Galactosemia

Galactosemia

Galactosemia is an inborn error of metabolism. Because of energy barriers, essentially none of the chemical reactions that take place in living things could occur at any measurable rate without the presence of a catalyst. Most catalysts in living things are enzymes that depend on their structure to be able to function. Their structure is determined by their coding on DNA. Inborn errors of metabolism, like the one seen in galactosemia, are caused by defective genes.

Galactosemia is an inherited metabolic disorder in which the transformation of galactose to glucose is blocked, allowing galactose to increase to toxic levels in the body (Chung 1997). Galactose epimerase, the enzyme in the liver that is required to break down galactose, is deficient in galactosemia patients ("Galactosemia" 1995 and Wohlers, Christacos, and Harreman 1999). This enzyme works as a catalyst to speed up the breakdown of galactose. When there is a deficiency of this enzyme, the body cannot metabolize galactose as quickly as needed, causing a toxic buildup (Olendore, Jenyan, and Bayden 1999).

This disease is inherited in an autosomal recessive manner, this means that galactosemia is only present in individuals with two defective copies of any one of the three genes that causes it (Chung 1997). These genes are the genes that code for the three enzymes, galactosemia-1-phosphate-uridyl transferase (GALT), galactokinase (GALK), and uridyl disphosphogalactose-4-epimerase (Olendore, Jenyan, and Bayden 1999). Although carriers have less than normal enzyme activity, carriers of the disease are unaware that they are carrying a defective gene since no symptoms are evident (Chung 1997). If two carriers of the same defective gene have children, the chance of their child getting galactosemia by having two copies of the same defective gene is 25% for each pregnancy (Elsas 1999). Every cell nucleus has two copies of each gene, therefore, if only one of the two copies is defective, enough of the enzyme is made and the pathway of galactose metabolism is not blocked (Olendore, Jenyan, and Bayden 1999).

Most states have now included testing for galactosemia in newborn screening programs ("Galactosemia" 1995). However, if galactosemia is not found in a screening program, some symptoms appear within the first couple of days of the newborn's life (Elsas 1999). Symptoms usually begin to appear quickly in newborns because their entire diet is made up of milk, which is made of 20% galactose (Olendore, Jenyan, and Bayden 1999). High levels of galactose cause vomiting diarrhea, lethargy, low blood sugar, brain damage, jaundice, liver enlargement, cataracts, malnutrition, rapid organ damage, susceptibility to infection especially to gram negative bacteria, and even death (Olendare, Jenyan, and Bayden 1999 and Chung 1997). Infants may also exhibit poor growth, feeding difficulties, encephalopathy, and renal tubular dysfunction (Berry et al. 1995).

The Human Genome Project has had a great impact on what is known about galactosemia. They have identified what causes the disease and on which chromosome the mutation occurs. Three enzymes are required to completely convert galactose to glucose-1-phosphate, which is able to enter the metabolic pathway and turn into energy. A separate gene encodes each of these three enzymes. If any of these enzymes fail to function galactose builds up and galactosemia result (Olendore, Jenyan, and Bayden 1999).

The first type of galactosemia is called galactosemia I or classic galactosemia. This form has been discovered to be caused by defects in both copies of the gene that codes the enzyme galactosemia-1-phosphate-uridyl transferase (GALT) (Olendore, Jenyan, and Bayden 1999). This enzyme is responsible for the second phase of galactose metabolism. Without this enzyme, the body cannot convert galactose to UDP galactose, which eventually leads to glucose formation causing hypoglycemia. Since this cannot occur, the galactose metabolite, galactose-1-phosphate remains unconverted and accumulates causing rapid damage to vital organs (Chung 1997). There are thirty known mutations in this gene that cause GALT to malfunction. The frequency of this form is relatively high, occurring in 1 in 50,000 to 70,000 births (Olendore, Jenyan, and Bayden 1999).

The second type of galactosemia is called galactosemia II. This form is caused by defect in the gene that codes for the enzyme galactokinase (GALK). Galactokinase normally acts as a catalyst that converts galactose-1-phosphate to glucose-1-phosphate using a series of reactions requiring uridine triphosphate (UTP) as a coenzyme. Without galactokinase, the reaction occurs too slowly and galactose-1-phosphate is not converted to glucose-1-phosphate (Oldenore, Jenyan, and Bayden 1999). A deficiency in galactokinase causes some physical problems such as nuclear cataracts before or shortly after birth. It also causes mental retardation in some. Biochemically, it results in the increased secretion of galactose and corresponding