A 2-week-old girl of north European descent is found to have a blood methionine concentration of 1500 μM (normal

Question

A 2-week-old girl of north European descent is found to have a blood methionine concentration of 1500 μM (normal <50) on newborn screening. Physical examination shows no abnormalities. Which of the following enzymes is most likely to be deficient in this patient's liver?

Quizplus · Accepted Answer

The elevated methionine level suggests a deficiency in cystathionine beta-synthase, which is involved in the conversion of homocysteine to cystathionine. This condition is known as homocystinuria. The most common causes of homocystinuria are defects in the cystathionine β-synthase (CBS) gene. CBS is involved in the conversion of methionine to cysteine and so a deficiency in the enzyme can be suspected in cases where there are elevated levels of methionine in the blood.

A 2-Week-Old Girl of North European Descent Is Found to Have