Galactosemia and impaired interconversions ?

A newborn infant presents with poor feeding, vomiting, jaundice and an enlarged liver. The urine test is positive for a reducing substance. Which of the following conversions is most likely to be normal?

A. Glucose to Galactose

B. Activated galactose to activated Glucose

C. Lactose to galactose

D. Excretion of galactose by kidney

E. Excretion of glucose by kidney

Answer- The correct answer is- B- Conversion of activated galactose to activate glucose.

As evident from the clinical features, the child is suffering from Galactosemia.

Galactosemia is associated with the following 3 enzyme deficiencies;

1) Classical galactosemia is a major symptom of three enzyme defects. It results from loss of the enzyme galactose-1-phosphate uridyl transferase.

2) The second form of galactosemia results from a loss of Galactokinase and

3) The third disorder results from a deficiency of UDP-galactose-4-epimerase (Figure)

Galactokinase

Galactose metabolism

Figure- Metabolism of galactose

Galactose-1-phosphate uridyl Transferase (GALT) deficiency is the most common enzyme deficiency that causes Galactosemia.

Galactose-1-phosphate uridyl Transferase (GALT) catalyzes conversion of galactose-1-p to UDP Galactose (Figure-1). This is an important reaction during the metabolism of galactose.

Galactokinase catalyzes the conversion of galactose to galactose-1-P; its deficiency is less common and is less severe.

UDP galactose epimerase catalyzes the interconversion of UDP galactose to UDP glucose.

Thus except for Galactokinase, the other two enzymes catalyze the interconversion of activated galactose and activated glucose.

As regards other options

As such the conversion of glucose to galactose is not possible. They can be interconverted only in the activated forms

Lactose to galactose conversion takes place by lactase enzyme which is deficient in lactose intolerance. Lactose intolerance does not produce the symptoms as described above.

Excretion of galactose or glucose by kidney can get impaired only in renal failure; enzymes are not involved in their excretion from the kidney.

Hence considering the diagnosis of galactosemia in this child, the most suitable option is- B, impairment of conversion of activated galactose to activated glucose.

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