Catecholamines Differ in Their Capacity to Form Melanin

Upon oxidation, the catechol amino acid L-DOPA polymerizes spontaneously to form the insoluble pigment melanin. Related catechols are less prone to polymerization, however, and this correlates with function. Comparison of the oxidation products of these catechols reveals that L-DOPA, dopamine, norepinephrine and epinephrine form a red-colored "chrome" upon oxidation, but epinephrine and norepinephrine, which act as neurohormones and neurotransmitters, do not go on to form an insoluble melanin. N-acetyldopamine (NADA), which functions in cuticle hardening in insects, does not even cyclize to the "chrome," so the quinone remains available to crosslink cuticular proteins without obscuring coloration by other pigments. Thus, the side-chains of catecholamines affect reactivity as well as function.