arginine, organic compound, one of the 20 amino acids commonly found in animal proteins. Only the l-stereoisomer participates in the biosynthesis of proteins. Its basic side chain adds a positive charge and hence a greater degree of water-solubility to proteins in neutral solution. Although arginine can be synthesized from cellular metabolites, it is usually considered essential to the diet of children for the maintenance of normal rates of growth. Arginine is the direct metabolic precursor of urea, the dominant nitrogenous waste product of most mammals. It was discovered in protein in 1895.