Hydrogen bonding requirements for the insulin-sensitive sugar transport system of rat adipocytes

(1) The t 1/2 for 1.3 mM D-allose uptake and efflux in insulin-stimulated adipocytes is 1.7 +/- 0.1 min. In the absence of insulin mediated uptake of D-allose is virtually eliminated and the uptake rate (t 1/2 = 75.8 +/- 4.99 min) is near that calculated for nonmediated transport. The kinetic parameters for D-allose zero-trans uptake in insulin-treated cells are Koizt = 271.3 +/- 34.2 mM, Voizt = 1.15 +/- 0.12 mM . s-1. (2) A kinetic analysis of the single-gate transporter (carrier) model interacting with two substrates (or substrate plus inhibitor) is presented. The analysis shows that the heteroexchange rates for two substrates interacting with the transporter are not unique and can be calculated from the kinetic parameters for each sugar acting alone with the transporter. This means that the equations for substrate analogue inhibition of the transport of a low affinity substrate such as D-allose can be simplified. It is shown that for the single gate transporter the Ki for a substrate analogue inhibitor should equal the equilibrium exchange Km for this analogue. (3) Analogues substituted at C-1 show a fused pyranose ring is accepted by the transporter. 1-Deoxy-D-glucose is transported but has low affinity for the transporter. High affinity can be restored by replacing a fluorine in the beta-position at C-1. The Ki for D-glucose = 8.62 mM; the Ki for beta-fluoro-D-glucose = 6.87 mM. Replacing the ring oxygen also results in a marked reduction in affinity. The Ki for 5-thio-D-glucose = 42.1 mM. (4) A hydroxyl in the gluco configuration at C-2 is not required as 2-deoxy-D-galactose (Ki = 20.75 mM) has a slightly higher affinity than D-galactose (Ki = 24.49 mM). A hydroxyl in the manno configuration at C-2 interferes with transport as D-talose (Ki = 35.4 mM) has a lower affinity than D-galactose. (5) D-Allose (Km = 271.3 mM) and 3-deoxy-D-glucose (Ki = 40.31 mM) have low affinity but high affinity is restored by substituting a fluorine in the gluco configuration at C-3. The Ki for 3-fluoro-D-glucose = 7.97 mM. (6) Analogues modified at C-4 and C-6 do not show large losses in affinity. However, 6-deoxy-D-glucose (Ki = 11.08 mM) has lower affinity than D-glucose and 6-deoxy-D-galactose (Ki = 33.97 mM) has lower affinity than D-galactose. Fluorine solution at C-6 of D-galactose restores high affinity. The Ki for 6-fluoro-D-galactose = 6.67 mM. Removal of the C-5 hydroxymethyl group results in a large affinity loss. The Ki for D-xylose = 45.5 mM. The Ki for L-arabinose = 49.69 mM. (7) These results indicate that the important hydrogen bonding positions involved in sugar interaction with the insulin-stimulated adipocytes transporter are the ring oxygen, C-1 and C-3. There may be a weaker hydrogen bond to C-6. Sugar hydroxyls in non-gluco configurations may sterically hinder transport.