Intestinal saccharidase deficiencies

Defects in in two enzymes required for intestinal digestion of dietary carbohydrate, lactase (LCT, a domain of lactase-phlorizin hydrolase protein) and sucrase-isomaltase (SI), are annotated here. The first affects nursing infants; the second affects individuals after weaning.<p>The disaccharide lactose is a major constituent of human breast milk. To be taken up from the gut in the nursing infant, this sugar must first be hydrolyzed by LCT present on the external face of enterocytes in microvilli of the small intestine. Mutations that disrupt LCT activity are associated with acute illness in newborn children as lactose fermentation by gut bacteria leads to severe diarrhea. The condition is effectively treated by feeding affected infants a lactose-free formula. This congenital disease is distinct from the down-regulation of LCT expression after weaning in many human populations that is associated with a milder form of lactose intolerance in adults (Jarvela et al. 2009).<p>The starch in a post-weaning diet is digested by amylases to di- and oligosaccharides that must be further digested to monosaccharides in order to be taken up from the lumen of the small intestine into endothelial cells of the intestinal brush border. If they are not digested, a process in which enterocyte-associated SI plays a central role, they remain in the gut lumen and are fermented by gut bacteria, leading to osmotic and fermentative diarrhea (Naim et al. 2012; Van Beers et al. 1995).

在此处对参与肠道消化膳食碳水化合物的两种酶——乳糖酶（LCT，乳糖-芸香苷水解酶蛋白的一个结构域）和蔗糖异麦芽糖酶（SI）——的缺陷进行了注释。第一种酶的缺陷影响哺乳婴儿；第二种酶的缺陷影响断奶后的个体。<p>乳糖作为人乳的主要成分，为了从肠道被哺乳婴儿吸收，这种糖首先必须在小肠绒毛的肠细胞外部表面上的LCT酶解。破坏LCT活性的突变与新生儿急性疾病有关，因为肠道细菌的乳糖发酵会导致严重腹泻。通过喂养受影响婴儿无乳糖配方奶粉，该病状可以得到有效治疗。这种先天性疾病与许多人类种群在断奶后LCT表达下调相关，这种下调与成人乳糖不耐症的轻度形式有关（Jarvela等，2009年）。<p>断奶后的饮食中的淀粉通过淀粉酶分解为双糖和寡糖，这些糖必须进一步分解为单糖，以便从小肠腔内被吸收到肠道刷状缘的内皮细胞中。如果不能被消化，其中与肠细胞相关的SI在其中的作用至关重要，这些糖将留在肠道腔内，被肠道细菌发酵，导致渗透性和发酵性腹泻（Naim等，2012年；Van Beers等，1995年）。