The breakdown of starch begins in the mouth, with salivary amylase. It is often assumed that as this is swallowed into an acid stomach the enzymic carbohydrate breakdown is then stopped (although acid hydrolysis may still occur) because salivary amylase is inhibited by a pH below 4. However, starch and its end products and the proteins and amino acids present in a mixed meal all buffer the acid of the stomach, allowing hydrolysis to continue. The quantitative involvement of salivary amylase in the breakdown of starch is thus probably underestimated. Pancreatic a-amylase added to the emptying gastric contents (chyle) in the duodenum cannot hydrolyze the (1-6) branching links and has little specificity for the (1-4) links adjacent to the branching points. Amylase action thus produces large oligosaccharides (a-limit dextrins) containing on average about eight glucose units with one or more (1-6) links. These a-limit dextrins are split by the enzymic action of glucoamylase (a-limit dextrinase), which sequentially removes a single glucose unit from the nonreducing end of a linear a(1-4)-glucosyl oligosaccharide. Maltose and maltotriose are then broken down by secreted and brush border disaccharidases, especially sucrase-isomaltase, into free glucose, which is then transported into and across the enterocytes by hexose transporters (Table 3,...,4).
Table 3.4 Human Facilitated-Diffusion Glucose Transporter Family, GLUT 1-5
The initial breakdown of starch into a-limit dextrins, the intraluminal or cavital digestion phase, occurs mainly in the bulk fluid phase of the intestinal contents. In man, there appears to be little of the so-called contact or membrane digestion in which adsorption of amylase onto the brush border surface of enterocytes facilitates its enzymatic activity (2).
Normally a-amylase is not a limiting factor in the assimilation of starches in humans, but newborn babies and especially premature ones cannot assimilate starch because the pancreas secretes insufficient a-amylase to digest it. However, after a month, the secretion of a-amylase is satisfactory for full digestion ( 3).
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