Iron chelation, known to block progression through the cell cycle, was examined for effects on the activity and subunit levels of the cyclin-dependent protein kinases (cdk). Treatment of asynchronous MDA-MB-453 cells with the iron chelators mimosine or desferrioxamine (DFO) for 24 h stopped cell division, but did not produce a single, synchronous block. DNA content analysis demonstrated that although a majority of the cells were blocked in G1 (87.3%), an unexpectedly large fraction of the cells were blocked in S phase (11.5%). Western blot analysis of the treated lysates demonstrated the presence of cyclin B, confirming that part of the cell population was blocked in S phase. After release from mimosine treatment, 84% of the cell population remained in G1 up to 8 h. Treating breast cancer cells with 400 microM mimosine for 24 h inhibited cyclin E- and cyclin A-associated kinase activity by 85% or more, although immunoblots using anti-cyclin A, cyclin E, cdc2, and cdk2 antibodies showed that these key subunits were still present in the cells at pretreatment levels. Interestingly, Western blot analysis also demonstrated that iron chelation decreased the protein levels of the cyclin D and cdk4 subunits as compared to control and produced a change in retinoblastoma protein phosphorylation. These results indicate that iron deprivation effects the activity and protein levels of the cyclin-dependent kinases, and ultimately, the pathways that control cell division.