Question

What is the enzyme that is defective in most patients with SCID? What are two enzymes of purine degradation that may be defective in SCID?

Answer 1

Adenosine deaminase (ADA) is commonly defective in SCID patients, affecting the immune system drastically. Two enzymes of purine degradation that may be defective in SCID are ADA and Hypoxanthine-Guanine Phosphoribosyltransferase (HGPRT). G6PD deficiency, another enzymatic disorder, causes hemolytic anemia, not SCID.

Step-by-step explanation:

The Enzyme Defective in SCID

The enzyme that is most commonly defective in patients with Severe Combined Immunodeficiency (SCID) is adenosine deaminase (ADA). ADA is critical for the purine degradation pathway, and its deficiency leads to the accumulation of deoxyadenosine, which is toxic to lymphocytes, affecting both the B cell and T cell arms of the adaptive immune system. Without a functional immune system, patients with SCID are highly susceptible to infections.

Enzymes of Purine Degradation Defective in SCID

Two enzymes that can be defective in SCID, in association with purine degradation, are:

1. Adenosine Deaminase (ADA) - As mentioned, a deficiency in ADA is a common cause of SCID.
2. Hypoxanthine-Guanine Phosphoribosyltransferase (HGPRT) - A deficiency in HGPRT can result in an SCID-like syndrome, as seen in Lesch-Nyhan syndrome, where purines are not adequately salvaged and consequently degraded to uric acid.

In contrast, defects in glucose-6-phosphate dehydrogenase (G6PD) lead to a different condition known as G6PD deficiency, which is characterized by hemolytic anemia and is the most common enzyme deficiency worldwide.