The intravenous infusion of fructose into healthy volunteers leads to a two - to fivefold increase in the level of lactate in the blood, a far greater increase than that observed after the infusion of the same amount of glucose. (a) why is glycolysis more rapid after the infusion of fructose? (b) based on your answer to question (a), why is the use of fructose in place of glucose for intravenous feeding unwise?

a) This is due to the fact that the fructose 1-phosphate pathway forms glyceraldehyde 3-phosphate. The glycolysis is rapid because the initial step in glycolysis, involving phosphofructokinase, a key control enzyme, is bypassed. There is little energy needed in the phosphorylation of the fructose as compared to beginning with glucose.

b) Phosphofructokinase is important in the regulation of the rate of the glycolysis hence its bypassing can result in too much glycolysis. Furthermore, fructose 1-phosphate stimulates pyruvate kinase as an allosteric effector. Therefore more pyruvate is formed as the glycolysis reaction is pushed forward. The regulation of the glycolysis process is therefore impeded by fructose IV.