The disaccharide sucrose is the primary source of fructose. When sucrose is broken down, equal amounts of fructose and glucose are released. The transportation of fructose into cells does not rely on insulin. Fructose is first transformed into fructose 1-phosphate by fructokinase, and then cleaved by aldolase B into dihydroxyacetone phosphate and glyceraldehyde. These enzymes are present in the liver, kidneys, and small intestine. If there is a deficiency of fructokinase, it results in a harmless condition called essential fructosuria. On the other hand, a deficiency of aldolase B leads to hereditary fructose intolerance (HFI), which causes severe hypoglycemia and liver failure. In such cases, fructose (and sucrose) must be eliminated from the diet to prevent death. Mannose, an important component of glycoproteins, is phosphorylated by hexokinase to form mannose 6-phosphate. This compound can be reversibly isomerized to fructose 6-phosphate by phosphomannose isomerase. Aldose reductase can convert glucose to sorbitol (glucitol) in various tissues including the lens, retina, peripheral nerves, kidneys, ovaries, and seminal vesicles. In the liver, ovaries, and seminal vesicles, sorbitol dehydrogenase can further oxidize sorbitol to produce fructose. Hyperglycemia leads to the accumulation of sorbitol in cells lacking sorbitol dehydrogenase. This accumulation results in cell swelling and may contribute to the development of cataracts, peripheral neuropathy, nephropathy, and retinopathy in individuals with diabetes. Lactose is the primary dietary source of galactose, and its transportation into cells does not require insulin. Galactose is first phosphorylated to galactose 1-phosphate by galactokinase. A deficiency of this enzyme can cause cataracts. Galactose 1-phosphate is then converted to uridine diphosphate (UDP)-galactose by galactose 1-phosphate uridylyltransferase (GALT), with UDP-glucose providing the nucleotide. Classic galactosemia occurs when there is a deficiency of this enzyme. Galactose 1-phosphate accumulates, and excess galactose is converted to galactitol by aldose reductase. This conversion leads to liver damage, brain damage, and cataracts. Treatment for classic galactosemia involves the elimination of galactose (and lactose) from the diet. In order for UDP-galactose to enter the main glucose metabolism pathway, it must first be isomerized to UDP-glucose by UDP-hexose 4-epimerase. This enzyme can also produce UDP-galactose from UDP-glucose when needed for the synthesis of glycoproteins and glycolipids. Lactose, a disaccharide composed of galactose and glucose, is found in milk and other dairy products. Lactose is synthesized by lactose synthase in the lactating mammary gland using UDP-galactose and glucose. The enzyme consists of two subunits, protein A (a galactosyltransferase present in most cells where it synthesizes N-acetyllactosamine) and protein B (α-lactalbumin, which is only found in lactating mammary glands and is stimulated by the hormone prolactin to produce lactose) When both subunits are present, the transferase synthesizes lactose.
The major source of fructose is the disaccharide sucrose, which, when cleaved, releases equimolar amounts of fructose and glucose (Fig. 12.8). Transport of fructose into cells is insulin independent. Fructose is first phosphorylated to fructose 1-phosphate by fructokinase and then cleaved by aldolase B to dihydroxyacetone phosphate and glyceraldehyde. These enzymes are found in the liver, kidneys, and small intestine. A deficiency of fructokinase causes a benign condition (essential fructosuria), whereas a deficiency of aldolase B causes hereditary fructose intolerance (HFI), in which severe hypoglycemia and liver failure lead to death if fructose (and sucrose) is not removed from the diet. Mannose, an important component of glycoproteins, is phosphorylated by hexokinase to mannose 6phosphate, which is reversibly isomerized to fructose 6-phosphate by phosphomannose isomerase. Glucose can be reduced to sorbitol (glucitol) by aldose reductase in many tissues, including the lens, retina, peripheral nerves, kidneys, ovaries, and seminal vesicles. In the liver, ovaries, and seminal vesicles, a second enzyme, sorbitol dehydrogenase, can oxidize sorbitol to produce fructose. Hyperglycemia results in the accumulation of sorbitol in those cells lacking sorbitol dehydrogenase. The resulting osmotic events cause cell swelling and may contribute to the cataract formation, peripheral neuropathy, nephropathy, and retinopathy seen in diabetes. The major dietary source of galactose is lactose. The transport of galactose into cells is insulin independent. Galactose is first phosphorylated by galactokinase (a deficiency results in cataracts) to galactose 1phosphate. This compound is converted to uridine diphosphate (UDP)galactose by galactose 1-phosphate uridylyltransferase (GALT), with the nucleotide supplied by UDP-glucose. A deficiency of this enzyme causes classic galactosemia. Galactose 1-phosphate accumulates, and excess galactose is converted to galactitol by aldose reductase. This causes liver damage, brain damage, and cataracts. Treatment requires removal of galactose (and lactose) from the diet. For UDP-galactose to enter the mainstream of glucose metabolism, it must first be isomerized to UDP-glucose by UDP-hexose 4-epimerase. This enzyme can also be used to produce UDP-galactose from UDP-glucose when the former is required for glycoprotein and glycolipid synthesis. Lactose is a disaccharide of galactose and glucose. Milk and other dairy products are the dietary sources of lactose. Lactose is synthesized by lactose synthase from UDP-galactose and glucose in the lactating mammary gland. The enzyme has two subunits, protein A (which is a galactosyltransferase found in most cells where it synthesizes N-acetyllactosamine) and protein B (α-lactalbumin, which is found only in lactating mammary glands, and whose synthesis is stimulated by the peptide hormone prolactin). When both subunits are present, the transferase produces lactose.
Choose the ONE best answer.
2.1. A nursing female with classic galactosemia is on a galactose-free diet. She is able to produce lactose in breast milk because:
A. galactose can be produced from fructose by isomerization.
B. galactose can be produced from a glucose metabolite by epimerization.