Fructose and Galactose Metabolism

Comprehensive vocabulary flashcards covering the key enzymes, pathways, metabolic intermediates, and clinical inborn errors of fructose and galactose metabolism.

GLUT-5

The transporter that mediates dietary fructose absorption across the intestinal apical membrane into mucosal cells via facilitated diffusion.

Fructokinase

An abundant liver enzyme with a high $V_{\max}$ that phosphorylates fructose to fructose 1-phosphate using ATP, bypassing hexokinase regulation.

Aldolase B

A hepatic enzyme that cleaves fructose 1-phosphate into dihydroxyacetone phosphate (DHAP) and glyceraldehyde, enabling fructose intermediates to enter glycolysis.

Essential Fructosuria

A benign, autosomal recessive inborn error of metabolism caused by a deficiency of fructokinase in the liver, resulting in asymptomatic urinary excretion of fructose without toxic metabolite accumulation.

Hereditary Fructose Intolerance (HFI)

An autosomal recessive deficiency of aldolase B that causes toxic accumulation of trapped fructose 1-phosphate in hepatocytes, depleting inorganic phosphate ($P_i$) and ATP, thereby inhibiting gluconeogenesis and glycogenolysis and producing postprandial hypoglycemia.

Polyol Pathway

A two-reaction alternative glucose utilization pathway that converts glucose to sorbitol via aldose reductase, and subsequently oxidizes sorbitol to fructose via sorbitol dehydrogenase.

Aldose Reductase

An enzyme present in the lens, retina, peripheral nerves, kidneys, and seminal vesicles that reduces glucose to sorbitol (or galactose to galactitol) using NADPH.

Sorbitol Dehydrogenase

An enzyme that oxidizes sorbitol to fructose using $\text{NAD}^+$, found in hepatocytes, ovaries, and seminal vesicles, where it produces fructose as the primary energy fuel for spermatozoa.

Osmotic Damage (Diabetic Microvascular Complications)

Cellular edema and damage caused by sorbitol accumulation in insulin-independent tissues lacking sorbitol dehydrogenase (lens, retina, Schwann cells, kidneys) during prolonged hyperglycemia.

Triose Kinase

An enzyme in fructose catabolism that phosphorylates glyceraldehyde to glyceraldehyde 3-phosphate using ATP so it can directly enter mainstream glycolysis.

SGLT-1

The sodium-dependent glucose transporter-1 that actively absorbs galactose and glucose from the intestinal lumen into intestinal mucosal enterocytes.

Galactokinase

An enzyme present in the liver, brain, and kidneys that phosphorylates galactose to galactose 1-phosphate using ATP.

Galactose-1-Phosphate Uridyltransferase (GALT)

An enzyme that catalyzes the exchange reaction converting galactose 1-phosphate and UDP-glucose into glucose 1-phosphate and UDP-galactose.

Galactokinase Deficiency

A relatively mild, autosomal recessive disorder characterized by elevated blood galactose (galactosemia) and urine galactose (galactosuria), resulting in early infantile cataract formation due to galactitol accumulation.

Classic Galactosemia

A severe, autosomal recessive disorder caused by absence of GALT, leading to toxic intracellular accumulation of galactose 1-phosphate and galactitol, presenting with failure to thrive, jaundice, hepatomegaly, cataracts, intellectual disability, and predisposition to neonatal Escherichia coli sepsis.

Galactitol

An osmotically active polyol produced from accumulated galactose by the action of aldose reductase; it becomes trapped in ocular lens cells, drawing water in and producing cataracts.

UDP-Galactose 4-Epimerase

An enzyme in galactose catabolism and nucleotide sugar recycling that interconverts UDP-galactose and UDP-glucose.

$\alpha$-Lactalbumin

Protein B of the lactose synthase complex whose synthesis is stimulated by prolactin in the lactating mammary gland, enabling mammary production of lactose during lactation.