Sucrase, intestinal

A case similar to the slow, practically irreversible inhibition of jack bean a-D-mannosidase by swainsonine is represented by the interaction of castanospermine with isomaltase and rat-intestinal sucrase. Whereas the association constants for the formation of the enzyme-inhibitor complex were similar to those of other slow-binding glycosidase inhibitors (6.5 10 and 0.3 10 M s for sucrase and isomaltase, respectively), the dissociation constant of the enzyme-inhibitor complex was extremely low (3.6 10 s for sucrase) or could not be measured at all (isomaltase), resulting in a virtually irreversible inhibition. Danzin and Ehrhard discussed the strong binding of castanospermine in terms of the similarity of the protonated inhibitor to a D-glucosyl oxocarbenium ion transition-state, but were unable to give an explanation for the extremely slow dissociation of the enzyme-inhibitor complex. 

The principle of active-site-directed inactivation of glycosidases by gly-con-related epoxides can be extended to compounds having an exocyclic oxirane ring, either directly attached to the six-membered ring (32) or at some distance (33,34). Studies with -o-glucosidase from sweet almonds and intestinal sucrase-isomaltase revealed that, in spite of the higher intrinsic reactivity of these epoxides, this shift of the position of the epoxide function causes a 10- to 30-fold decrease of kj(max)/Ki, an effect which probably reflects the limited flexibility of the catalytic groups involved in the epoxide reaction. 

V al-Met-Ser-Asp-T rp-Ala-Ala-His-His-Ala-Gly-Val-)S-D-Glucosidase A (bitter almonds) lle-Thr-Glx-Glx-Val-Phe-Gly-Asp-Ser-(Ala, Asxj, Glx, Pro)-Lys ff-D-Glucosidase (human placenta, lysosomal) -Val-Ala-Ser-Gln-Lys-Asn-Asp-Leu-Asp-Ala-Val-Ala-a-D-Glucosidase (sucrase, rabbit small intestine) -lle-Asp-Met-Asn-Glu-Pro-Asn-a-D-Glucosidase (isomaltase, rabbit small intestine) -Gly-Gly-Gln-lle-Asp-Met-)ff-D-Galactosidase (Escherichia co//) -Ser-Leu-Gly-Asn-Glu-Ser-Gly-His-Gly-Ala-... 

Santini, R., Jr., Aviles, J., and Sheehy, T. W., Sucrase activity in the intestinal mucosa of patients with sprue and normal subjects. Am. J. Digest. Diseases 5, 1059-1062 (1960). 

G. Hanozet, H.-P. Pircher, P. Vanni, B. Oesch, and G. Semenza, An Example of Enzyme Hysteresis The Slow and Tight Interaction of Some Fully Competitive Inhibitors with Small Intestinal Sucrase , J. Biol. Chem. 1981,256, 3703-3711. 

Fructose is found in honey and fruit and as part of the disaccharide sucrose (common table sugar). Sucrose is hydrolyzed by intestinal brush border sucrase, and the resulting monosaccharides, glucose and fructose, are absorbed into the portal blood. The liver phosphorylates frurtose and cleaves it into glyceraldehyde and DHAP. Smaller amounts are metabolized in renal proximal tubules. The pathway is shown in Figure 1-12-7 important enzymes to remember are ... 

Miglitol (Glyset) is another a-glucosidase inhibitor, but in contrast to acarbose, mightol is systemically absorbed prior to its activity in the small intestine. It also appears to inhibit the enzymes sucrase and maltase to a greater extent than does acarbose. It does not undergo metabolism and is renally excreted unchanged. 

Insulin secretion stimulation. Oil, administered orally to young suckling rabbits, quickened and strengthened the rise of immunoreactive serum insulin ". Intestinal brush border membrane. Oil, administered orally to rats at a dose of 10% for 5 weeks, produced an increase in level of saturated fatty acids in the brush border membrane from coconut oil-fed animals. Membrane fluidity was as follows coconut oil less than commercial pellet diet less than corn oil less than fish oil. The membrane hexose content was high in the coconut-fed rats. Hexamines were elevated in coconut-treated rat brush borders. The activities of alkaline phosphatase, sucrase, and lactase were increased "". 

Fig. 8. The preventive effects of chitosan on doxorubicin-induced gastrointestinal toxicity in sarcoma 180-beanng mice, a) small intestine weight b) sucrase activity in small intestinal mucosa.

Source and kinds of disaccharidases The final digestive processes occur at the mucosal lining of the small intestine. Several disaccharidases [for example, lactase (p-galactosidase), sucrase, maltase, and isomal-tase] produce monosaccharides (glucose, galactose, and fructose). These enzymes are secreted by and remain associated with the luminal side of the brush border membranes of intestinal mucosal cells. Absorption of the monosaccharides requires specific trans porters. 

Intestinal Disaccharidase Activity The results of the determination of small intestine mucosal maltase and sucrase activity for all animals fed treatment diets for 18 months are given in Table VII. No difference was detected in the activities of these enzymes from the animals fed the instant breakfast product or the hydrolyzed egg albumin. The animals fed the browned egg albumin,... 

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