Question 4

A new born diagnosed with hyperphenylalaninemia at birth and placed on special alanine-restricted diet. Several months later, the infant has a normal serum alanine level; however, some neurological abnormalities are observed. His serum prolactin level is elevated. Which of the following enzymes are mostly deficient in this patient?
a. Phenylalanine hydroxylase
b. Dihydrobiopterin reductase
c. Dopamine hydroxylase
d. Homogentisate oxidase
e. N-methyltransferase
f. Tyrosinase

The answer is: b

This enzyme is responsible for reduction of dihydrobiopterin (BH2) to BH4.
(BH4) is a co factor for enzymes that participate in the synthesis of tyrosine, Dopamine and serotonin, as well as nitric oxide.

Under normal conditions dopamine from the tuberoinfundibular system typically inhibits the release of prolactin. Decreased BH4 causes decreased levels of dopamine, which therefore cause increased levels of prolactin.