SYNTHESIS, REDOX PROPERTIES, IN-VIVO FORMATION, AND NEUROBEHAVIORAL EFFECTS OF N-ACETYLCYSTEINYL CONJUGATES OF DOPAMINE - POSSIBLE METABOLITES OF RELEVANCE TO PARKINSONS-DISEASE

A very early event in the pathogenesis of idiopathic Parkinson's disea se (PD) has been proposed to be an elevated translocation of L-cystein e (CySH) and/or glutathione (GSH) into pigmented dopaminergic cell bod ies in the substantia nigra (SN) in which cytoplasmic dopamine (DA) is normally autoxidized to DA-o-quinone as the first step in a reaction leading to black neuromelanin polymer. Such an elevated influx of CySH and GSH would be expected to initially result in formation of 5-S-cys teinyldopamine (5-S-CyS-DA) and 5-S-glutathionyldopamine (5-S-Glu-DA), respectively, and might account for the massive irreversible loss of GSH and progressive depigmentation of SN cells that occurs in the Park insonian brain. However, 5-S-Glu-DA has not been detected in the Parki nsonian brain. Furthermore, although the 5-S-CyS-DA/DA and 5-S-CyS-DA/ homovanillic acid concentration ratios increase significantly in the S N and cerebrospinal fluid, respectively, of PD patients, the absolute concentrations of 5-S-CyS-DA are extremely low and similar to those me asured in age-matched control patients. One explanation for these obse rvations is that 5-S-CyS-DA might be intraneuronally oxidized to more complex cysteinyldopamines and a number of dihydrobenzothiazines (DHBT s) and benzothiazines (BTs). Similarly, 5-S-Glu-DA might be intraneuro nally oxidized to more complex glutathionyldopamines. In this investig ation, however, it is demonstrated that 5-S-Glu-DA is rapidly metaboli zed in rat brain to 5-S-CyS-DA and 5-S-(N-acetylcysteinyl)dopamine (5) in reactions mediated by gamma-glutamyl transpeptidase (gamma-GT) and cysteine conjugate N-acetyltransferase. Similarly, 5-S-CyS-DA is meta bolized to 5 in rat brain although more slowly than 5-S-Glu-DA. These reactions occur most rapidly in the midbrain, a region that contains t he SN. Furthermore, 5, 2-S-(N-acetylcysteinyl)dopamine (6) and 2,5-di- S-(N-acetylcysteinyl)dopamine (9) are toxic when administered into mou se brain having LD(50) values of 14, 25, and 42 mu g, respectively, an d evoke a profound hyperactivity syndrome. These results suggest that the failure to detect 5-S-Glu-DA and the presence of only very low lev els of 5-S-CyS-DA in Parkinsonian SN tissue and CSF might be related t o both their intraneuronal oxidation and extraneuronal metabolism to N -acetylcysteinyl conjugates of DA. Furthermore, the toxic properties a nd neurobehavioral responses evoked by 5, 6, and 9 raise the possibili ty that these N-acetylcysteinyl conjugates of DA, in addition to certa in cysteinyldopamines, DHBTs and BTs, might include endotoxins that co ntribute to SN cell death and other neuronal damage that occurs in PD. Methods are described for the synthesis of several N-acetylcysteinyl conjugates of DA, and their redox behaviors have been studied using cy clic voltammetry.