Vertebrate intestine apical membrane mechanisms of organic nutrient transport

This paper presents the current understanding of comparative vertebrate intestine basic mechanisms of brush-border membrane transport. Animals control the uptake of monosaccharides and amino acids at three levels: 1) mucosal hyperplasia increases uptake nonselectively, 2) individual enterocytes increase the transport capacity of specific transporter systems, and 3) the transporters themselves are modulated by solute and ion electrochemical gradients. In light of the current literature, This paper summarizes the kinetics, thermodynamics, and the physical arrangement of one mode of transport, the prototype Na(+)-solute cotransporter. The model presented is experimentally consistent with "preferred random" kinetics, with Na+ binding preferentially before solute at the extracellular face. In the case of glucose, the cotransporter system may be physically arranged in the membrane as a tetramer comprising 73,000 Da subunits. All vertebrates may have evolved with a similar mechanism, with particular variations reflecting selected arrangements from a pool of polypeptide sequence blocks. The same fundamental transport mechanisms may be observed in the intestines of animals ranging from lower vertebrates through humans.