Brush-border inositol transport by intestines of carnivorous and herbivorous teleosts

Transport characteristics of myoinositol by isolated brush-border membrane vesicles of two fish, the herbivorous tilapia (Oreochromis mossambicus) and the carnivorous eel (Anguilla anguilla), were measured. [3H]myoinositol uptake by vesicles of both fish was stimulated by a transmembrane Na gradient, was electrogenic, and was inhibited by phloridzin. Kinetic analysis of myoinositol influx disclosed species differences (tilapia, K = 0.15 mM, J(max) = 0.2 nmol·mg protein-1·min-1; eel, K = 2.6 mM, J(max) = 0.8 nmol·mg protein-1·min-1). D-Glucose inhibition of myoinositol influx was shown to be noncompetitive. Additional inhibition studies with a range of sugars demonstrated that aldohexoses in the C-1 chair conformation were preferred substrates. Myoinositol had no effect on D-glucose transport. Preloading vesicles with myoinositol transstimulated [3H]myoinositol uptake, while the use of internal D-glucose was without effect. These results suggest that the intestinal brush border may have a pathway for myoinositol transport entirely separate from that for D-glucose but inhibited by D-glucose via binding to a regulator site on the myoinositol transporter. Markedly dissimilar influx kinetic constants suggest possible differences in myoinositol needs by carnivorous and herbivorous fish.