The Secretion and Action of Brush Border Enzymes in the Mammalian Small Intestine

Abstract

Microvilli are conventionally regarded as an extension of the small intestinal absorptive surface, but they are also, as latterly discovered, a launching pad for brush border digestive enzymes. Recent work has demonstrated that motor elements of the microvillus cytoskeleton operate to displace the apical membrane toward the apex of the microvillus, where it vesiculates and is shed into the periapical space. Catalytically active brush border digestive enzymes remain incorporated within the membranes of these vesicles, which shifts the site of BB digestion from the surface of the enterocyte to the periapical space. This process enables nutrient hydrolysis to occur adjacent to the membrane in a pre-absorptive step. The characterization of BB digestive enzymes is influenced by the way in which these enzymes are anchored to the apical membranes of microvilli, their subsequent shedding in membrane vesicles, and their differing susceptibilities to cleavage from the component membranes. In addition, the presence of active intracellular components of these enzymes complicates their quantitative assay and the elucidation of their dynamics. This review summarizes the ontogeny and regulation of BB digestive enzymes and what is known of their kinetics and their action in the peripheral and axial regions of the small intestinal lumen.

Intestinal NCoR1, a regulator of epithelial cell maturation, controls neonatal hyperbilirubinemia

“…The results demonstrated that NCoR1 deletion accelerated cell proliferation and promoted epithelial cell migration in ΔIEC UN mice. Sucrase isomaltase (Sis), a brush border glucosidase, only exists in differentiated duodenal and jejunal enterocytes and has been used as a marker of enterocyte maturation (36,37). Sis gene expression along the CVA was up-regulated in ΔIEC UN mice (Fig.…”

Crystal structure and mechanism of human carboxypeptidase O: Insights into its specific activity for acidic residues

“…While hCPA1 and hCPA2 show a higher preference for small aliphatic and bulky aromatic side chains, respectively (14), hCPB preferentially cleaves substrates with C-t basic amino acids (15)(16)(17). Although their combined action can release a wide range of amino acids that are absorbed in the intestinal tract, pancreatic MCPs are unable to release acidic C-t amino acids, such as Asp or Glu (13,18,19), even though these acidic residues are two of the most abundant amino acids in alimentary proteins (20)(21)(22).…”

Evaluation of the impact of a rat small intestinal extract on the digestion of four different functional fibers

“…44,45 In this sense, mucosal maltase-glucoamylase and sucrase-isomaltase complexes could hydrolyze to α(1 → 6) bonds between the monomers of both AlphaGOS P and melibiose. 20,21,46 These enzymatic structures could also have more versatility in terms of hydrolytic activity, as was showed elsewhere. 16,47 To the best of our knowledge, these data are the first evidence about digestibility of α-GOS with small intestine enzymes.…”

“…20 Sucrase site splits glucose and fructose, while isomaltase site splits Glc-Glc α(1 → 4) and α(1 → 6) linkages, being one of the most common complexes in the small intestine. 21 Inulinase activity showed the lowest value, which could be related to the low digestibility of prebiotic fructans. 14,22…”