DOPAMINE, IN THE PRESENCE OF TYROSINASE, COVALENTLY MODIFIES AND INACTIVATES TYROSINE-HYDROXYLASE

Dopamine has been implicated as a potential mediating factor in a vari ety of neurodegenerative disorders, Dopamine can be oxidized to form a reactive dopamine quinone that can covalently modify cellular macromo lecules including protein and DNA, This oxidation can be enhanced thro ugh various enzymes including tyrosinase and/or prostaglandin H syntha se, One of the potential targets in brain for dopamine quinone damage is tyrosine hydroxylase, the rate-limiting enzyme in catecholamine bio synthesis, The present studies demonstrated that dopamine quinone, the formation of which was enhanced through the activity of the melanin b iosynthetic enzyme, tyrosinase, covalently modified and inactivated ty rosine hydroxylase, Dihydroxyphenylalanine (DOPA; the catechol-contain ing precursor of dopamine) also inactivated tyrosine hydroxylase under these conditions, Catecholamine-mediated inactivation occurred with b oth purified tyrosine hydroxylase as well as enzyme present in crude p heochromocytoma homogenates, Inactivation was associated with covalent incorporation of radiolabelled dopamine into the enzyme as assessed b y immunoprecipitation, size exclusion chromatography, and denaturing s odium dodecylsulfate (SDS)-poly-acrylamide gel electrophoresis, Furthe rmore, the covalent modification and inactivation of tyrosine hydroxyl ase was blocked by antioxidant compounds (dithiothreitol, reduced glut athione, or NADH). In addition to kinetic feedback inhibition and the formation of an inhibitory dopamine/Fe+3 complex, these findings sugge st that a third mechanism exists by which dopamine (or DOPA) can inhib it tyrosine hydroxylase, adding further complexity to the regulation o f catecholamine biosynthesis, (C) 1998 Wiley-Liss,Inc.