The Secretion and Action of Brush Border Enzymes in the Mammalian Small Intestine.
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.
Institute of Food Nutrition and Human Health, Massey University, Private Bag 121222, Palmerston North, New Zealand.|The New Zealand Institute for Plant & Food Research Limited, Food Industry Science Centre, Batchelar Road, Palmerston North, 4474, New Zealand.
Diane
D
Hooton
Institute of Food Nutrition and Human Health, Massey University, Private Bag 121222, Palmerston North, New Zealand. R.G.Lentle@massey.ac.nz.
Roger
R
Lentle
The New Zealand Institute for Plant & Food Research Limited, Food Industry Science Centre, Batchelar Road, Palmerston North, 4474, New Zealand.
John
J
Monro
Reacta Biotech Limited, Manchester, UK.
Martin
M
Wickham
The New Zealand Institute for Plant & Food Research Limited, Food Industry Science Centre, Batchelar Road, Palmerston North, 4474, New Zealand.
Robert
R
Simpson
D000818:Animals; D004798:Enzymes / Q000096:biosynthesis / Q000378:metabolism*; D007421:Intestine, Small / Q000201:enzymology*; D007700:Kinetics; D008871:Microvilli / Q000201:enzymology*
D004798:Enzymes
Digestion; Enterocyte; Membrane; Microvillus; Vesicles