Galactose

Galactose is a monosaccharide, an aldohexose with chemical formula C6H12O6. In biology, the D-form is most common in nature. It is an epimer of glucose, differing at carbon 4.

Natural occurrence and dietary sources: Galactose occurs primarily as a constituent of lactose, the disaccharide in milk, where it is bound to glucose. It is released during digestion by lactase activity. It is also found in small amounts in some foods and is produced industrially from lactose.

Metabolism: In humans the main route is the Leloir pathway. Galactose is phosphorylated by galactokinase to galactose-1-phosphate. Galactose-1-phosphate uridylyltransferase exchanges galactose-1-phosphate with UDP-glucose to yield UDP-galactose and glucose-1-phosphate. UDP-galactose is converted to UDP-glucose by UDP-galactose 4-epimerase. Glucose-1-phosphate is converted to glucose-6-phosphate and enters glycolysis or glycogenesis. Disorders in these steps cause galactosemia.

Clinical significance: Galactosemia is a metabolic disorder caused by deficiencies in GALT, GALK, or GALE. Classic galactosemia (GALT deficiency) presents in newborns with failure to thrive, vomiting, jaundice, hepatomegaly, and risk of cataracts; treatment is strict dietary restriction of lactose- and galactose-containing foods. Less severe variants include Duarte and GALE-related deficiencies.

Other notes: Galactose is used in the synthesis of glycoproteins and glycolipids, and serves as a precursor for UDP-galactose in biosynthetic pathways. It has applications in research and industry as a substrate for enzymatic assays and to study carbohydrate metabolism.