The apparent autoxidation rate of catechols in dopamine-rich regions of human brains increases with the degree of depigmentation of substantia nigra.

The concentrations of the 5-S-cysteinyl adducts of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and 3,4-dihydroxyphenylalanine (DOPA) and the levels of noradrenaline (NA), DA, DOPAC and DOPA were determined in the putamen (PUT), caudate nucleus (CN) and substantia nigra (SN) of human post mortem brains with or without depigmentation and degeneration of the SN. The levels of DA, DOPAC and DOPA decreased with the degree of depigmentation and degeneration in the three brain regions while NA levels only decreased in SN and PUT. In general, the concentrations of the 5-S-cysteinyl adducts did not differ, but the ratios of 5-S-cysteinyl-DA/DA, 5-S-cysteinyl-DOPAC/DOPAC and 5-S-cysteinyl-DOPA/DOPA were higher in patients with a more depigmentated and degenerated SN, except for the 5-S-cysteinyl-DA/DA ratio in the PUT. Higher ratios were also found in the cell body areas compared to the neuron terminal areas. Thus depigmentation and degeneration of dopaminergic SN neurons, seem to be correlated to enhanced rates of autoxidation, possibly due to an impaired antioxidant capacity.