GALACTOSE

'Galactose' (Gal) (also called 'brain sugar') is a type of sugar which is less sweet than glucose and not very water-soluble. It is considered a nutritive sweetener because it has food energy.
Galactan is a polymer of the sugar galactose. It is found in hemicellulose and can be converted to galactose by hydrolysis.

Contents

Sources

Relationship to lactose

Clinical significance

Structure and isomerism

Liver galactose metabolism

Metabolic disorders

References

Sources

It is found in dairy products, in sugar beets and other gums and mucilages.
It is also synthesized by the body, where it forms part of glycolipids and glycoproteins in several tissues.

Relationship to lactose

Galactose is a monosaccharide constituent, together with glucose, of the disaccharide lactose. The hydrolysis of lactose to glucose and galactose is catalyzed by the enzyme lactase, a β-galactosidase. In the human body, glucose is changed into galactose in order to enable the mammary glands to secrete lactose.
Galactose and glucose are produced by hydrolysis of lactose by ß-galactosidase. This enzyme is produced by the ''lac'' operon in ''Escherichia coli'' (''E. coli'').

Clinical significance

Two studies have suggested a possible link between galactose in milk and ovarian cancer.^[1][2] Other studies failed to show such a link.
There are some ongoing studies which suggest that galactose may have a role in treatment of focal segmental glomerulosclerosis (a kidney disease resulting in kidney failure and proteinuria). This effect is likely to be a result of binding of galactose to FSGS factor.

Structure and isomerism

The first and last -OH groups point the same way and the second and third -OH groups point the other way. D-Galactose has the same configuration at its penultimate carbon as D-glyceraldehyde.
Galactose is an optical isomer of glucose.

Liver galactose metabolism

In the liver, galactose is converted to glucose 6-phosphate 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

Metabolic disorders

There are 3 important disorders involving galactose:

'Name'	'Enzyme'	'Description'
''Galactosemia''	Galactokinase	Causes cataracts and mental retardation. If a galactose-free diet starts sufficiently early, the cataracts will regress without complications however neurological damage is permanent.
''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 do not have considerable long-term effects.

References

1. Lactase persistence and milk consumption as determinants of ovarian cancer risk, Cramer D, , , Am J Epidemiol, 1989
2. Galactose consumption and metabolism in relation to the risk of ovarian cancer, Cramer D, Harlow B, Willett W, Welch W, Bell D, Scully R, Ng W, Knapp R, , , Lancet, 1989