Digestion and absorption of a sulphoxide analogue of triacylglycerol in the rat

The stereochemistry of fat digestion and absorption was studied by feeding a triacylglycerol analogue to rats with a thoracic duct cannula. The analogue, rac-1,2-dioleoyl-3-S-tetradecyl-3-thioglycerol-S-oxide was chosen since its enantiomers exhibited high rotation in optical rotatory dispersion (ORD) and circular dichroism (CD). In the chyle, triacylglycerol was the major lipid but X-1,2-diacyl-3-S-tetradecyl-3-thioglycerol-S-oxide constituted 8% of lipid weight. It was resolved by thin-layer chromatography (TLC) into two diastereomers. Each of the diastereomers were analyzed for the proportions of 1-thio-sn-glycerol/3-thio-sn-glycerol isomers by ORD and CD. The 1-thio-sn-glycerol isomers dominated for both compounds indicating that they were enriched during the absorption processes, since a racemic compound was fed. The stereospecificities are probably exerted by acyltransferase(s) during chyle lipid synthesis. The methods used will be valuable tools in studies on the metabolism of enantiomeric glycerides, and also for characterization of naturally occurring sulphur-containing lipids.     

食物中核酸的消化与吸收

人和动物体摄入的三大营养物质淀粉经消化分解成葡萄糖,脂肪经消化分解为甘油和脂肪酸,蛋白质经消化分解成各种氨基酸并被小肠上皮细胞吸收。食物中的核酸在小肠内被核酸酶、二酯酶、核苷酸酶水解为核苷酸、核苷、磷酸、核糖、碱基。人体内核苷酸有“从头合成途径”和利用游离的碱基合成核苷酸的“补救途径”。外源核酸不可能直接被人体细胞吸收利用,人体细胞中的核酸都是自己合成的。正常人无需额外补充核酸类物质,不需任何核酸保健品。     

Digestion and absorption of polyunsaturated fatty acids.

Polyunsaturated fatty acids play an important part in the structure and function of cellular membranes and are precursors of lipid mediators which play a key role in cardiovascular and inflammatory diseases. Dietary sources of essential fatty acids are vegetable oils for either linoleic or alpha-linolenic acids, and sea fish oils for eicosapentaenoic and docosahexaenoic acids. Because of the specificity of the pancreatic lipid hydrolases, triglyceride fatty acid distribution is an essential parameter in the digestibility of fats. The efficiency of the intestinal uptake depends on the hydrolysis and especially on their micellarization. n-3 polyunsaturated fatty acid ethyl ester digestion is recognized to be impaired, but n-3 polyunsaturated fatty acid triglyceride hydrolysis remains a controversial point, and to some authors explains differences observed between vegetable and fish oil absorption. So additional studies are required to investigate this intestinal step. In enterocytes, morphological and biochemical absorption processes involve reesterification of long-chain fatty acids and lipoprotein formation. At this level, specific affinity of I- and L-FABPc (cytosolic fatty acid binding proteins) to polyunsaturated fatty acids requires further investigation. A better understanding of the role of these FABPc might bring to light the esterification step, particularly the integration of polyunsaturated fatty acids into phospholipids. With reference to differences published between fish and vegetable oil absorption, longer-term absorption studies appear essential to some authors. Polyunsaturated fatty acid absorption is thought to be not very dissimilar to that of long-chain mono-unsaturated fatty acid absorption. However, several digestion and absorption specific steps are worth studying with reference to the crucial role of polyunsaturated fatty acids in the organism, and for example adaptation of possible dietary supplements.     

Digestion and gastrointestinal absorption of the 14-16-kDa rice allergens

The digestibility and gastrointestinal absorption of 14-16-kDa rice allergens (RAs) were investigated. RAs and bovine serum albumin (BSA) were first evaluated for their digestibility. BSA was digested completely by in vitro incubation with some proteases, but RAs remained almost intact. Administered orally (20 mg per mouse), intact RAs were clearly detected in the small intestine even 60 min after the administration, the amount of total RAs in the small intestine being estimated to be 0.59 mg. RAs were then biotinylated and infused into the duodenal lumen of anesthetized mice, and portal blood was collected. The RA concentrations in the portal plasma were respectively estimated to be 0.4-0.9 and 0.3-2.5 microg/ml for 0.4 and 4 mg doses. These results suggest that RAs are highly resistant to digestive enzymes and that about 1/100 of orally administered RAs remain intact in the small intestine, while at least 1/1,000-1/10,000 is absorbed and delivered into circulated blood.     

Digestion rate, gut passage time and absorption efficiency in the Antarctic spiny plunderfish

The absolute gut evacuation rate (GER) (g day⁻¹) of Harpagifer antarcticus increased with increasing ration mass, fish mass only influenced the absolute GER at a daily ration level of 0·3% wet fish mass (approximately a maintenance ration). The relative GER (% of meal fed day⁻¹) was also affected differently by fish and ration mass depending on the relative ration level being fed; at rations of 0·7% wet fish mass or above the relative GER decreased with increasing fish or ration mass (in such a way that the absolute GER remained constant and unaffected by fish mass). At maintenance (0·3% wet fish mass) rations the relative GER was not affected by fish size or ration mass. Thus, there appears to be a ration threshold above which the digestion physiology alters. Mass-specific GER (% g fish⁻¹ day⁻¹) decreased with increasing fish mass. Within a set relative ration level (% wet fish mass) an increase in fish mass decreased the mass-specific GER. At a fixed ration mass, an increase in fish mass (i.e. a reduction in the ration expressed as % fish mass) resulted in a decrease in mass-specific GER. Gut evaluation time (GET) decreased and absorption efficiency (A) increased with increasing absolute GER. The effect of ration and fish mass on the absolute and relative GER followed the same pattern irrespective of the diet, however the A and GER (% day⁻¹ and g day⁻¹) were higher and the GET shorter when the fish were fed shelled krill rather than amphipods.     

Iron absorption in the iron-deficient rat

Iron absorption in the iron-deficient rat was compared with that in the normal rat to better understand the regulation of this dynamic process. It was found that: Iron uptake by the iron-deficient intestinal mucosa was prolonged as a result of slower gastric release, particularly when larger doses of iron were employed. The increased mucosal uptake of ionized iron was not the result of increased adsorption, but instead appeared related to a metabolically active uptake process, whereas the increased mucosal uptake of transferrin iron was associated with increased numbers of mucosal cell membrane transferrin receptors. Mucosal ferritin acted as an iron storage protein, but its iron uptake did not explain the lower iron absorption in the normal rat. Iron loading the mucosal cell (by presenting a large iron dose to the intestinal lumen) decreased absorption for 3 to 4 days. Iron loading of the mucosal cell from circulating plasma transferrin was proportionate to the plasma iron concentration. Mucosal iron content was the composite of iron loading from the lumen and loading from plasma transferrin versus release of iron into the body. These studies imply that an enhanced uptake-throughout mechanism causes the increased iron absorption in the iron-deficient rat. Results were consistent with the existence of a regulating mechanism for iron absorption that responds to change in mucosal cell iron, which is best reflected by mucosal ferritin.     

Digestion of plant monogalactosyldiacylglycerol and digalactosyldiacylglycerol in rat alimentary canal

We investigated digestion of orally fed galactoglycerolipids such as monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) from wheat flour in the rat alimentary canal, especially focusing on the digestive fates of deacylated galactosylglycerol structures. After a single oral administration of MGDG (20 mg/rat), monogalactosylmonoacylglycerol and monogalactosylglycerol (MGG) were found to be major digestion products in the intestinal tract. Similarly, digalactosylmonoacylglycerol and digalactosylglycerol (DGG) were confirmed to be present in the intestinal tract after DGDG ingestion (20 mg/rat). In rats fed wheat flour glycolipids (42 mg MGDG and 81 mg DGDG per rat), completely deacylated galactosylglycerols (MGG and DGG) were not detected in portal plasma. Although the deacylated galactosylglycerols were not significantly decomposed by intestinal mucosa in vitro, they were hydrolyzed by cecal contents. The results demonstrated that orally ingested plant galactoglycerolipids in the rat alimentary canal are rapidly hydrolyzed into constituent fatty acids and that hydrophilic galactosylglycerols and the hydrophilic backbone galactosylglycerols are not absorbed from intestine or degraded into galactose and glycerol in the intestinal tract. Therefore, the presence of deacylated galactosylglycerols may affect the fermentative activity of enterobacteria in the cecum and colon.     

Intestinal absorption of biotin and biocytin in the rat

The uptake of biotin and biocytin was investigated in rat intestine using the everted sac technique. It has been shown that at biotin and biocytin concentrations !ess than 40 and 50 nM respectively, absorption proceeds by a saturable process, whereas at higher concentrations uptake by passive diffusion predominates. Fractionation of solublized brush border preparations indicates that biotinidase is the only protein which binds biotin in this preparation.