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The effect of different concentrations of aluminum sulphate on the nonenzymatic oxidation of dopamine was studied in order to evaluate the action of this metal on neuromelanin synthesis. Data shows that under the studied conditions, aluminum partially inhibits dopamine self-oxidation, decreasing the formation of some intermediate compounds, namely dopaminequinone and dopaminochrome. If neuromelanins have a cytoprotective function in the central nervous system, possibly acting as intracellular scavengers of free radicals and redox metal ions, their decrease due to aluminum could be responsible for serious damage to neuronal tissues.

The oxygen uptake during the reaction of dopamine autoxidation was studied and it was found that it may occur through two distinct oxidative pathways. One involves the univalent reduction of oxygen, forming the superoxide radical, the other involves a bivalent reduction which generates hydrogen peroxide. The detection of the univalent reaction was followed by the study of oxygen consumption, in the presence of cytochrome c. The detection of the bivalent reaction was made in the presence of catalase. It was found that manganese increases the oxygen uptake of the reaction, by increasing the production of both oxygen active forms. It seems probable that the neurotoxicity of this metal may be related with the increased production of activated oxygen in dopaminergic neurons, which undergo an oxidative stress.

To investigate functional changes in Trypanosoma cruzi parasites induced during their interaction with the vertebrate host, we compared the blood clearance profiles of blood forms isolated from infected normal mice (Reg-Tc) or from infected mice immunodepressed after treatment with cyclophosphamide (Cy-Tc). Parasite blood numbers were measured at various time intervals in animals injected intravenously (i.v.) with 1-2 x 10(6) T. cruzi of either isolate. In the absence of added immune sera (spontaneous clearance), Reg-Tc and Cy-Tc were cleared from blood at similar rates. However, when acute immune mouse serum (Ac-IMS) was injected i.v. 2 min after inoculation of parasites, a significant proportion of Cy-Tc only was cleared from the blood an hour later, whereas Reg-Tc were not, their clearance profile being identical to that observed in mice injected with normal mouse serum. Cy-Tc susceptibility to Ac-IMS was not the result of a toxic effect of cyclophosphamide over T. cruzi as parasites recovered from animals immunodepressed by irradiation before infection were cleared similarly by acute serum. Contrary to Ac-IMS, chronic immune mouse serum induced similar rates of disappearance of Reg-Tc and Cy-Tc from blood. Our results suggest the occurrence of T. cruzi selection or modification during the acute phase, which leads to an increased parasite resistance to the clearance properties of acute-phase antibodies.