Galaktose

Galactose is a simple sugar, a monosaccharide of the aldohexose class. In animals, the naturally occurring form is D-galactose, which is the C-4 epimer of glucose. It occurs most prominently in lactose, the disaccharide in milk, where galactose is linked to glucose. Free galactose can be formed by hydrolysis of lactose during digestion.

Chemistry and isomerism: Galactose exists in solution as cyclic hemiacetals and can adopt alpha and beta anomeric forms. It is one of several hexoses that can participate in biological processes through mutarotation and linkage in glycans. Its preferred natural form in humans is D-galactose.

Metabolism: After intestinal absorption, galactose is phosphorylated by galactokinase to galactose-1-phosphate. In the Leloir pathway, galactose-1-phosphate reacts with UDP-glucose to yield UDP-galactose and glucose-1-phosphate, catalyzed by galactose-1-phosphate uridylyltransferase. UDP-galactose is interconvertible with UDP-glucose by UDP-galactose 4-epimerase. Glucose-1-phosphate is converted to glucose-6-phosphate and enters glycolysis or glycogenesis. Defects in these steps disrupt galactose metabolism.

Clinical relevance: Impaired galactose metabolism causes galactosemia, a group of disorders that can present in infancy with liver dysfunction, jaundice, cataracts, and feeding difficulties. Classic galactosemia results from GALT deficiency; milder forms involve GALK or GALE deficiencies. Management relies on early detection and dietary restriction of lactose and galactose.

Dietary sources and roles: Beyond milk, galactose is found in some dried fruits and certain polysaccharides. In the body, it serves as a building block for glycoproteins, glycolipids, and other glycoconjugates and contributes to energy production via standard carbohydrate pathways.