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Partial Purification and Characterization of Tumor and Liver S-Adenosylmethionine Synthetases

The Department of Biochemistry, The University of Texas System Cancer Center, M. D. Anderson Hospital and Tumor Institute, Houston, Texas 77030

The S-adenosylmethionine synthetase activities of rat liver and Novikoff ascites tumor have been partially purified and characterized by chromatographic behavior, kinetic analysis, sulfhydryl dependency, and response to inhibitors. We have shown that the tumor contains a single form of the enzyme, with a K_m (methionine) of 21 µM, and that the liver contains two isofunctional forms, a minor form with a K_m (methionine) of 21 µM, as well as a major form with a K_m of 1 mM. The tumor contained more of the low K_m form of the enzyme than the liver, although the total enzyme activity of liver (measured at high substrate concentrations) exceeded that of the tumor severalfold. The tumor enzyme also corresponded to the minor form of liver enzyme in elution position from Sephadex G-150 and diethylaminoethyl cellulose, and both had a K_m (adenosine 5'-triphosphate) of 0.14 mM. The tumor enzyme differed from the major form of the liver enzymes in elution position, and the K_m (adenosine 5'-triphosphate) for the latter was 1.5 mM. In contrast to the major liver enzyme, the tumor enzyme did not appear to require sulfhydryl agents for the activity to be detected, was inhibited by S-adenosylmethionine, and was inhibited to a greater degree by tripolyphosphate. It is suggested that the two forms of the enzyme are involved in the production of S-adenosylmethionine for different biological functions, and their different properties may allow selective inhibition of tumor growth by chemotherapeutic agents.

¹ Recipient of Grants CA-15086, awarded by the National Cancer Institute, Department of Health, Education and Welfare; G-635, from the Robert A. Welch Foundation; and IN-36, from the Institutional Grant.

Received 6/ 2/76. Accepted 11/ 1/76.