Inhibitory effect of dibutyryl cyclic adenosine monophosphate on the induction of alkaline phosphatase in human fetal liver cell line

When human fetal liver cells (HuL-1-317), cultured continuously in a serum-free medium, were incubated with a combination of prednisolone, butyrate and a hypertonic concentration of NaCl at 37 degrees C, alkaline phosphatase activity increased. However, the addition of dibutyryl adenosine cyclic monophosphate (Bt2cAMP) to these agents inhibited the increase in alkaline phosphatase activity in a dose-dependent manner: the inhibitory effect of Bt2cAMP was significant at 0.05 mM, but disappeared at 0.01 mM. Both cycloheximide and actinomycin D inhibited the increase in alkaline phosphatase activity with the combination described above. Western blotting showed that this enzyme activity increase was a consequence of greater biosynthesis of enzyme molecules in HuL-1-317 cells, and that Bt2cAMP regulated the synthesis of enzyme molecules. We conclude that the changes in alkaline phosphatase activity under various conditions are based on the changes in the number of enzyme molecules in HuL-1-317 cells.     

A study of adenosine 3':5'-cyclic monophosphate, sodium butyrate and cortisol as inducers of HeLa alkaline phosphatase

The role of adenosine 3′:5′-cyclic monophosphate in the cortisol-mediated induction of HeLa 65 alkaline phosphatase was investigated. Although growth of these cells with 0.5–1.0 mmN₆,O²′-dibutyryl adenosine 3′:5′-cyclic monophosphate induces a 5- to 8-fold increase in cellular phosphatase activity after 72 hr, neither cAMP nor theophylline induce at concentrations up to 1 mm. Sodium butyrate induces the enzyme as well as dibutyryl cAMP. Moreover, induction kinetics show sodium butyrate to be a more efficient inducer than dibutyryl cAMP, inducing activity as quickly as cortisol. This suggests that the butyric acid cleaved from dibutyryl cAMP by HeLa cells is the mediator of induction when the cyclic nucleotide derivative is used.     

Induction of alkaline phosphatase in choriocarcinoma cells by 1-beta-D-arabinofuranosyl-cytosine, mitomycin C, phleomycin, and cyclic nucleotides.

Alkaline phosphatase is induced in cultured human choriocarcinoma cells by three inhibitors of DNA synthesis which alter DNA structure: 1-β-D-arabinofuranosyl-cytosine, mitomycin C, and phleomycin. No induction is observed with the inhibitors, hydroxyurea and thymidine, which do not alter DNA structure. Cyclic AMP, analogs of cyclic nucleotides, and sodium butyrate also induce alkaline phosphatase in these cells. Among the cyclic nucleotides tested, dibutyryl cyclic AMP is the best inducer, whereas dibutyryl cyclic GMP is a poor inducer. Induction of alkaline phosphatase by inhibitors of DNA synthesis or by exposure to dibutyryl cyclic AMP appears to utilize different mechanisms. Maximum induction is observed after simultaneous addition of both types of inducers at the concentrations found to be optimal for each inducer alone. Under these conditions, the induced activity is equal to or greater than the sum of the activities induced by each inducer. RNA synthesis and protein synthesis are required for induction. Dibutyryl cyclic AMP added to cultures of choriocarcinoma cells is not degraded in the culture medium, but is extensively degraded in the cells. Nevertheless, significant amounts of dibutyryl and monobutyryl cyclic AMP are found intracellularly throughout the experiment. Since the cellular uptake of dibutyryl cyclic AMP is extremely slow, the amount of butyrate released by intracellular degradation cannot account for the observed induction. Neither the rate of uptake nor the stability of dibutyryl cyclic AMP are changed by the addition of 1-β-D-arabinofuranosyl-cytosine to the culture medium. Furthermore, 1-β-D-arabinofuranosyl-cytosine inhibits the induction by sodium butyrate. The results indicate that butyrate is not the major mediator of induction by dibutyryl cyclic AMP.     

Effect of cyclic AMP on chromatin-bound protein kinases in WI-38 fibroblasts stimulated to proliferate.

When resting confluent monolayers of WI-38 fibroblasts are stimulated to proliferate by serum, DNA synthesis begins to increase between 15-18 h after stimulation. Chromatin-bound protein kinase activity increases in stimulated cells within 1 h after the nutritional change, concomitant with an increase in the template activity of nuclear chromatin. Addition of dibutyryl 3' : 5'-cyclic adenosine monophosphate (dibutyryl cyclic) AMP to the stimulating medium inhibits the entrance of cells into S phase, but only if dibutyryl cyclic AMP (5-10(-4) M) is added before the onset of DNA synthesis. The increases in chromatin template activity and in the chromatin-bound kinase activity are not inhibited by dibutyryl cyclic AMP in the early hours after stimulation, but are completely inhibited after the 5th hour from the nutritional change. This seems to indicate that in stimulated WI-38 cells, dibutyryl cyclic AMP exerts its inhibitory action somewhere between 5 and 12 h after stimulation. A number of protein kinase activities were extracted from chromatin with 0.3 M NaCl and partially resolved on a phosphocellulose column. Two distinct peaks of protein kinase activity appeared to be markedly increased in WI-38 cells 6 h after serum stimulation. Both peaks of increased activity were inhibited by dibutyryl cyclic AMP in vivo. Adenosine, sodium butyrate and adenosine 5'-monophosphate (AMP) do not inhibit the increase in DNA synthesis nor the increase in protein kinase activity. The results suggest that stimulation of cell proliferation in confluent monolayers of WI-38 cells causes an increase (or the new appearance) of certain chromatin-bound protein kinases, and that this increase is inhibited by cyclic AMP in vivo.     

Opposite effects of dibutyryl adenosine 3':5' cyclic monophosphate and serum on growth of Chinese hamster cells

The effect of dibutyryl adenosine cyclic 3′:5′ monophosphate and testosterone on growth, response to serum and transport properties of Chinese hamster ovary cells was studied. There was a marked depression of growth in the presence of both compounds. A change of medium was sufficient to permit a partially synchronized burst of growth of the treated cells either in the presence or absence of dibutyryl adenosine cyclic 3′:5′ monophosphate plus testosterone. However, in the presence of these compounds a second round of cell division was prevented. Initiation of the cell cycle by cells exposed to dibutyryl adenosine cyclic 3′:5′ monophosphate plus testosterone displayed a greater serum requirement than untreated cells. It is concluded that serum and cyclic AMP could have antagonistic interactions in growth regulation. The treated cells had a reduced ability to accumulate amino-isobutyrate and glutamine, but no difference was observed with uridine uptake. The data suggest that a functional alteration of the membrane is induced by the exposure to dibutyrl adenosine cyclic 3′:5′ monophosphate plus testosterone.     

Induction of alkaline phosphatase in cultured human fibroblasts. Comparison of normal cells and those from patients with cystic fibrosis.

The membrane glycoprotein enzyme, alkaline phosphatase was induced in cultured human fibroblasts by dibutyryl cyclic AMP, sodium butyrate, the serum glycoprotein fetuin, the Tamm-Horsfall urinary glycoprotein, and by a number of inhibitors of DNA synthesis. The uninduced basal enzyme activity increased at later stages of growth when the cells became confluent. Induction by dibutyryl cyclic AMP or fetuin was most effective when the agents were added after the cells had reached stationary phase and was maximal after at least two days of exposure. The levels of induction resulting from the addition of pairs of the agents, dibutyryl cyclic AMP, n-butyrate and fetuin were additive indicating that these have different modes of action. The inhibitors of DNA synthesis, cytosine arabinoside, hydroxyurea, and methothrexate were less effective inducers. Bromodeoxyuridine which also has non-DNA mediated effects induced to the same extent as dibutyryl cyclic AMP. Similar experiments with sex- and age-matched cell strains derived from patients with cystic fibrosis failed to detect differences in the levels of induction from those observed in normal cells. In addition, the combined inductive effects of Tamm-Horsfall glycoprotein, isoproterenol and theophylline, were similar with normal and cystic fibrosis cells.     

Alkaline phosphatase relieves desensitization of adenylate cyclase-coupled beta-adrenergic receptors in avian erythrocyte membranes.

Desensitization of adenylate cyclase-coupled beta-adrenergic receptors in avian erythrocytes results in a 40-65% decrease in agonist-stimulated adenylate cyclase activity and correlates with increased phosphorylation of beta-adrenergic receptors. To assess the role of phosphorylation in desensitization, membranes from isoprenaline- and dibutyryl cyclic AMP-desensitized turkey erythrocytes were incubated with alkaline phosphatase for 30 min at 37 degrees C, pH 8.0. In both preparations alkaline phosphatase treatment significantly decreased desensitization of agonist-stimulated adenylate cyclase activity by 40-75% (P less than 0.05). Similar results were obtained after alkaline phosphatase treatment of membranes from isoprenaline- and dibutyryl cyclic AMP-desensitized duck erythrocytes. Moreover, alkaline phosphatase treatment of membranes from duck erythrocytes desensitized with 12-O-tetradecanoylphorbol 13-acetate returned agonist-stimulated adenylate cyclase activity to near control values. In all experiments, inclusion of 20 mM-sodium phosphate to inhibit alkaline phosphatase during treatment of membranes attenuated the enzyme's effect on agonist-stimulated adenylate cyclase activity. In addition, alkaline phosphatase treatment of membranes from control and isoprenaline-desensitized turkey erythrocytes increased the mobility of beta-adrenergic-receptor proteins, specifically photoaffinity-labelled with [125I]iodocyanopindolol-diazirine, on SDS/polyacrylamide-gel electrophoresis. The increased mobility of the beta-adrenergic-receptor proteins after alkaline phosphatase treatment of membranes was again inhibited by 20 mM-phosphate. These results provide additional evidence for a direct role for phosphorylation in desensitization of adenylate cyclase-coupled beta-adrenergic receptors in avian erythrocytes.     

Induction of osteoblastic cell differentiation by forskolin. Stimulation of cyclic AMP production and alkaline phosphatase activity

The effects of forskolin on differentiation of osteoblastic cells (clone MC3T3-E1) cultured in alpha-minimum essential medium containing 0.1% bovine serum albumin were investigated by assays of intracellular cyclic AMP level and alkaline phosphatase activity in the cells. Forskolin increased cyclic AMP production in the cells in a dose-related manner, the maximum increase being 250-fold above that of the controls. Alkaline phosphatase activity in the cells was also elevated as early as 24 h and rose to nearly its maximum at 48 h. The elevation was dose-dependent, with a maximum increase at 5 X 10(-6) M forskolin. Forskolin and prostaglandin E2 showed a supraadditive effect on cyclic AMP production in the cells and had an additive effect on alkaline phosphatase activity, whereas forskolin and dibutyryl cyclic AMP had little additive effect on either cyclic AMP production or enzyme activity. These results suggest that cyclic AMP is closely linked to the differentiation of osteoblastic cells in vivo.     

Long acting cAMP analogues enhance sulfate incorporation into matrix proteoglycans and suppress cell division of fetal rat chondrocytes in monolayer culture.

The relationship between replication and the synthesis of matrix sulfated proteoglycans was investigated with fetal rat chondrocytes grown in monolayer culture. The effect of N6 O2' dibutyryl adenosine 3', 5' cyclic monophosphate (DBcAMP), adenosine 3', 5' cyclic monophosphate (cAMP), 8 Bromo adenosine 3', 5' cyclic monophosphate (8 Br-cAMP), sodium butyrate and hydroxyurea was examined. Between 0.05 and 0.5 mM DBcAMP, a dose related inhibition of cell division and stimulation of [35SO=/4] incorporation into matrix proteoglycans was demonstrated. At the higher concentrations of DBcAMP, cell division was completely inhibited and the enhancement of [35SO=/4] incorporation into matrix proteoglycans ranged between 40 and 120% (P less than 0.01). Utilizing 14C-glucosamine and photometric determination of proteoglycans with Alcian Blue, it was demonstrated that the increase in sulfate incorporation reflected enhanced accumulation of extracellular matrix. The effects of DBcAMP were mimicked by 8 Br-cAMP, suggesting they were mediated by the adenylyl cyclase system. cAMP (0.05-0.5 mM), sodium butyrate (0.1-0.5 mM) and hydroxyurea (0.5-5 mM) partially or fully inhibited cell division, but either failed or only slightly enhanced sulfate incorporation. The enhanced sulfated proteoglycan deposition promoted by DBcAMP began 8 to 12 hours after serum stimulation, its onset occurred prior to thymidine incorporation and the effect persisted for 28 hours. Determination of cell volume demonstrated an increase in size of DBcAMP treated chondrocytes between 8 to 12 hours, coincident with the onset of increased sulfate incorporation. These results are consistent with a model where matrix sulfated proteoglycan deposition by chondrocytes is mediated by intracellular cAMP levels and occurs in the G1 phase of the cell cycle.     

Induction of alkaline phosphatase in cultured human fibroblasts: Comparison of normal cells and those from patients with cystic fibrosis

The membrane glycoprotein enzyme, alkaline phosphatase was induced in cultured human fibroblasts by dibutyryl cyclic AMP, sodium butyrate, the serum glycoprotein fetuin, the Tamm-Horsfall urinary glycoprotein, and by a number of inhibitors of DNA synthesis. The uninduced basal enzyme activity increased at later stages of growth when the cells became confluent. Induction by dibutyryl cyclic AMP or fetuin was most effective when the agents were added after the cells had reached stationary phase and was maximal after at least two days of exposure. The levels of induction resulting from the addition of pairs of the agents, dibutyryl cyclic AMP, n-butyrate and fetuin were additive indicating that these have different modes of action. The inhibitors of DNA synthesis, cytosine arabinoside, hydroxyurea, and methothrexate were less effective inducers. Bromodeoxyuridine which also has non-DNA mediated effects induced to the same extent as dibutyryl cyclic AMP.Similar experiments with sex- and age-matched cell strains derived from patients with cystic fibrosis failed to detect differences in the levels of induction from those observed in normal cells. In addition, the combined inductive effects of Tamm-Horsfall glycoprotein, isoproterenol and theophylline, were similar with normal and cystic fibrosis cells.     

Effects of parathyroid hormone and cyclic AMP analogues on the activity of ornithine decarboxylase and expression of the differentiated phenotype of chondrocytes in culture

Parathyroid hormone (PTH) greatly increased the level of adenosine 3', 5' cyclic monophosphate (cAMP) in rabbit costal chondrocytes in culture 2 minutes after its addition. PTH, as well as N6 O2' dibutyryl adenosine 3', 5' cyclic monophosphate (DBcAMP) and 8 Bromo adenosine 3', 5' cyclic monophosphate (8 Br-cAMP) induced ornithine decarboxylase (ODC; L-ornithine carboxylyase; EC 4.1.1.17), which reached a maximum 4 hours after their addition. Neither cAMP, N6 O2' dibutyryl guanosine 3', 5' cyclic monophosphate (DBcGMP), nor sodium butyrate increased the activity of the enzyme. PTH had no effect on DNA synthesis, while DBcAMP and 8 Br-cAMP decreased DNA synthesis. Expression of the differentiated phenotype of chondrocytes in culture was also induced by PTH, DBcAMP, and 8 Br-cAMP, but not by cAMP, DBcGMP, or sodium butyrate, as judged by morphological change. Glycosaminoglycan synthesis, a characteristic of the cartilage phenotype, began to increase 8 hours after addition of PTH or DBcAMP, reaching a plateau 32 hours after their addition. These findings suggest that PTH induces increase of ODC activity and expression of the differentiated phenotype of chondrocytes through increase of cAMP and that induction of OCD is closely related to expression of the differentiated phenotype of chondrocytes.     

Induction of alkaline phosphatase activity by synergistic action of dibutyryl cyclic adenosine monophosphate, prednisolone, butyrate and sodium chloride in cultured cells

Human urinary bladder carcinoma cells (JTC-32) retain a low alkaline phosphatase activity. Prednisolone or a hypertonic concentration of NaCl caused a moderate increase in the activity (10- to 15-fold of control), but dibutyryl cAMP or butyrate did not. Examination of the combined effect of these four agents revealed that they acted synergistically in any combination. When the cells were incubated with the four agents together, the enzyme activity increased 60- to 250-fold. Serum also contributed to this synergistic increase. These agents slightly inhibited cell growth and protein synthesis. The enzyme induction was completely inhibited by cycloheximide or actinomycin D. The synergistic effect of the four agents on the enzyme activity was also observed in other strains of carcinoma cells, human urinary bladder carcinoma cells (JTC-30) and monkey hepatocarcinoma cells (NCLP-6E). Thus, it is concluded that the coexistence of the four agents provides general and superior conditions for the induction of alkaline phosphatase in cultured carcinoma cells.     

The effect of N6-2'-O-dibutyryl-adenosine 3',5'-monophosphate on rat liver calciferol 25-hydroxylase.

Liver calciferol 25-hydroxylase activity of vitamin-D deficient rats was enhanced 24 hours following the intravenous injection of N⁶-2′-O-dibutyryl adenosine 3′,5′-monophosphate. Sodium butyrate administered in the same way had no effect on this enzyme system. Administration of actinomycin D with N⁶-2′-O-dibutyryl adenosine 3′,5′-monophosphate abolished the stimulatory effect of the cyclic nucleotide. Direct addition to the incubation medium of adenosine 3′,5′-cyclic monophosphate or of its dibutyryl derivative did not influence the hepatic conversion of cholecalciferol to 25-hydroxycholecalciferol. These results suggest a possible role for the cyclic nucleotide in the regulation of this enzyme system.     

Studies on cyclic adenosine 3' ,5'-monophosphate levels, Adenylate cyclase and phosphodiesterase activities in the dimorphic fungus Mucor rouxii.

The germination of spores of Mucor rouxii into hyphae was inhibited by 2 mm dibutyryl cyclic adenosine 3′,5′-monophosphate or 7 mm cyclic adenosine 3′,5′-monophosphate; under these conditions spores developed into budding spherical cells instead of filaments, provided that glucose was present in the culture medium. Removal of the cyclic nucleotides resulted in the conversion of yeast cells into hyphae. Dibutyryl cyclic adenosine 3′,5′-monophosphate (2 mm) also inhibited the transformation of yeast to mycelia after exposure of yeast culture to air.Since in all living systems so far studied adenylate cyclase and cyclic adenosine 3′,5′-monophosphate phosphodiesterase are involved in maintaining the intracellular cyclic adenosine monophosphate level, the activity of both enzymes and the intracellular concentration of cyclic adenosine monophosphate were investigated in yeast and mycelium extracts. Cyclic adenosine monophosphate phosphodiesterase and adenylate cyclase activities could be demonstrated in extracts of M. rouxii. The specific activity of adenylate cyclase did not vary appreciably with the fungus morphology. On the contrary, cyclic adenosine monophosphate phosphodiesterase activity was four- to sixfold higher in mycelial extracts than in yeast extracts and reflected quite accurately the observed changes in intracellular cyclic adenosine monophosphate levels; these were three to four times higher in yeast cells than in mycelium.     

Parathyroid hormone-induced changes in alkaline phosphatase expression in fetal calvarial osteoblasts: Differences between rat and mouse

We studied the effects of parathyroid hormone (PTH) on two markers of the osteoblast phenotype: alkaline phosphatase (AP) (activity and mRNA) and cyclic adenosine monophosphate (cAMP) accumulation. Osteoblast-like cells derived from fetal rat (ROB) and mouse (MOB) calvariae were isolated by collagenase treatment. Cells were cultured in α-Minimal Essential Medium (MEM) with 2% fetal calf serum (FCS) for 4 days. In ROB and MOB bPTH(1-34) induced a fast increase (up to 5 minutes) in cAMP accumulation. When equal amounts of cells were seeded, the cAMP accumulation was higher in MOB than in ROB. No difference in basal AP activity was observed between ROB and MOB. When bPTH(1-34) was added to ROB for the last 24 or 48 hr, AP activity decreased dose dependently. However, MOB treated with bPTH(1-34) for the last 24 or 48 hours showed an increase of AP activity. Basal AP activity was positively correlated with the seeding density of ROB and MOB cultures. Basal AP activity influenced the degree of inhibition (ROB) or stimulation (MOB) after incubation with bPTH(1-34). © 1993 Wiley-Liss, Inc.     

Effects of dibutyryl adenosine 3':5'-cyclic monophosphate and other agents on induction of alkaline phosphatase activity in monkey kidney cells

The induction of alkaline phosphatase (ALP) by dibutyryl adenosine 3':5'-cyclic monophosphate (Bt2cAMP) was investigated in strain JTC-12 . P3 cells derived from monkey (Maccaca irus) kidney cortex. ALP activity was increased by Bt2cAMP in a dose-dependent manner, reaching a plateau at concentrations higher than 5 mM with the activity being about 4 times that of the controls. The concentration of Bt2cAMP required for half-maximal induction of ALP activity was about 0.8 mM. ALP activity was increased rapidly by Bt2cAMP for the first 5 days and then continued to increase gradually towards a plateau level. Removal of Bt2cAMP from the medium caused a rapid decrease in the activity, suggesting that the induction of ALP activity by Bt2cAMP is reversible. ALP activity was induced synergistically in the presence of 1 mM sodium butyrate together with Bt2cAMP at concentrations from 0.01 to 1 mM. It was also found that in the presence of 1 mM Bt2cAMP, sodium butyrate increased ALP activity in the same manner as Bt2cAMP did in the presence of 1 mM sodium butyrate. Although dexamethasone, a potent glucocorticoid, had no effect on ALP activity in control cells, the hormone suppressed the ALP activity induced by Bt2cAMP in a dose-dependent manner. At concentrations above 0.2 mM, two xanthine derivatives, theophylline and 3-isobutyl-1-methyl-xanthine (IBMX), also inhibited the induction of ALP activity by 1 mM Bt2cAMP. Inhibitors of protein synthesis, cycloheximide (1.5 micrograms/ml) and pactamycin (10 micrograms/ml), as well as inhibitors of RNA synthesis, actinomycin D (2 micrograms/ml) and alpha-amanitin (50 micrograms/ml), suppressed the induction of ALP activity.     

Regulation of alkaline phosphatase activity in rat hepatoma cells. Effects of serum proteins, cycloheximide, actinomycin D, chloroquine, dinitrophenol and potassium cyanide

Alkaline phosphatase activity in rat hepatoma cells (R-Y121B) cultured in a monolayer at 0.5% serum was enhanced by serum, bovine serum albumin, casein and gamma-globulin, but ovalbumin, polyvinylpyrrolidone, dexamethasone, insulin and dibutyrylcyclic AMP showed little effect on alkaline phosphatase activity. In addition, cycloheximide, actinomycin D, chloroquine, dinitrophenol and potassium cyanide also increased the enzyme activity, although the incorporation of [14C]leucine into cellular proteins was almost completely inhibited in the presence of these cytotoxic substances. When R-Y121B cell homogenates were incubated at 37 degrees C, alkaline phosphatase activity increased in a pH-dependent manner: the maximal increase was observed at pH 7.1. The magnitudes of the increase differed among cell homogenates and a 4- to 10-fold increase was observed. Alkaline phosphatase in R-Y121B cells was apparently heat-stable, but that in the cells obtained from various treatments was heat labile and the latter activity decreased to less than 50% of the initial activity after 15 min of incubation at 56 degrees C. Alkaline phosphatase in the control and also in the treated cells was more sensitive to L-homoarginine than L-phenylalanine. The Lineweaver-Burk plot showed that the increases in the enzyme activity were accompanied by changes not only in V but also in Km for alkaline phosphatase reaction. Finally, it has been suggested that the increases in alkaline phosphatase activity under various conditions are due to the conversion of the molecule with a low enzyme activity to the molecule with a high enzyme activity in R-Y121B cells.     

Changes in histone phosphorylation and associated early metabolic events in pig lymphocyte cultures transformed by phytohaemagglutinin or 6-N,2′-O-dibutyryladenosine 3′:5′-cyclic monophosphate

1. Pig lymphocytes were transformed by dibutyryl cyclic AMP (6-N,2'-O-dibutyryladenosine 3':5'-cyclic monophosphate) at concentrations of 0.01-0.1mum. The pattern of incorporation of label from [5-(3)H]uridine and [6-(3)H]thymidine into RNA and DNA respectively was identical with that obtained with unpurified phytohaemagglutinin. 2. Chlorpromazine (0.1mum) prevented the stimulation of [5-(3)H]uridine incorporation into RNA by phytohaemagglutinin, but only slightly lowered the lymphocyte response to dibutyryl cyclic AMP. 3. An increase in the size and specific radioactivity of the intracellular P(i) pool was found immediately after stimulation by both phytohaemagglutinin and dibutyryl cyclic AMP. This was followed after some 30min by a rise in the specific radioactivity and concentration of ATP. 4. There was an immediate increase in the specific radioactivity of phosphate groups of histones; by about 45min after stimulation only the histones remaining after extraction of histone fraction F1 continued to incorporate (32)P from [(32)P]P(i). 5. Histone kinase activity increased in the first 30min after stimulation; subsequently histone F1 kinase activity decreased, but activity with the other histones as substrate continued to increase for a further 30min. Kinase activation was effected by cyclic AMP (adenosine 3':5'-cyclic monophosphate). 6. Histone phosphatase activity behaved similarly to that of the kinase.