**4.3 Biological significances of endocytosis of pancreatic α-amylase**

Based on the SGLT1 inhibition of pancreatic α-amylase described in Section 3.2, the biological significance of endocytosis of the pancreatic α-amylase in Sections 4.1 and 4.2 is discussed as follows. **Figure 6** shows a hypothetical scheme of the biological function of α-amylase internalization [25]. (i) Within 30 min after food intake, pancreatic α-amylase has begun to be secreted from the pancreas into the duodenum, but its amount is not high. Therefore, the α-amylase on the surface of the duodenal BBM interacts with the SI on the BBM to promote starch digestion by both the α-amylase and SI. (ii) Approximately 30 min after food intake, the concentration of pancreatic α-amylase in the small intestinal lumen reaches a maximum, and the α-amylase binds to the glycoproteins in the BBM. At this time, the glucose uptake of SGLT1 is inhibited by the α-amylase, whereby rapid glucose uptake is suppressed, and postprandial hyperglycemia is corrected. (iii) At 30–60 min after food intake, the α-amylase bound to the BBM is internalized into early endosomes in the epithelial cells.

### **Figure 6.**

*A hypothetical schematic of the biological function of pancreatic α-amylase internalization.*

**209**

**Figure 7.**

*shown with liver expression as 1.*

*Regulatory Functions of α-Amylase in the Small Intestine Other than Starch Digestion…*

That is, the inhibition of glucose uptake of SGLT1 in BBM by α-amylase is released, which means the blood glucose level can be maintained even when the blood glucose level drops during fasting. (iv) At 60 min or more after food ingestion, the α-amylase internalized into cells is transported to lysosomes and

**5. Expression of endogenous α-amylase in the human duodenum**

α-Amylase is abundantly expressed in the pancreas and salivary glands, and has been detected in liver [26], thyroid [27], parotid gland [28], white blood cells [29], lung carcinoma tissues [30], and brain [3] in humans. There are five isoforms of α-amylase including three salivary α-amylases (isoforms *AMY1A*, *AMY1B*, and *AMY1C*) and two pancreatic α-amylases (isoforms *AMY2A* and *AMY 2B*) [31]. The α-amylases in liver, white blood cells, and lung carcinoma tissues are encoded by *AMY2B* [2, 29, 30]. The brain amylases are encoded by *AMY1A* and *AMY2B* [3]. The parotid gland amylase is encoded by *AMY1C* [28]. It is reported that α-amylase activity is detected in small intestinal tissues [32]. It is not clear whether this α-amylase is an exogenous one that endocytoses pancreatic α-amylase into small intestinal tissues or an endogenous α-amylase that is expressed in the small intestine itself. An endogenous α-amylase was stained in differentiated Caco-2 cells by immunostaining with an anti-α-amylase antibody during the experiments on internalization using fluorescence-labeled α-amylase in the cells described in Section 4. In this study, we demonstrated the expression of endogenous α-amylase in human duodenal epithelial cells and identified the isoform of the α-amylase expressed in the duodenal epithelial cells. Furthermore, the biological significances of α-amylase expression in the

*Expression of α-amylase in human tissues. A reaction mixture was prepared by mixing Human MTC panel I, II or Digestive system MTC panel (Clontech) as a cDNA (5 μl), Power SYBER Green PCR Master mix (6.25 μl), 50 μM forward and reverse primers for AMY2B (0.2 μl each), and putting it in the 96-well reaction plate. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was used as the internal standard. The sequences of primer sets for AMY2B and GAPDH were described previously [33]. Normalized data by GAPDH are* 

*DOI: http://dx.doi.org/10.5772/intechopen.92660*

undergoes degradation.

duodenum are shown [33].

*Regulatory Functions of α-Amylase in the Small Intestine Other than Starch Digestion… DOI: http://dx.doi.org/10.5772/intechopen.92660*

That is, the inhibition of glucose uptake of SGLT1 in BBM by α-amylase is released, which means the blood glucose level can be maintained even when the blood glucose level drops during fasting. (iv) At 60 min or more after food ingestion, the α-amylase internalized into cells is transported to lysosomes and undergoes degradation.
