82 - NIMS MEDICINE february 2009 paper - 7

7q: what is the enzyme deficiency observed in SCID ( severe combined immunodeficiency syndrome ) ?

a. adenosine deaminase
b. peroxidase
c. glucose 6 phosphatase
d. none

answer: a . adenosine deaminase .

The second most common form of SCID after X-SCID is caused by a defective enzyme, adenosine deaminase (ADA), necessary for the breakdown of purines. Lack of ADA causes accumulation of dATP. This metabolite will inhibit the activity of ribonucleotide diphosphate reductase, the enzyme that reduces ribonucleotides to generate deoxyribonucleotides. The effectiveness of the immune system depends upon lymphocyte proliferation and hence dNTP synthesis. Without functional ribonucleotide reductase, lymphocyte proliferation is inhibited and the immune system is compromised.