描述
This pathway depicts the conversion of galactose into glucose, lactose and other sugar intermediates that may be used for a range of metabolic process. The milk sugar, lactose, is hydrolyzed in the intestine to its constituent monosaccharides, glucose and galactose. Galactose is metabolized primarily in the liver, in a sequence of three reactions that yield one molecule of glucose 1-phosphate per molecule of galactose. First, galactose is phosphorylated to yield galactose 1-phosphate. Then, galactose 1-phosphate and UDP-glucose react to form UDP-galactose and glucose 1-phosphate, and UDP-galactose undergoes epimerization to form UDP-glucose. The UDP-glucose can be used in gucuronidation reactions and other pentose interconversions. In a reaction shared with other pathways, glucose 1-phosphate can be converted into glucose 6-phosphate. Also shown in this pathway is how glucose is changed into galactose via a process called hexoneogenesis in order to enable the mammary glands to secrete lactose. However, most galactose in breast milk is synthesized from galactose taken up from the blood, and only 35% is made by de novo synthesis. Glycerol also contributes some to the mammary galactose production. Also depicted in this pathway are the conversions of other dietary di and tri-saccharides (raffinose, manninotriose, melbiose, stachyose) into galactose, glucose and fructose.