An unknown cysteine-S-conjugate N-acetyltransferase (NAT) catalyses the transfer of an acetyl group from acetyl CoA (Ac-CoA) to the exo and endo forms of aflatoxin B1-cysteinyl conjugates (AFXBO-C, AFNBO-C). The resultant N-acetylcysteine-S-conjugate is termed a mercapturic acid which is readily excreted in urine (Hinchman & Ballatori 1994).