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Studies on Mammalian Ribonucleotide Reductase Inhibition by Pyridoxal Phosphate and the Dialdehyde Derivatives of Adenosine, Adenosine 5'-Monophosphate, and Adenosine 5'-Triphosphate¹

Department of Medical Microbiology, College of Medicine, University of South Florida, Tampa, Florida 33620

Ribonucleotide reductase activity in a partially purified enzyme preparation from Ehrlich tumor cells was inhibited by the dialdehyde derivatives of adenosine, 5'-adenylic acid, and adenosine 5'-triphosphate (prepared by the periodate oxidation of adenosine, 5'-adenylic acid, and adenosine 5'-triphosphate). The borohydride-reduced derivative of periodate-oxidized adenosine was not inhibitory to the ribonucleotide reductase activity, showing that the aldehyde moiety was important in the inhibitory interactions of these compounds. This suggested the formation of a Schiff base between the dialdehyde derivative and an amino group (presumably, the -amino group of lysine). Pyridoxal phosphate, which is known to inhibit enzymes that have lysyl residues in the catalytic or allosteric sites, was an inhibitor of ribonucleotide reductase. Pyridoxal, pyridoxamine phosphate, pyridoxamine, and pyridoxine were not inhibitors. Borohydride reduction of the enzyme in the presence of pyridoxal phosphate produced a protein fraction that had little reductase activity remaining. The inhibition by pyridoxal phosphate was not influenced by increasing the substrate concentration (cytidine 5'-diphosphate or adenosine 5'-diphosphate), but was diminished by increasing the ratio of allosteric effector to pyridoxal phosphate concentrations, suggesting an interaction of pyridoxal phosphate at the regulatory site of ribonucleotide reductase. The addition of adenosine 5'-triphosphate to the pyridoxal phosphate-enzyme mixture, which was subsequently treated with borohydride, partially prevented the inhibition by pyridoxal phosphate. Heat treatment of the ribonucleotide reductase enzyme preparation in the presence of pyridoxal phosphate protected the enzyme against loss of cytidine 5'-diphosphate and adenosine 5'-diphosphate reductase activities.

¹ This research was supported by Grant CA 10380 from the USPHS, National Cancer Institute.

² Recipient of Research Career Development Award CA 17199 from the USPHS, National Cancer Institute. Present Address: Department of Biochemistry, University of South Florida College of Medicine, Tampa, Fla.

Received 7/25/74. Accepted 10/25/74.