A review of metabolism of labeled glucoses for use in measuring glucose recycling.

The fate of tritium from each carbon of D-glucose and the metabolism of L-glucose and 2-deoxy-D-glucose are known. Differences in metabolism of labeled glucoses can be used to quantify physical and chemical recycling of glucose. Only physical recycling is measured by [1-3H]-L-glucose, whereas [U-14C]-D-glucose measures total recycling. The difference between [1-3H]-L-glucose and [U-14C]-D-glucose, therefore, is chemical recycling. Recycling from extracellular binding sites and hepatic glucose 6-phosphate can be measured by difference between [1,2-3H]-2-deoxy-D-glucose and [1-3H]-L-glucose, and the difference in irreversible loss of the two will measure extrahepatic uptake of D-glucose. Recycling via Cori-alanine cycle plus CO2 is the difference in irreversible loss measured by using [6-3H]-glucose and [U-14C]-D-glucose. Recycling via the hexose monophosphate pathway can be determined by difference in irreversible loss between [1-3H]-D-glucose and [6-3H]-D-glucose. Recycling via CO2 and glycerol must be measured directly with [U-14C]glucose, bicarbonate, and glycerol. Recycling via hepatic glycogen can be estimated by subtracting all other measured chemical recycling from total chemical recycling. This review describes means to quantify glucose recycling in vivo, enabling studies of mechanisms for conservation and utilization of glucose.