“…3B), e.g., the K, value with respect to ATP of Ca2+/calmodulin-dependent protein kinase 2 is about 19 pM using MAP-2 as substrate [Gupta et al, 19921. Rabbit pancreas protein kinase C, casein kinase 2, cGMPdependent protein kinase 1 alpha, p4.5 casein kinase, have K, values in the same range [Ederveen et al, 1992;Jakoby and Traugh, 1992;Huggins et al, 1994;Angelov , 19941. As demonstrated in this work, the cellular ATP concentration varies through the cell cycle, being high during G,+M-phase (about 3.5 mM) and decreasing during GI-phase of the cell cycle, reaching its minimum at late G,/early S-phase (corresponding to about 2.2 mM). However, these changes are somewhat masked by the incomplete synchronization of our cell population, so the cellular ATP concentration during G, + M-phase may be considerably higher than 3.5 mM, and thereby exceed the ATP concentration needed to depolymerize microtubules ( Figs.…”