Why is arginine classified as an essential amino acid in humans when it is synthesized in the urea cycle?
Arginine participates in a number of metabolic pathways depending on the cell type. It is synthesized as an intermediate in the urea cycle pathway and is also obtained from dietary proteins. A number of key metabolites such as nitric oxide, phosphocreatine, spermine, and ornithine are derived from arginine. During normal growth and development, under certain pathological conditions (e.g., endothelial dysfunction) and if the endogenous production of arginine is insufficient, a dietary supplement of arginine may be required. Thus, arginine is considered a semi-essential amino acid.