Molecular and cellular aspects of hydrolysis and absorption

Digestion and hydrolysis of macronutrients by the gastrointestinal tract are required to provide substrates for absorption and subsequent utilization. Carbohydrates are hydrolyzed by brush border enzymes of the small intestine, leading to monosaccharides that are then absorbed across the microvillus membrane by specific transport proteins. During development, the activities of disaccharidases and glucose transporters evolve in a similar time sequence, which is subject to individual regulation. Biosynthesis of disaccharidases involves initial synthesis of enzyme precursor molecules, followed by post-translational modification and intracellular trafficking. Various patterns of genetic defects of disaccharidases have been described, involving defects in enzyme synthesis, alterations in intracellular transport, and catalytically altered apoenzyme. The absorption of specific monosaccharides is facilitated by different transport proteins, with glucose using the sodium-dependent glucose transporter. A steady supply of monosaccharides, derived from varying nutritional sources, is assured by the interplay between sugar hydrolases and transport proteins within the microvillar membrane of the enterocyte in association with amino acid transport proteins.