Use of glucose transport mutants to examine the intrinsic properties of glucose transport processes in rat myoblasts.

Glucose transport mutants were used to examine the intrinsic properties of glucose transport processes in rat myoblasts. Studies with mutants devoid of any functional glucose transporter revealed that substantial amount of sugar analogues was internalized via simple diffusion; however, equilibration of these analogues across the plasma membrane was not achieved after 1 min of incubation at 23 degrees C. The rates of internalization were substantially higher with sugar analogues that were phosphorylated by intracellular kinases. Mutants harbouring only one functional GLUT transporter were also used to examine the intrinsic properties of specific GLUT transporters. The preferred substrate for the GLUT 1 transporter was 2-deoxy-D-glucose (dGlc); the transport affinity for this substrate was reduced by energy uncouplers. Studies with mutants possessing only the GLUT 4 transporter revealed that this transporter existed in a high and a low affinity form. The former was responsible for dGlc uptake; whereas the latter was for the uptake of both 3-O-methyl-D-glucose (MeGlc) and dGlc; only the former was affected by energy uncouplers. These studies illustrated the usefulness of mutants in characterizing glucose transport processes.