Fructose metabolism in the human erythrocyte. Phosphorylation to fructose 3-phosphate.

In human erythrocytes, the first step in the metabolism of fructose is generally thought to be phosphorylation to fructose 6-phosphate catalysed by hexokinase. In variance with this assumption, we show here that fructose in these cells is metabolized primarily to fructose 3-phosphate by a specific 3-phosphokinase. This process has an overall estimated Km of 30 mM with respect to extracellular fructose and an apparent Vmax. of 0.6 mumol/h per ml. At a fixed concentration of fructose in the medium, the accumulation of fructose 3-phosphate was linearly dependent on the duration of incubation up to 5 h and was not affected by glucose. Once accumulated, fructose 3-phosphate appears to be degraded and/or relatively slowly metabolized, decreasing by only approximately 30% after a 12 h incubation in a fructose-free medium.