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Galactose 1-phosphate is not converted to galactose 1-phosphate by hexokinase, and galactose is not converted to galactose 1-phosphate. Instead, UDP-glucose is converted to UDP-galactose by UDP-hexose 4-epimerase, providing the appropriate form of galactose for lactose synthesis. Isomerization of fructose to galactose does not occur in the human body. Galactosemia is the result of a deficiency in the enzyme galactose 1-phosphate uridylyltransferase.

A 5-month-old boy, who was being weaned off breast milk, developed symptoms such as vomiting, night sweats, and tremors after introducing fruit juices to his diet. The presence of reducing sugar in his urine suggests a deficiency in aldolase B, indicating hereditary fructose intolerance. Deficiencies in fructokinase or galactokinase lead to elevated levels of fructose or galactose, respectively, in the blood and urine. A deficiency in β-galactosidase results in a decreased ability to degrade lactose, but this is not the case in this baby.

Lactose synthesis, essential for milk production by mammary glands, involves the transfer of galactose from galactose 1-phosphate to glucose by galactosyltransferase (protein A), generating lactose. Protein A is not exclusively used in lactose synthesis. α-Lactalbumin (protein B) regulates the specificity of protein A by decreasing its affinity for glucose. The expression of protein B is stimulated by the hormone prolactin.

A 3-month-old girl who is developing cataracts exhibits signs such as not having a social smile or being able to track objects visually. However, all other aspects of her examination are normal. Tests on her urine show positive results for reducing sugar but negative results for glucose. The most likely enzyme deficiency in this girl is Galactose 1-phosphate uridylyltransferase.

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C. hexokinase can efficiently phosphorylate galactose to galactose 1phosphate.
D. the enzyme affected in galactosemia is activated by a hormone produced in the mammary gland.
Correct answer = B. Uridine diphosphate (UDP)-glucose is converted to UDPgalactose by UDP-hexose 4-epimerase, thereby providing the appropriate form of galactose for lactose synthesis. Isomerization of fructose to galactose does not occur in the human body. Galactose is not converted to galactose 1phosphate by hexokinase. A galactose-free diet provides no galactose. Galactosemia is the result of an enzyme (galactose 1-phosphate uridylyltransferase) deficiency.
2.2. A 5-month-old boy is brought to his physician because of vomiting, night sweats, and tremors. History revealed that these symptoms began after fruit juices were introduced to his diet as he was being weaned off breast milk. The physical examination was remarkable for hepatomegaly. Tests on the baby’s urine were positive for reducing sugar but negative for glucose. The infant most likely suffers from a deficiency of:
A. aldolase B.
B. fructokinase.
C. galactokinase.
D. β-galactosidase.
Correct answer = A. The symptoms suggest hereditary fructose intolerance, a deficiency in aldolase B. Deficiencies in fructokinase or galactokinase result in relatively benign conditions characterized by elevated levels of fructose or galactose in the blood and urine. Deficiency in β-galactosidase (lactase) results in a decreased ability to degrade lactose (milk sugar). Congenital lactase deficiency is quite rare and would have presented much earlier in this baby (and with different symptoms). Typical lactase deficiency (adult hypolactasia) presents at a later age.
2.3. Lactose synthesis is essential in the production of milk by mammary glands. In lactose synthesis:
A. galactose from galactose 1-phosphate is transferred to glucose by galactosyltransferase (protein A), generating lactose.
B. protein A is used exclusively in lactose synthesis.
C. α-lactalbumin (protein B) regulates the specificity of protein A by decreasing its affinity for glucose.
D. protein B expression is stimulated by prolactin.
Correct answer = D. α-Lactalbumin (protein B) expression is increased by the hormone prolactin. Uridine diphosphate–galactose is the form used by the galactosyltransferase (protein A). Protein A is also involved in the synthesis of the amino sugar N-acetyllactosamine. Protein B decreases the Michaelis constant (Km) and, so, increases the affinity of protein A for glucose.
2.4. A 3-month-old girl is developing cataracts. Other than not having a social smile or being able to track objects visually, all other aspects of the girl’s examination are normal. Tests on the baby’s urine are positive for reducing sugar but negative for glucose. Which enzyme is most likely deficient in this girl?
A. Aldolase B
B. Fructokinase
C. Galactokinase
D. Galactose 1-phosphate uridylyltransferase