Thiopurine methyltransferase (TPMT) is one of the enzymes involved in the catabolism of 6-mercaptopurine and azathioprine. An assay procedure was developed for rat TPMT, and the activity of the enzyme was measured in erythrocyte lysates and in homogenates of three solid tissues of Sprague-Dawley rats. Apparent Michaelis-Menten (Km) values for the two co-substrates of the reaction, 6-mercaptopurine (6-MP) and S-adenosyl-L-methionine (SAM), were 2.4, 1.8, 1.4 and 1.5 × 10-3M for 6-MP and 5.3, 3.3, 4.3 and 4.3 × 10-6M for SAM in erythrocyte, intestine, kidney, and spleen respectively. The pH optima were 6.7 in all four tissues. Sprague-Dawley rat kidney TPMT activity increased 28-fold and spleen activity increased 2-fold from birth to adulthood when expressed per gram of tissue. The increases were 13- and 2.2-fold, respectively, when expressed per milligram of protein. Enzyme activity was measured in tissues of adult male animals of nine inbred and two outbred rat strains. The rank order of tissue TPMT activity in all strains was: kidney > intestine > spleen > blood. There was a significant correlation between relative erythrocyte and relative kidney enzyme activities. Two-fold variations among strains in renal TPMT activity were found.
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