1. This study investigates the acetylation of 5′-aminosalicylic acid by isolated human colonic epithelial cells.

2. After incubation of intact cells with 0.1 mmol/l 5-aminosalicylic acid, N-acetyl-5-aminosalicylic acid was detected in a concentration of 141 (± 23.8 sem) nmol/g dry weight in the incubation medium, and 34.8 (± 5.5 sem) nmol/g dry weight intracellularly. No unchanged 5-aminosalicylic acid was detected inside the cell.

3. Acetylation of 5-aminosalicylic acid by a cell homogenate was very poor, but the addition of 1 mmol/l acetyl-CoA resulted in complete conversion of 0.1 mmol/l 5-aminosalicylic acid to N-acetyl-5-aminosalicylic acid.

4. N-Acetyltransferase activity was detected in the cytosol, with a mean of 3.3 nmol min−1 mg−1 of protein. There was no N-acetyltransferase activity in the brush border. There was no difference in enzyme activity between epithelial cells derived from normal, Crohn's disease or ulcerative colitis patients.

5. Preliminary characterization of the N-acetyltransferase enzyme suggests a molecular weight of 150 000.

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