A 20-year-old, Gravida 1, Para 0 woman at 16 weeks’ gestation presents to the obstetrician’s office to discuss her amniocentesis test results. Her past medical history is unremarkable. Her father has a history of recurrent foot pain and kidney stones. Her vital signs are blood pressure 114/78 mm Hg and heart rate of 82 beats/minute. Fetal heartbeat is heard using fetal Doppler and has a normal rate of 143 beats/minute. The obstetrician explains that the amniocentesis revealed a deficiency in hypoxanthine-guanine phosphoribosyltransferase (HPRT) activity.
Which of the following statements is correct regarding the action of this enzyme?
D) Phosphoribosyl pyrophosphate (PRPP) is needed for the conversion of guanine into guanosine monophosphate
Hypoxanthine-guanine phosphoribosyltransferase (HPRT) catalyzes the salvage pathway of inosine monophosphate (IMP) and guanosine monophosphate (GMP) from hypoxanthine and guanine, respectively, with PRPP as a co-substrate. The HPRT abnormality seen in this patient results in the accumulation of hypoxanthine and guanine, which are converted into uric acid by means of xanthine oxidase. The abnormalities can range from clinically unnoticeable in the mildest form to severe action dystonia, choreoathetosis, mild to moderate intellectual disability, and self-mutilating behavior in the complete form of Lesch-Nyhan syndrome.
Answer choice A: It converts guanine into inosine monophosphate with help of nicotinamide adenine dinucleotide (NADH), is incorrect. Inosine monophosphate (IMP) and guanosine monophosphate (GMP) are made from hypoxanthine and guanine, respectively.
Answer choice B: It converts hypoxanthine and guanine into xanthine, is incorrect.
Hypoxanthine and guanine are converted into xanthine by enzyme xanthine oxidase and guanine deaminase, respectively.
Answer choice C: It converts hypoxanthine and guanine, is incorrect. Both of these are free purine bases.
Answer choice E: Thiamine is needed for its action, is incorrect. Thiamine is used in many biochemical reactions but not this one.
Key Learning Point
Phosphoribosyl pyrophosphate (PRPP) is needed for the conversion of guanine into guanosine monophosphate in the purine salvage pathway.
