Enzyme Defects in Urea Cycle

Defects of the enzymes mediating all four reactions of the urea cycle proper have now been established, and there is some evidence of the existence of a fifth enzyme defect, involving carbamyl phosphate synthetase, mediating the initial reaction of the pathway. As the first report of a metabolic disorder involving the urea cycle was only in 1958, it is not surprising that there have been very few reviews of this topic, that of Efron (El) being the most complete to date. 

Since one of the most important, if not the most important, result of a defect of the biosynthesis of urea is an increased level of blood ammonia, it is essential to consider other conditions that might affect indirectly the urea cycle or in some other way raise the blood ammonia. For example, it has been suggested that since lysine can act as a competitive inhibitor of the conversion of arginine to ornithine and urea, an increased level of plasma lysine may therefore inhibit the urea cycle (B12). 

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