Fructose 3-phosphate, a novel monosaccharide phosphate, has been identified in the lens of diabetic rats. This compound, which is not present in normal lenses, is a protein glycosylating agent and enzyme inactivator. In addition, because of its structural features, this metabolite is relatively labile and undergoes hydrolysis to yield inorganic phosphate and the potent glycosylating agent, 3-deoxyglucosone. The increase in the concentration of fructose 3-phosphate in the lens of diabetic rats suggests that it and its hydrolysis product, 3-deoxyglucosone, may be responsible in part for the development of some diabetic complications in the lens.

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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.

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