Fructose occurs naturally in foods as a free monosaccharide and as a component of the disaccharide sucrose. It is also widely used as a sweetener. In the body, fructose catabolism occurs in the liver and to a lesser extent in the kidney and small intestine. In these tissues, it is converted to dihydroxyacetone phosphate and D-glyceraldehyde 3-phosphate, two intermediates in the glycolytic pathway, in a sequence of three reactions. It is phosphorylated to form fructose 1-phosphate, which is cleaved by aldolase to yield dihydroxyacetone phosphate and D-glyceraldehyde, and the latter compound is phosphorylated to yield D-glyceraldehyde 3-phosphate. Other pathways exist for the conversion of D-glyceraldehyde to intermediates of glycolysis, but these appear to play only a minor role in normal fructose metabolism (Sillero et al. 1969).
The cleavage of fructose 1-phosphate is catalyzed by the same enzyme that catalyzes the reversible cleavage of fructose 1,6-bisphosphate in glycolysis. The isoform of this enzyme found in liver, kidney, and intestine (B) is approximately equally active with fructose 1-phosphate and fructose 1,6-bisphosphate as substrates, while the muscle and brain isoforms (A and C, respectively), have little activity with fructose 1-phosphate.