ornithine cycle

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A cyclic series of reactions in which potentially toxic, nitrogenous products from protein catabolism are converted to urea that is innocuous. In the cycle diagrammed below, ammonia is removed from the system and used in the conversion of ornithine to citrulline. Aspartic acid enters the cycle, and its amino group is incorporated into arginosuccinic acid before it can form ammonia. Arginosuccinic acid is converted to arginine, and the fumaric acid released enters the citric acid cycle (q.v.). Urea splits off arginine and regenerates ornithine. In humans, mutants are known that block the cycle at any one of its steps, as shown in the diagram. Blocking produces disorders that include: ornithine transcarbamylase deficiency, from blocking of step 1; citrullinuria (condensing enzyme deficiency), step 2; arginosuccinic aciduria (arginosuccinase deficiency), step 3; lysine intolerance (inhibition of arginase by excess lysine), step 4.

Ornithine cycle

Subjects: Genetics and Genomics.

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