Differential detoxification of two thioether conjugates of menadione in confluent monolayers of rat renal proximal tubular cells

Abstract

This in vitro study describes proximal tubular toxicity of quinone thioethers after incubation of these compounds on either side of a renal proximal tubular cell (RPTC) monolayer. These cells were cultured to confluency on porous supports of tissue culture inserts, and the apically and basolaterally induced toxicity of two thioether conjugates of menadione (2-methyl-1,4-naphthoquinone) was investigated. As judged by lactate dehydrogenase (LDH) leakage, the glutathione (GSH) conjugate of menadione (MGNQ) was only toxic after basolateral challenge. However, after inhibition of γ-glutamyl transpeptidase by acivicin, MGNQ was also toxic after apical challenge. The mercapturic acid of menadione [M(NAC)NQ] displayed cytotoxicity both after apical and basolateral challenge. From the basolateral side, MGNQ and M(NAC)NQ-induced cytotoxicity could be enhanced by inhibition of the organic anion transport system with probenecid. Inhibition of β-lyase did not influence M(NAC)NQ-induced cytotoxicity. In addition, inhibition of intracellular N-deacetylation of M(NAC)NQ using paraoxon potentiated the observed toxic effect. Thus, it appears that the MGNQ and M(NAC)NQ-induced cytotoxicity is the result of extracellular events, presumably redox cycling. Putative uptake of the conjugates is likely to be associated with detoxification of these compounds.