Lactose is a disaccharide contained mainly in the milk of mammals (including the human species). 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.
Lactase is an inducible enzyme, present in the newborn, able to hydrolyse the Β-1, 4-glycosidic bond of lactose (Β-galactosyl-α-glucoside) and then split the latter into galactose and glucose. The inducibility of this enzyme can be lost in the case of non-use of milk; if an individual "loses" completely the gene that codes for lactase, he becomes lactose intolerant, that is, lactose, when ingested, is not degraded and is therefore not assimilable by the cells: lactose accumulates in the intestine and draws back liquids. 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 is manifested by diarrhea, meteorism and other gastrointestinal disorders.
Galactose follows glucose metabolism, after an activated form of it is converted by an enzyme epimerase (galactose is an epimer on the fourth carbon of glucose) to an activated form of glucose.
The galactase kinase which converts galactose into galactose 1-phosphate acts on galactose. The second enzyme in the process is galactose 1-phosphate uridyl transferase: it is a dependent UDP-glucose enzyme that works at NAD +. From the action of this enzyme glucose 1-phosphate and UDP-galactose are obtained: the galactose 1-phosphate attacks the phosphoryl α of UDP-glucose and thus, the formation of the just mentioned products occurs. UDP-galactose then goes under the action of the UDP- galactic enzyme 4-epimerase (it is the key enzyme of the process) and is converted into UDP-glucose which returns to circulation; this enzyme acts on the fourth carbon of galactose: it transforms the hydroxyl into carbonyl and then regenerates the hydroxyl function with a configuration inversion.
A serious illness of the newborn is due to the deficiency of galactose 1-phosphate uridyl transferase: galactosemia. This pathology occurs at birth and can be cured by removing lactose from the child'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.
