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Unit Code 8628:
Galactokinase, Blood

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Useful For

Diagnosis of the second most common cause of galactosemia

(ie, galactokinase deficiency)

Clinical Information

Three clinically important inborn errors of galactose metabolism

that result in galactosemia have been described. The genetic

disturbance is expressed as a deficiency of galactokinase,

galactose-1-phosphate uridyltransferase, (GPUT) or UDP

galactose-4-epimerase, the enzymes catalyzing the reactions in

converting galactose to glucose:

 

                 Kinase               Transferase                 Epimerase         

Gal ATP  ----------> Gal-1-P ------------> UDP-Gal ----------> UDP-Glu

                                                                   

                               UDP-Glu                    Glu-1-P

 

                                 

The transferase deficiency is the most commonly occurring of

the 3 disorders and is characterized by inanition, failure to thrive,

vomiting, liver disease, cataracts, and ultimately, mental

retardation if galactose ingestion is continued.

                                               

Galactokinase deficiency results in a milder variant of

galactosemia than that which results from GPUT deficiency.

Galactokinase deficiency is very rare and usually is expressed

by occurrence of juvenile cataracts in the absence of the mental

retardation (which occurs in transferase deficiency).

Manifestations, such as listed immediately above, along with

high serum or urine galactose levels are indications for ordering

galactokinase determinations.

 

The epimerase deficiency is like the kinase deficiency and is

much more rare than the transferase deficiency.

                                                                               

Inborn errors of galactose metabolism are transmitted by

autosomal recessive inheritance. Clinical manifestations result

from exposure to galactose or its alternative metabolites.

 

The presumptive diagnosis of galactosemia may be made by the

finding of galactosuria. The presence of galactose in the urine is

suggested by a positive test for reducing substances (Benedict's

test) and can be confirmed by thin-layer chromatography (TLC).

Reference Values

<2 years:  20.1-79.8 mU/g of hemoglobin

> or =2 years:  12.1-39.7 mU/g of hemoglobin

(literature values)

Interpretation

Values <20.1 mU/g of hemoglobin suggest galactokinase

deficiency.

Cautions

It should be noted that the presence of galactose in blood and

urine is dependent upon dietary consumption, and a test for

galactose is useful only after consumption of a source of

galactose.

 

The most common cause of galactosemia is GPUT deficiency

(see #8333 "Galactose-1-Phosphate Uridyltransferase(GALT),

Blood").

Clinical Reference

Holton JB, Walter JH, Tyfield LA:  Galactosemia. In The Metabolic

and Molecular Basis of Inherited Disease. 8th edition. Edited by

CR Scriver, AL Beaudet, WS Sly, et al. New York, McGraw-Hill

Book Company, 2001, pp 1553-1587

Key