A-limit dextrin

I This enzyme contains a molecule of covalently bound pyridoxal I phosphate that is required as a coenzyme.] The resulting structure is called a limit dextrin, and phosphorylase cannot degrade it any further (Figure 11.8). r -... 

In a similar use of a-amylase, Parrish and Whelan249 treated potato starch with crystalline human salivary a-amylase and obtained a phosphorylated maltotetraose that had previously been reported by Postemak250 and that was the smallest phospho-dextrin formed. They determined its structure to be 63-phosphomaItotetraose, similar in structure to the smallest a-limit dextrin, 63-a- D-glucopyranosylmaltotriose, formed by this enzyme and porcine pancreatic a-amylase. 

The inhibition of aZp/ta-amylases from malt barley and hog pancreas by maltose has been shown to be noncompetitive, as has the inhibition of the malt-barley enzyme by a limit dextrin obtained by... 

We will discuss first whether there is an absolutely definite limit of action for all amylases. In the case of the action of /5-amylase on starch and on a-dextrins this question seems to be settled, but in the case of the malt a-amylase the answer is less certain. But certainly the action of the malt amylase practically stops at a certain limit. There is, however, almost always a very slow further action. It is possible that this slow saccharification of the limit dextrins is due not to the amylases but to other carbohydrases which have no action on starch but which are capable of attacking products with short chains. Under all circumstances it must be kept in mind that when in an experiment the saccharification for practical purposes has stopped and the limit dextrins have been isolated, this does not necessarily mean that the limit dextrins will not be further attacked by the enzyme used. But the velocity of this action is certainly very small compared with the velocity of the action on starch. Thus, it must be admitted that experiments involving the isolation of the limit dextrins after the action of a certain amylase on starch are in most cases not strictly reproducible. TJie total yield and chain length distribution of limit dextrins may vary, but their general character is not affected. If a limit dextrin produced by a certain amylase is treated with the same enzyme for a very long time, it is very often transformed to another limit dextrin of lower molecular weight with concomi-... 

The brush border enzymes with disaccharidase and ohgosaccharidase activity are listed in Table 48-1. The sucrase-isomaltase complex comprises most of the sucrase, isomaltase, and maltase (80%) activity of the small intestine. It hydrolyzes sucrose to its constituent monosaccharides, cleaves glucose from a-limit dextrins with 1,6 bonds, and hydrolyzes maltose. The activity of the complex is fourfold to fivefold greater in the jejunum than in the ileum. Changes in diet have a marked effect on the expression of the complex starvation leads to a rapid decline in activity, which is rapidly restored on refeeding. AH small intestinal saccha-ridases may decrease with infection or inflammation of the small bowel to the extent that carbohydrate malabsorption... 

In addition to their actions on disaccharides, the brush border enzymes further hydrolyze the products of amylase action, including maltose, maltotriose, and a-limit dextrins. The brush border enzymes appear to act in an integrated manner in that there is a flow of substrate from glucoamy-lase and isomaltase to sucrase with the production of the monosaccharides glucose, galactose, and fructose. These monosaccharides are transported into the ehterocyte by... 

Hydrolysis of a-limit dextrin to glucose by brush border membrane enzymes. 

D. a-Amylase hydrolyzes a( 1 4) glycosidic bonds present in starch (amylose and amylopectin) in a random fashion leaving primarily the disaccharide maltose, the trisaccharide maltotriose, and an oligosaccharide known as the a-limit dextrin, which is composed of 6 to 8 glucose residues with one or more a(l—>6) glycosidic bonds. Galactose and fructose are not present in starch. 

During the incubation of starch a-dextrins with maltase-containing saliva for 2-3 months, small proportions of panose are formed. This probably represents the action of the maltase impurity on true tetra- and pentasaccharide a-limit dextrins, rather than a third stage of hydrolysis. 

In applying the Kuhn methylation to a pentasaccharide isolated from amylopectin a-limit dextrin, the maximum methoxyl content that French, E. E. Smith, and Whelan were able to obtain was 41.8X (theoretical, 49.5%). Quantitative analysis of the hydrolyzate showed that there was no undermethylation, and they concluded that it is essential to wash the product with cyanide after each methylation in order to eliminate chloroform-soluble impurities containing iodine. Kabat and coworkers found that a Kuhn methylation of hexosamines using silver oxide did not give interpretable results, but no problem was experienced when barium oxide was employed. 

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