Melanins from tetrahydroisoquinolines: spectroscopic characteristics, scavenging activity and redox transfer properties.

Tetrahydroisoquinolines (TIQs) are endogenous compounds deriving from the nonenzymatic Pictet-Spengler condensation of catecholamines (CA) with aldehydes. TIQs have been extensively studied in the last years not only because they have been found in the brain of postmortem specimens of Parkinson's patients, but also because they are able to induce parkinsonian symptoms if injected in animals. In the present article we demonstrate that TIQs bearing a catecholic moiety (tetrahydropapaveroline, salsolinol, laudanosoline, and apomorphine) are easily oxidized in the presence of hydrogen peroxide by various enzymes--i.e., peroxidase (POD), lipoxygenase (LOX), and xanthine oxidase (XO)--into the corresponding TIQ-melanins. The kinetic parameters of the above-mentioned reactions and some spectroscopic characteristics of the synthetized pigments are reported. In particular, UV-VIS and EPR spectra emerge as very similar to those exhibited by dopa-melanin. Furthermore, TIQ-melanins appear to be similar to dopa-melanin regarding some specific physico-chemical properties: NADH-oxidizing properties, oxy-radicals scavenging activity, and ability to form soluble mixed polymers with melanins from opioid peptides.