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Differential Turnover of the Subunits of Ribonucleotide Reductase in Synchronized Leukemia L1210 Cells¹

Department of Biochemistry, University of South Florida College of Medicine, Tampa, Florida 33612

Ribonuleotide reductase catalyzes the rate-limiting step in the de novo synthesis of 2'-deoxyribonucleoside 5'-triphosphates that is required for DNA replication. The mammalian enzyme consists of two nonidentical protein subunits that are both required for enzyme activity. In leukemia L1210 cells, enriched in G₁-phase cells by centrifugal elutriation, it was found that ribonucleotide reductase activity increased as the cells progressed to S-phase. The two subunits making up the holoenzyme did not increase coordinately. The nonheme iron subunit increased much more rapidly than the effector-binding (EB) subunit. The activity of the holoenzyme paralleled the level of the EB subunit, which was limiting. The half-livers of the holoenzyme and its subunits were determined in S-phase cells by treatment with cycloheximide. The half-lives of the holoenzyme and the nonheme iron and EB subunits, as determined by enzyme activity, were 3.5, 7.6, and 4 h, respectively. These half-lives are consistent with the data that indicate that the EB subunit is the limiting component in L1210 cells.

¹ This work was supported by Grant CA 27398 from the USPHS, National Cancer Institute.

² To whom requests for reprints should be addressed, at Department of Biochemistry, University of South Florida College of Medicine, Box 7, 12901 North 30th Street, Tampa, FL 33612.

Received 3/20/86. Revised 7/24/86. Accepted 8/21/86.