Molecular Pharmacology, Vol 9, 542-560, Copyright © 1973 by the American Society for Pharmacology and Experimental Therapeutics

Effects of Inhibitors of Adenosine Triphosphate:L-Methionine S-Adenosyltransferase on Levels of S-Adenosyl-L-methionine and L-Methionine in Normal and Malignant Mammalian Tissues

¹ Department of Pharmacology and Experimental Therapeutics, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205

Low levels of ATP:L-methionine S-adenosyltranferase (adenosyltransferase, EC 2.5.1.6) have been detected by a sensitive radioactive assay procedure in transplantable neoplasms, including the Walker 256 tumor of the rat, and the Lewis lung tumor and B-16 melanoma of C₅₇ black mice. These enzymatic activities and those of a number of normal rodent tissues are all significantly inhibited by 1-aminocyclopentanecarboxylic acid (cycloleucine) and by L-2-amino-4-hexynoic acid, which are structural and conformational analogues of L-methionine.

Following single intraperitoneal injections to rats, the plasma cycloleucine levels remain almost constant for at least 96 hr, whereas the L-2-amino-4-hexynoic acid plasma levels fall with a half-life of about 60 hr. Both cycloleucine and L-2-amino-4-hexynoic acid are highly concentrated in the Walker 256 tumor, while the acetylenic amino acid is concentrated in the Lewis lung tumor.

Administration of pharmacological doses of cycloleucine (5.19 mmoles/kg) or L-2-amino-4-hexynoic acid (2.64 mmoles/kg) elevates plasma and tissue levels of these amino acids in a dose- and time-depenent manner to concentrations which are probably sufficiently high to achieve significant inhibition of the tissue adenosyltransferases. Mice and rats treated with these adenosyltransferase inhibitors display dramatic elevations of free L-methionine levels in all tissues examined (liver, spleen, kidney, brain, plasma, Walker 256 tumor, and Lewis lung tumor). These effects are dose-dependent. The levels of five other amino acids (isoleucine, leucine, tyrosine, phenylalanine, and valine) in liver or spleen show only minor changes in response to this treatment, except for a doubling of the phenylalanine level in the spleen. Treatment of animals with high doses of other amino acid analogues that do not inhibit the adenosyltransferases (L-norvaline, L-valine, DL-homoserine) had no effect on the L-methionine levels.

Treatment with pharmacological doses of cycloleucine and L-2-amino-4-hexynoic acid depresses the levels of (-)-S-adenosyl-L-methionine (Ado-Met) by 20-40% in most normal and malignant tissues (including spleen, kidney, pancreas, brain, adrenal, Walker 256 tumor of rats, and Lewis lung and L1210 tumors of mice). These effects are time- and dose-dependent. Amino acids (L-norvaline, L-serine, and L-valine) that are not adenosyltransferase inhibitors are without effect on tissue Ado-Met concentrations. In contrast to the findings in other tissues, the Ado-Met levels of liver are invariably increased following administration of adenosyltranferase inhibitors.

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ACKNOWLEDGMENTS The authors are grateful to Mary Karen Burch for expert technical assistance. Susan Roland Webb kindly performed some of the tissue analyses for S-adenosyl-L-methionine. Dr. A. W. Coulter and Patricia Margulies supplied generous quantities of L-2-amino-4-hexynoic acid. Dr. Ann Marie Benson provided advice on amino acid determinations. The chemotherapeutic studies on L1210 leukemia were kindly carried out by the Cancer Chemotherapy National Service Center (Drs. Saul Schepartz, J. M. Venditti, and H. B. Wood, Jr.). Mr. Isidore Wodinsky of Arthur D. Little supplied tumor-bearing animals upon request of the National Cancer Institute.