Molecular Pharmacology, Vol 11, 174-184, Copyright © 1975 by the American Society for Pharmacology and Experimental Therapeutics

Adenosylcobalamin Analogues as Inhibitors of Ribonucleotide Reductase and Vitamin B₁₂ Transport

¹ Department of Biochemistry, Scripps Clinic and Research Foundation, La Jolla, California 92037

Nucleoside analogues of 5'-deoxy-5'-adenosylcobalamin have been synthesized by reduction of cyanocobalamin with zinc in ammonium chloride, followed by reaction with the 5'-chloro derivatives of inosine, thioinosine (6-mercaptopurine riboside), adenine arabinoside, cytosine arabinoside, 2-fluoroadenosine, homoadenosine, 1,N⁶-ethenoadenosine, tubercidin, and formycin. These analogues, several alkylcobalamins (CH₃-, CCl₂F-, CClF₂, aminoethyl-, and carboxypentyl-), and adenosyl- and methylcobinamide were tested as inhibitors of the adenosylcobalamin-dependent ribonucleotide reductase (EC 1.17.4.2) from Lactobacillus leichmannii. The fluoroadenosine derivative had full coenzymatic activity (K_m = 6.7 x 10^-7 M), but all the other cobalamin analogues were inhibitory; most effective were the formycin (K_i, = 1.0 x 10^-6 M) and adenine arabinoside (K_i = 3.0 x 10^-6 M) analogues. The cobinamides were very weak inhibitors. When tested as inhibitors of the transcobalamin II-mediated transport of vitamin B₁₂ into L1210 cells, all the cobalamins were effective competitors (50% inhibition at analogue to vitamin B₁₂ ratios of less than 10). The cobinamides, however, were poor inhibitors of vitamin B₁₂ transport.

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ACKNOWLEDGMENTS The authors are indebted to Lawrence Troxell and Bettie Bowen for expert technical assistance. Dr. R. Reisfeld kindly provided the original L1210 culture.