Microsomal cysteine-S-conjugate N-acetyltransferase catalyses the N-acetylation of various S-substituted cysteines in liver and kidney. We describe here the purification and more detailed characterization of this enzyme catalysing the final reaction of mercapturic acid biosynthesis, and thus playing a crucial role in the detoxicating metabolism of many xenobiotics. The solubilization of cysteine-S-conjugate N-acetyltransferase by deoxy-BIGCHAP [N,N´-bis-(3-d-gluconamidopropyl)deoxycholamide] was the prerequisite for partial purification by means of anion-exchange chromatography. The molecular mass of the enzyme was determined by gel filtration. A polyclonal antiserum was raised against the excised protein band from SDS/PAGE and purified antibodies were used for the complete purification of native cysteine-S-conjugate N-acetyltransferase by immunoaffinity chromatography. A dimeric form of the enzyme was sometimes detected on SDS/PAGE, depending on the degree of purification. For further characterization of cysteine-S-conjugate N-acetyltransferase, the stability of catalytic activity, the pH optimum and Km values were determined. The inhibitory effects of various agents were tested, revealing a substantial, yet not complete, loss of cysteine-S-conjugate N-acetyltransferase activity after treatment with cysteine proteinase inhibitors or probenecid under various conditions.

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