MERCAPTURIC ACID FORMATION IN CULTURED OPOSSUM KIDNEY-CELLS

We investigated the last step of mercapturic acid formation, the N-ace tylation of cysteine S-conjugates, in the established opossum kidney ( OK) cell line which exhibits characteristics of the proximal tubule. S -Benzyl-L-cysteine was used as a model substance for such a cysteine S -conjugate. We succeeded in showing that OK cells absorb S-benzyl-L-cy steine via an active transport system which is inhibitable by phenylal anine. This transport follows Michaelis-Menten kinetics and the two ch aracterizing parameters were determined: the Michaelis-Menten constant K-m = 1.8mmol/l, and the maximum of the difference between the intrac ellular and the extracellular concentration of Sbenzyl-L-cysteine Delt a c(max) = 19.4 mmol/l. S-Benzyl-L-cysteine is converted to N-acetyl-S -benzyl-L-cysteine at a constant rate, which is independent of the ext racellular S-benzyl-L-cysteine concentration. Under the tested experim ental conditions this is probably due to saturation of the microsomal N-acetyltransferase catalyzing this reaction. In conclusion, we have s hown that OK cells are a suitable model for studying mercapturate form ation. They take up S-benzyl-L-cysteine mainly via the same carrier as phenylalanine, which is known to be transported in the rat by the hig h-capacity, low-affinity neutral amino acid carrier, and convert it to N-acetyl-L-benzyl-S-cysteine.