Nutrition researchers investigating the relationship between fructose consumption and cardiovascular disease conduct a prospective cohort study on a population of randomly selected young adults. Study participants undergo semiannual measurement of waist circumference, blood pressure, and serum cholesterol and triglyceride concentrations. Dietary fructose consumption is assessed through the use of questionnaires and by measuring urinary fructose excretion. A 23-year-old man enrolled in the study is found to excrete large amounts of fructose in his urine compared to other study participants despite maintaining a moderate fructose intake. Further evaluation shows a hereditary defect in fructose metabolism, but he is asymptomatic and has no other medical problems. This patient most likely remains able to metabolize fructose due to the compensatory activity of which of the following enzymes?

Question

Nutrition researchers investigating the relationship between fructose consumption and cardiovascular disease conduct a prospective cohort study on a population of randomly selected young adults.  Study participants undergo semiannual measurement of waist circumference, blood pressure, and serum cholesterol and triglyceride concentrations.  Dietary fructose consumption is assessed through the use of questionnaires and by measuring urinary fructose excretion.  A 23-year-old man enrolled in the study is found to excrete large amounts of fructose in his urine compared to other study participants despite maintaining a moderate fructose intake.  Further evaluation shows a hereditary defect in fructose metabolism, but he is asymptomatic and has no other medical problems.  This patient most likely remains able to metabolize fructose due to the compensatory activity of which of the following enzymes?

Quizplus · Accepted Answer

11ec160b_369c_58fc_9249_e9e5b5c8a66d_MD0001_00 Fructose is obtained in the diet primarily from fruits and food sweeteners such as table sugar (sucrose) and high-fructose corn syrup. Fructose is absorbed in the proximal intestine through the GLUT5 fructose transporter. It is normally phosphorylated by fructokinase in the liver, yielding fructose-1-phosphate, which is converted by aldolase B to dihydroxyacetone phosphate (DHAP) and glyceraldehyde (Choice C). Glyceraldehyde and DHAP can be converted to glyceraldehyde-3-phosphate, which can then be metabolized in the glycolytic pathway. Fructokinase deficiency (essential fructosuria) is an asymptomatic, autosomal recessive disorder that causes dietary fructose to be excreted unchanged in the urine. In fructokinase deficiency, hexokinase takes over the role of fructose metabolism, converting dietary fructose into fructose-6-phosphate. Fructose-6-phosphate can be metabolized in the glycolytic pathway or converted to glucose-6-phosphate or glucose-1-phosphate, which can be used in the pentose phosphate pathway or for glycogen synthesis, respectively. (Choice A) Aldolase B plays a nonessential role in glycolysis due to the redundant function of aldolase A. However, it is particularly important during fructose metabolism as deficiency of this enzyme leads to toxic accumulation of fructose-1-phosphate (hereditary fructose intolerance). This life-threatening disorder presents in infancy after the introduction of fructose-containing foods. (Choice B) Aldose reductase is the enzyme that converts glucose to sorbitol. Aldose reductase has a low affinity for glucose, and normally only very small amounts of glucose are metabolized by this enzyme. The amount of glucose metabolized by the aldose reductase pathway increases significantly in diabetes mellitus and contributes to chronic complications such as neuropathy and retinopathy. (Choice E) UDP-galactose-4-epimerase is involved in the metabolism of galactose; it does not play a role in fructose metabolism. Educational objective: Essential fructosuria is a benign disorder of fructose metabolism caused by fructokinase deficiency. In patients with essential fructosuria, some of the dietary fructose load is converted by hexokinase to fructose-6-phosphate, which can then enter glycolysis; this pathway is not significant in normal individuals.

Nutrition Researchers Investigating the Relationship Between Fructose Consumption and Cardiovascular