Brain tissue from normal individuals with incidental Lewy bodies and cell loss in pigmented substantia nigra neurons (asymptomatic Parkinson's disease) and age-matched control subjects without nigral Lewy bodies was examined biochemically. There was no difference in dopamine levels or dopamine turnover in the caudate and putamen of individuals with incidental Lewy body disease compared to control subjects. There were no differences in levels of iron, copper, manganese, or zinc in the substantia nigra or other brain regions from the individuals with incidental Lewy body disease compared to those from control subjects. Similarly, ferritin levels in the substantia nigra and other brain areas were unaltered. There was no difference in the activity of succinate cytochrome c reductase (complexes II and III) or cytochrome oxidase (complex IV) between incidental Lewy body subjects and control subjects. Rotenone-sensitive NADH coenzyme Q1 reductase activity (complex I) was reduced to levels intermediate between those in control subjects and those in patients with overt Parkinson's disease, but this change did not reach statistical significance. The levels of reduced glutathione in substantia nigra were reduced by 35% in patients with incidental Lewy body disease compared to control subjects. Reduced glutathione levels in other brain regions were unaffected and there were no changes in oxidized glutathione levels in any brain region. Altered iron metabolism is not detectable in the early stages of nigral dopamine cell degeneration. There may be some impairment of mitochondrial complex I activity in the substantia nigra in Parkinson's disease.(ABSTRACT TRUNCATED AT 250 WORDS)