GLUTATHIONE S-CONJUGATE FORMATION AND METABOLISM IN HEPG2 CELLS - A CELL MODEL OF MERCAPTURIC ACID BIOSYNTHESIS

Mercapturic acid biosynthesis is mediated by a series of at least four enzymatic steps and three cell membrane transport events, and is beli eved to require the interorgan shuttling of the various metabolic inte rmediates. To identify a single cell type that can carry out all of th ese metabolic and transport steps, the present study examined whether HepG2 cells, a human hepatoma-derived cell line, can convert an electr ophilic chemical (1-chloro-2,4-dinitrobenzene, CDNB) to its correspond ing mercapturic acid (S-dinitrophenyl-N-acetylcysteine, DNP-NAC). The results demonstrate that HepG2 cells are able to convert CDNB to DNP-N AC in a dose-and time-dependent fashion. Intracellular conjugation wit h glutathione occurred rapidly, and the resulting glutathione S-conjug ate was promptly transported into the culture medium, where it was seq uentially degraded to the cysteinylglycine and cysteine S-conjugates. The cysteine conjugate was then presumably reabsorbed, and N-acetylate d intracellularly to form the mercapturic acid. The mercapturic acid w as found to accumulate slowly in the culture medium, such that after 4 h of incubation, 4-10% of the CDNB dose was recovered as the mercaptu ric acid. These data provide the first demonstration that a single cel l type can carry out all of the transport and enzymatic steps required for mercapturic acid biosynthesis. HepG2 cells may provide a useful m odel system for studying this important detoxification pathway.