Purine Salvage Pathway & Lesch-Nyhan Syndrome


Purine Salvage Lesch-Nyhan SCID Gout

Nucleic acids are constantly being recycled in the body. This salvages free purine bases which can be reused to make new nucleic acids. DNA or RNA breakdown releases free Guanosine Monophosphate (GMP) & Adenosine Monophosphate (AMP). The sugar and phosphate groups are removed to give us Adenosine & Guanine. Adenosine is then converted to an intermediate, Inosine. When needed Inosine & Guanine are recycled by Hypoxanthine Guanine Phosphoribosyl Transferase so they can be used in the formation of new DNA & RNA.

Sometimes you have more nucleic acids than you need. So Hypoxanthine & Guanine can also be degraded. This process involves converting Hypoxanthine and Guanine into Xanthine. Xanthine is then acted on to form Uric Acid which can be excreted in the urine. In small amounts this process does not cause problems. However, if an excess amount of purines are being degraded the amount of uric acid will build up in the body as the kidney’s ability to excrete it is no longer sufficient. This uric acid builds up and can form crystals in the joints (Gout) and kidney (Uric Acid Renal Stones). Allopurinol is a gout drug which inhibits the formation of uric acid from xanthine. Each of these items will be discussed more in chapters on the appropriate organ system.

A deficiency of Adenosine Deaminase leads to a buildup of adenosine which is toxic to B & T cells. This is one mechanism of developing Severe Combined Immunodeficiency (SCID), a deficiency of B & T cells. This will be covered in more detail in the Immunology section.

Lesch-Nyhan Syndrome is an X-linked recessive deficiency of Hypoxanthine Guanine Phosphoribosyl Transferase (HGPT). This deficiency prevents proper purine salvage. As purines cannot be recycled back into new nucleic acids, all of the purines from degraded DNA are converted to uric acid. This excess in uric acid exceeds the body’s ability to excrete uric acid. The buildup of uric acid can be detected in the blood and causes urate crystal formation resulting in gout and renal stones. Lesch-Nyhan is also characterized by poor muscle control (spasticity), mild mental retardation and Lip/Finger Biting (self-mutilation).


Now that you are done with this video you should check out the next video in the Genetics section which covers Neurofibromatosis, Tuberous Sclerosis & Von Hippel Lindau

Leave a Reply

Free USMLE Step1 Videos