An assay of adenosine(5′)tetraphospho(5′)adenosine (Ap4A), based on the luciferin/luciferase method for ATP measurement, was developed, which allows one to determine picomolar amounts of unlabeled Ap4A in cellular extracts. In eukaryotic cells this method yielded levels of Ap4A varying from 0.01 μM to 13 μM depending on the growth, cell cycle, transformation, and differentiation state of cells. After mitogenic stimulation of G1‐arrested mouse 3T3 and baby hamster kidney fibroblasts the Ap4A pools gradually increased 1000‐fold during progression through the G1 phase reaching maximum Ap4A concentrations of about 10 μM in the S phase. Quiescent 3T3 cells reach a high level of Ap4A (1 μM) in a “committed” but prereplicative state if exposed to an external mitogenic stimulant (excess of serum) and simultaneously to a synchronizer which inhibits entry into the S phase (hydroxyurea). When the block for DNA replication was removed at varying times after removal of the stimulant decay of commitment to DNA synthesis was found correlated with a shrinkage of the Ap4A pool. Cells lacking a defined G1 phase (V79 lung fibroblasts, Physarum) possess a constitutively high base level of Ap4A (about 0.3 μM) even during mitosis. From this high level, Ap4A concentration increases only about tenfold during the S phase. Temperature‐down‐shift experiments, using chick embryo cells infected with transformation‐defective temperature‐sensitive viral mutants(td‐ts), have shown that the expression of the transformed state at 35°C is accompanied by a tenfold increase of the cellular Ap4A pool. Treatment of exponentially growing human cells with interferon leads, concomitantly with an inhibition of DNA syntheses, to a tenfold decrease in intracellular Ap4A levels within 20 h. The possibility of Ap4A being a “second messenger” of cell cycle and proliferation control is discussed in the light of these results and those reported previously demonstrating that Ap4A is a ligand of mammalian DNA polymerase α, triggers DNA replication in quiescent mammalian cells and is active in priming DNA synthesis.

DNA synthesis and adenosine(S')tetraphosphate(S ')adenosine (Ap.A) levels decrease in cells treated with EDTA . The inhibitory effect of EDTA can be reversed with micro molar amounts of ZnCI 2 • ZnCh in micromolar concentrations also inhibits Ap.A hydrolase and stimulates amino acid-dependent Ap.A synthesis, suggesting that Zn2+ is modulating intracellular Ap.A pools. Serum addition to GI-arrested cells enhances uptake of Zn, whereas serum depletion leads to a fivefold decrease of the rates of zinc uptake. These results are discussed by regarding Zn 2 + as a putative 'second messenger' of mitogenic induction and Ap.A as a possible 'third messenger' and trigger of DNA synthesis.

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