Arginine is a complex amino acid that is often found at the active (or catalytic) site in proteins and enzymes due to its amine-containing side chain. Although arginine is considered an essential amino acid (it must be obtained through the diet), this is true only during the juvenile period in humans. Arginine is incorporated in proteins at about a 4.7 percent on a per-mole basis when compared to the other amino acids. Natural sources of arginine are brown rice, nuts, popcorn, raisins, and whole-wheat products.