and Schneider, 1988). The free bases are then reoxidized by a soluble NADPH-dependent mixed function... 

The importance of hydrogen bond contribution to permeability and absorption processes was also studied on a heterogeneous set of chemicals [22] as well as for the transport of a series of passively absorbed dmgs in humans [23],... 

It has to be emphasized that this expression is only applicable to compounds that are absorbed by diffusion and do not suffer a first-pass effect. The compounds that are actively absorbed (at least in part) along the enterocyte membrane, as well as those that are actively and substantially excreted to luminal fluid from the membrane or even the cytoplasm of the absorbing cells (i.e. P-gp substrates), should not be used for such approaches. For the compounds tested, the fraction of dose absorbed in vivo coincides with oral bioavailability (/ ), since they are passively absorbed and do not suffer first-pass metabolism. 

Q13 Osteomalacia and osteoporosis are complications of celiac disease. The mineral in bone is mainly calcium phosphate a supply of calcium is therefore needed for bone growth and replacement. Calcium is absorbed by active mechanisms in the duodenum and jejunum, facilitated by a metabolite of vitamin D. It is also passively absorbed in the ileum and specific calcium binding proteins are present in the intestinal epithelial cells. Loss of absorptive cells and calcium binding proteins markedly decreases calcium uptake and limits its availability for bone growth and repair. 

Passively absorbed compounds diffuse either through the cell itself (transcellular pathway) or in between cells (paracellular pathway). The lipid bilayers of which the mucosal and basolateral epithelial cell membranes are composed of, define the primary transcellular diffusion resistance to solute transport across the intestinal barrier. Transcellular permeabihty, particularly of lipophilic solutes, depends on their partitioning between intestinal membrane and aqueous compartments (Fig. 1). 

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