Lactose is a disaccharide consisting of glucose and galactose. After the ingestion of lactose, most commonly from breast milk for an infant or cow milk and any milk from an animal, the enzyme lactase hydrolyzes the sugar into its monosaccharide constituents, glucose and galactose. In the first step of galactose metabolism, galactose is converted to galactose-1-phosphate (Gal-1-P) by the enzyme galactokinase. Gal-1-P is converted to uridine diphosphate galactose (UDP-galactose) by the enzyme galactose-1-phosphate<unk> , with UDP-glucose acting as the UDP donor. UDP-galactose can then be converted to lactose, by the enzyme lactose synthase or to UDP-glucose by UDP-galactose epimerase (GALE). Normal metabolic pathway for galactose in humans. In classic galactosemia, galactose-1-phosphate<unk>  activity is reduced or absent; leading to an accumulation of the precursors, galactose, galactitol, and Gal-1-P. The elevation of precursors can be used to differentiate GALT deficiency from galactokinase deficiency, as Gal-1-P is typically not elevated in galactokinase deficiency. If the activity of the enzyme galactose<unk> -phosphate uridyltransferase is decreased, there is a buildup of the precursors in the pathway, mainly galactose<unk> -phosphate and galactose.
Galactose-1-phosphate uridylyltransferase deficiency
Arginine:glycine amidinotransferase deficiency
Ribose-5-phosphate isomerase deficiency
Malonyl-CoA decarboxylase deficiency
Ornithine aminotransferase deficiency
Galactose epimerase deficiency
Guanidinoacetate methyltransferase deficiency
Adenine phosphoribosyltransferase deficiency