STUDIES ON THE CHARACTERIZATION OF THE INHIBITORY MECHANISM OF 4'-ALKYLATED 1-METHYL-4-PHENYLPYRIDINIUM AND PHENYLPYRIDINE ANALOGS IN MITOCHONDRIA AND ELECTRON-TRANSPORT PARTICLES

1-Methyl-4-phenylpyridinium (MPP(+)), the toxic agent in MPTP-induced dopaminergic neurotoxicity, is thought to act by inhibiting mitochondr ial electron transport at complex I. This study examined this latter a ction further with a series of 4'-alkylated analogues of MPP(+). These derivatives had IC50 values that ranged from 0.5 to 110 mu M and from 1.6 to 3,300 mu M in mitochondria and electron transport particles (E TPs), respectively. The IC50 values of corresponding 4'-alkylated phen ylpyridine derivatives to inhibit NADH-linked oxidation ranged from 10 to 205 mu M in mitochondria and from 1.7 to 142 mu M in ETPs. The pot encies of both classes of inhibitors directly correlated with their ab ility to partition between 1-octanol and water. In mitochondria, incre ased hydrophobicity resulted in greater inhibition of NADH dehydrogena se but a smaller dependence on the transmembrane electrochemical gradi ent for accumulation of the pyridiniums as evidenced by an similar to 600-fold, versus only a 36-fold, increase in the ICS, of MPP(+) versus 4'-pentyl-MPP(+), respectively, in the presence of uncoupler. In ETPs , the analogous increase in potencies of the more hydrophobic analogue s was also consistent with an inhibitory mechanism that relied on diff erential partitioning into the lipid environment surrounding NADH dehy drogenase. However, the pyridinium charge must play a major role in ex plaining the inhibitory mechanism of the pyridiniums because their pot encies are much greater than would be predicted based solely on hydrop hobicity. For example, in ETPs, 4'-decyl-MPP(+) was nearly 80-fold mor e potent than phenylpyridine although the latter compound partitions t wice as much into 1-octanol. In addition, the lipophilic anion TPB- wa s a more effective potentiator of inhibition by pyridiniums possessing greater hydrophilicity (0-5 carbons), consistent with facilitation of accumulation of these analogues within the membrane phase of complex I, probably via ion pairing. These studies delineate further the mecha nisms by which this class of compounds is able to accumulate in mitoch ondria, inhibit complex I activity, and thereby, effect neurotoxicity.