Xylose Transport Research Articles

Active transport of D-xylose in the isolated small intestine of the bullfrog.

Fluxes of D-xylose-1-C(14) (xylose) across the wall of the isolated intestine of the bullfrog were studied. When sodium was the principal cation in the mucosal bathing fluid, the transport rate of xylose from the mucosa to the serosa was about 5 times greater than the transport rate from the serosa to the mucosa, indicating an active intestinal transport for this sugar. With potassium as the principal cation on the mucosal side, the transport rate of xylose from the mucosal to the serosal compartment is reduced about 5 to 6 times without appreciable change in the serosal to mucosal transport. The asymmetry was also considerably reduced when ouabain was added to the mucosal and serosal compartments. The data confirm the in vitro and in vivo observations indicating active transport of xylose and are also in accord with the earlier findings that active transport of sugars in the intestine is dependent upon the presence of sodium ions in the mucosal compartment and is inhibited by cardioactive steroids. Since the chemical constitution of xylose does not meet the requirements which were hitherto considered necessary for active transport of sugars in the intestine, this structural requirement has to be revised.

D-xylose active transport in the hamster small intestine

D-Xylose is accumulated by rings of hamster small intestine, in vitro, through the same equilibrating mobile-carrier mechanism involved in glucose active transport. It is not necessary to postulate the presence of a distinct accumulation step in order to explain energy-dependent sugar accumulation. The experimental evidence is as follows: 1.1. Under aerobic conditions, tissue/medium xylose concentration ratios may be higher than 1.2.2. Xylose transport exhibits saturation kinetics, with a Km of the order of 100–300 mM, which is about 100 times higher than that of substrates such as glucose.3.3. Phlorizin which is a competitive inhibitor of glucose transport also competitively inhibits xylose transport.4.4. Xylose exhibits reciprocal (competitive) inhibition with glucose and its analogs.5.5. Other compounds, e.g., α-methylmannoside and d-lyxose also serve as substrates for the glucose carrier, but with even smaller affinities than xylose. For most practical purposes, they are inert, but their characteristics add weight to the evidence for xylose as a substrate.6.6. In accord with the mobile-carrier concept, xylose countertransport (substrate-induced counterflow) can be demonstrated using glucose and other actively transported sugars as elicitors.7.7. As is true for glucose, the apparent affinity of xylose for the carrier is enhanced by Na+ and depressed by K+, and the direction of net xylose transport reflects the medium/tissue Na+ concentration ratio.

Induced uphill and downhill transport: relationship to the Ussing criterion.

DURING the entry of glucose into the human red cell there is an induced outward transport of radioglucose against its concentration gradient1. Similarly in the rabbit erythroeyte the entry of glucose induces an outward transport of xylose or galactose against a concentration gradient2. It is the purpose of this communication to show that this phenomenon is directly related to the ratio of the inward and outward fluxes across the membrane and to give a reversible thermodynamic derivation of the Ussing3 criterion.

Effect of insulin on xylose transport in human leukocytes

Effect of insulin on xylose transport in human leukocytes