Lactose is a disaccharide contained mainly in the milk of mammals (including humans). It is formed by the union of two simple sugars (monosaccharides), called respectively glucose and galactose. To be digested, lactose must necessarily be broken down into these two simpler units.
There lactase is an inducible enzyme, present in the newborn, able to hydrolyze the Β-1,4-glycosidic bond of lactose (Β-galactosyl-α-glucoside) and thus break the latter into galactose and glucose. this enzyme can be lost if milk is not used; if an individual completely "loses" the gene that codes for lactase, he becomes intolerant to lactose, that is, lactose, when ingested, is not degraded and therefore cannot be assimilated by the cells: lactose then accumulates in the intestine and recalls This inactivation of the gene that codes for lactase is responsible for a disorder, known as lactose intolerance, which arises following the ingestion of milk or other foods containing lactose and manifests itself with diarrhea, meteorism and other gastrointestinal disorders.
Galactose follows the metabolism of glucose, after an activated form of it is converted by an epimerase enzyme (galactose is an epimer on the fourth carbon of glucose) to an activated form of glucose.
Galactose acts on galacto kinase which converts galactose to galactose 1-phosphate. The second enzyme in the process is the galactose 1-phosphate uridyl transferase: is a UDP-glucose dependent enzyme that works on NAD +. From the action of this enzyme glucose 1-phosphate and UDP-galactose are obtained: galactose 1-phosphate attacks the phosphoryl α of UDP-glucose and thus the products just mentioned are formed. The UDP-galactose then goes under the action of the enzyme UDP-galactose 4-epimerase (it is the key enzyme of the process) and is converted into UDP-glucose which returns to the circulation; this enzyme acts on the fourth carbon of galactose: it transforms the hydroxyl into carbonyl and then regenerates the hydroxyl function with an inversion of configuration.
A serious illness of the newborn is due to the deficiency of galactose 1-phosphate uridyl transferase: galactosemia. This disease occurs at birth and can be cured by eliminating lactose from the baby's diet. The deficiency of galactose 1-phosphate uridyl transferase causes an accumulation of galactose which is not toxic as such but, by reduction on the first carbon, leads to the formation of galactosyl polyalcohol which is able to act on neurons and cause serious brain damage.
