This pathway depicts the conversion of galactose into glucose, lactose, and other sugar intermediates that may be used for a range of metabolic process. Dietary sources of galactose are numerous, but some of the primary sources in the human diet can be found in milk and milk derivative products. This is because during digestion milk sugars and lactose are hydrolyzed into their molecular constituents (e.g. base monosaccharides). In milk, such monosaccharides include glucose and galactose. The metabolism of the sugar Galactose is occurs almost entirely in the liver, and its metabolism is the consequence of three steps or reactions. First, the phosphorylation of galactose is induced by a special enzyme with the predictable name, galactokinase, and produces galactose 1-phosphate. Second, this biproduct and a second molecule, UDP-glucose, undergo a reaction which leads to the formation of UDP-galactose and glucose 1-phosphate. Thus, this reaction produces 1 molecule of glucose 1-phosphate per molecule of galactose. This is mediated by the enzyme galactose-1-phosphate uridylyltransferase (GALT). The resulting UDP-galactose undergoes epimerization to form UDP-glucose via the enzyme UDP-galactose-4 epimerase (GALE). The UDP-glucose can be used in glucuronidation reactions and other pentose interconversions. In a reaction shared with other pathways, glucose 1-phosphate can be converted into glucose 6-phosphate. There are other pathways associated with galactose metabolism. For instance, galactose can be converted into UDP-glucose by the sequential activities of GALK, UDP-glucose pyrophosphorylase 2 (UGP2), and GALE. Galactose can also be reduced to galactitol by NADPH-dependent aldose reductase. Also shown in this pathway is the conversion of glucose to galactose vis a vis a different process to the ones described earlier. This pathway, called hexoneogenesis, allows mammary glands to produce galactose. It should be noted however, that despite the existence of this pathway of galactose production, the vast majority of galactose in breast milk is actually the result of direct uptake up from the blood, whereas only a small fraction, ~35%, is the result of this de novo process hexoneogenesis. Also depicted in this pathway are the conversions of other dietary di and tri-saccharides (raffinose, manninotriose, melibiose, stachyose) into galactose, glucose and fructose as well as and dietary sugar alcohols (melibitol, galactinol, galactosylglycerol) into sorbitol, myo-inositol, and glycerol.

Galactose Metabolism References