Urea cycle disorder and protein metabolism

BOSTON — Protein metabolism presents some serious waste management challenges for the body. When protein is broken down, one of the by-products is ammonia. Ammonia is toxic, so our bodies have elaborate systems for getting rid of it. Most of the detoxifying work falls to our liver. There, ammonia travels a multistep pathway featuring five enzymes that turn it into urea, to be excreted in urine. People with rare genetic deficits that interfere with the urea cycle often die in childhood. Other mutations, though, may be responsible for some problems in adults, reports the May 2007 issue of the Harvard Health Letter.

One in 8,000 American children has a genetic defect that causes one or more of the enzymes involved in ammonia processing to be defective or scarce. As these children start to consume protein, the ammonia begins to pile up. Treatment includes protein restriction, medications that sop up extra ammonia, dialysis, and possibly liver transplant. The death rate is high.

Urea cycle disorders are viewed as rare and primarily pediatric conditions, but there might be a whole range of unrecognized, genetically determined problems with protein metabolism experienced by adults. Some people may have mild mutations that compromise a gene's function and cause slight symptoms. This may explain why one person eschews meat while another loves nothing more than a steak meal. Defects in protein metabolism may also explain why some people have bad reactions to high-protein diets like the Atkins diet.

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