They cannot synthesize isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Phenylalanine, tryptophan, and histidine are essential amino acids for animals. Tryptophan and 5-HTP are precursors to serotonin. Consuming purified tryptophan increases brain serotonin whereas eating foods containing tryptophan does not. Phenylalanine, tyrosine, and tryptophan, the aromatic amino acids, arise from chorismate. Nicotinic acid can be synthesized from tryptophan or aspartic acid. It catalyzes the hydrolysis of peptide bonds in L-isomers of tyrosine, phenylalanine, and tryptophan. There is increased dopaminergic activity, whereas there is reduced serotonin due to reduced tryptophan. Low levels of tryptophan and serotonin in the brain were already associated to depression. Some of the fecal odor is due to indoles, metabolized from the amino acid tryptophan. "Pichia" is used to produce the amino acid tryptophan and the vitamin pyridoxine. The intermediate in the synthesis from tryptophan to serotonin, 5-hydroxytryptophan, was not found. Some conditions can prevent the absorption of dietary niacin or tryptophan and lead to pellagra. The conversion is catalyzed by the enzyme tryptophan synthase. The resulting decarboxylated tryptophan analog is tryptamine.