Galactolysis

Galactolysis refers to the catabolism of galactose.

In the liver, galactose is converted through the Leloir pathway to glucose 6-phosphate^[1] in the following reactions:

       galacto-                uridyl                phosphogluco-
        kinase               transferase                mutase
   gal --------> gal 1 P ------------------> glc 1 P -----------> glc 6 P
                            ^           \
                           /             v
                        UDP-glc       UDP-gal
                           ^             /
                            \___________/
                              epimerase

Mutations in the enzymes involved in galactolysis result in metabolic disorders.^[2]

Metabolic disorders

There are 3 types of galactosemia or galactose deficiencies:

Name	Enzyme	Description
galactokinase deficiency	Galactokinase	Causes cataracts, which form due to the elevation of galactitol that accumulates when galactose is metabolized in an alternative pathway that is not the Leloir pathway.[2] These are treatable by restricting galactose from the diet.
UDPgalactose-4-epimerase deficiency	UDPgalactose-4-epimerase	Is extremely rare (only 2 reported cases). It causes nerve deafness.
Galactose-1-phosphate uridyl transferase deficiency	Galactose-1-phosphate uridyl transferase	Is the most problematic, as galactose-free diets are not effective in treating neurocognitive deficiencies (in particular language disorders such as verbal dyspraxia) and ovarian failure. If a galactose-free diet is administered, cataracts and acute symptoms such as kidney and liver failure respond immediately. Formation of cataracts is similar to that in galactokinase deficiency.[2]