Metabolism and Functions of Amino Acids in Sense Organs.

Sense organs (eyes, ears, nose, tongue, and skin) provide senses of sight, hearing, smell, taste, and touch, respectively, to aid the survival, development, learning, and adaptation of humans and other animals (including fish). Amino acids (AAs) play an important role in the growth, development, and functions of the sense organs. Recent work has identified receptor-mediated mechanisms responsible for the chemosensory transduction of five basic taste qualities (sweet, sour, bitter, umami and salty tastes). Abnormal metabolism of AAs result in a structural deformity of tissues and their dysfunction. To date, there is a large database for AA metabolism in the eye and skin under normal (e.g., developmental changes and physiological responses) and pathological (e.g., nutritional and metabolic diseases, nutrient deficiency, infections, and cancer) conditions. Important metabolites of AAs include nitric oxide and polyamines (from arginine), melanin and dopamine (from phenylalanine and tyrosine), and serotonin and melatonin (from tryptophan) in both the eye and the skin; γ-aminobutyrate (from glutamate) in the retina; and urocanic acid and histamine (from histidine) in the skin. At present, relatively little is known about the synthesis or catabolism of AAs in the ears, nose, and tongue. Future research should be directed to: (1) address this issue with regard to healthy ageing, nasal and sinus cancer, the regulation of food intake, and oral cavity health; and (2) understand how prenatal and postnatal nutrition and environmental pollution affect the growth, development and health of the sense organs, as well as their expression of genes (including epigenetics) and proteins in humans and other animals.