Modification of protein thiol by mixed disulfide formation with low molecular weight cellular thiols has been proposed as one of the post-translational modifications of amino acid side chains and is known to be catalyzed by thioltransferase. Intestinal mucosa is susceptible to oxidative injury and is likely to form protein mixed disulfide during oxidative stress. In the present study thioltransferase was purified from monkey small intestinal mucosa and its role in protein-s-thiolation was investigated. The purified enzyme was homogeneous, as judged by polyacrylamide gel electrophoresis under reducing conditions. The enzyme, with a molecular weight of 52 kDa, was a monomeric protein, which showed optimum activity at pH 8.0 with hydroxyethyl disulfide as substrate. The enzyme specifically cleaved the disulfide bond of the synthetic substrate, hydroxyethyl disulfide, in the presence of reduced glutathione (GSH) with the formation of oxidized glutathione (GSSG) as shown by high performance liquid chromatography. The enzyme also catalyzed protein thiolation of monkey intestinal mitochondria when incubated with glutathione disulfide. These studies have shown that thioltransferase purified from intestinal mucosa could catalyze dethiolation and thiolation.