Regulation Of Alkaline Phosphatase Activity Research Articles

Mineralization initiation of MC3T3-E1 preosteoblast is suppressed under simulated microgravity condition.

Microgravity decreases the differentiation of osteoblast. However, as this process is multistage and complex, the mechanism by which microgravity inhibits osteoblast differentiation is still unclear. We have previously found that 24 h acute treatment of simulated microgravity (SM) with a random positioning machine (RPM) significantly inhibited the differentiation of preosteoblasts and have explored whether osteoblasts show different response to microgravity condition at other stages, such as the mineralizing-stage. Murine MC3T3-E1 preosteoblasts induced for osteogenic differentiation for seven days were cultured either under normal gravity or SM conditions for 24 h. SM treatment significantly suppressed mineralized nodule formation. Alkaline phosphatase (ALP) activity was dramatically decreased, and the expression of ALP gene was downregulated. Expression of well-known markers and regulators for osteoblasts differentiation, including osteocalcin (OC), type I collagen α1 (Col Iα1), dentin matrix protein 1 (DMP1) and runt-related transcription factor 2 (Runx2), were downregulated. Western blot analysis showed that the phosphorylated extracellular signal-regulated kinase (p-ERK) level was lower under SM condition. Thus, the initiation of osteoblast mineralization is suppressed by SM condition, and the suppression may be through the regulation of ALP activity and the osteogenic gene expression. ERK signaling might be involved in this process. These results are relevant to the decrease of osteoblast maturation and bone formation under microgravity condition.

Temporal-controlled Release of Bovine Serum Albumin from Chitosan Nanoparticles: Effect on the Regulation of Alkaline Phosphatase Activity in Stem Cells from Apical Papilla

IntroductionThe controlled delivery of bioactive molecules is crucial for the regulation of stem cell differentiation. In this study, we examined the effects of temporal-controlled release of bovine serum albumin (BSA) from chitosan nanoparticles (CSnp) to regulate the alkaline phosphatase activity (ALP) in stem cells from apical papilla (SCAP). MethodsBSA-loaded CSnp were synthesized by 2 methods to achieve the variant temporal-controlled release: (1) the encapsulation technique (BSA-CSnpI) and (2) the adsorption technique (BSA-CSnpII). After characterization of the size, charge, and release kinetics, SCAP were cultured in the presence of these bioactive molecule–loaded nanoparticles. SCAP viability was analyzed at 1, 7, 14, 21, and 28 days, and ALP activity was analyzed every 7 days until 21 days to determine the effect of these bioactive molecule–releasing nanoparticles on the cytotoxicity and differentiation potential, respectively. ResultsBSA-CSnpI and BSA-CSnpII presented distinct in vitro release profiles of BSA in a time-controlled manner. Cell viability was significantly enhanced over time in the presence of BSA-CSnpI and BSA-CSnpII (P < .01), when compared with BSA nonloaded CSnp. ALP activity was significantly higher (P < .01) in the presence of BSA-CSnpI after 3 weeks than in BSA-CSnpII. ConclusionsBSA-loaded CSnps were synthesized and characterized in this study. Based on the physical/chemical interaction of BSA with CSnp (encapsulation or surface adsorption), different time-controlled release profiles were observed that influenced the ALP activity of SCAP in vitro. This study highlighted the potential of temporal-controlled bioactive molecule release technology in the differentiation of stem cells in dentin pulp regeneration.

Role of sea water DIP and DOP in controlling bulk alkaline phosphatase activity in N.W. Mediterranean Sea (Toulon, France)

The regulation of alkaline phosphatase activity by dissolved inorganic (DIP) and organic phosphorus (DOP) and the contribution of DOP as phosphorus source were studied monthly in Toulon Bay (NW Mediterranean, France) in 2005–2006. The concentrations of DIP and DOP varied respectively from 0 to 0.185μM and from 0 to 0.329μM. The bulk activities (Vm, Km, Vm/Km) were measured using MUFP as substrate. Its high affinity component (Km: 0.05–1.00μM) was negatively correlated with the sum of the concentrations of DIP and DOP but not with these compounds taken independently. A negative correlation with DIP was found when the concentrations of DOP were lower than 0.08μM. A negative correlation with DOP was shown when the concentrations of DIP were lower than 0.05μM. This high affinity component can be considered as a valuable indicator for the potential utilization of the compounds which contribute to the intracellular phosphorus pool.

Presence and regulation of alkaline phosphatase activity in eukaryotic phytoplankton from the coastal ocean: Implications for dissolved organic phosphorus remineralization

The biologically important constituents of the dissolved organic phosphorus (DOP) pool, their bioavailability, and their cycling in coastal systems are still poorly understood. Here we use the enzyme alkaline phosphatase as a metric of DOP bioavailability and track the activity of this enzyme in a coastal system. We observed alkaline phosphatase activity (APA) in the >0.2-μm size fraction of all surface samples tested during an Oregon coast cruise in August 2001. Although there was not a significant trend between APA and phosphate concentration in the data set as a whole, chlorophyll a-normalized APA was elevated at the station with the lowest dissolved inorganic phosphate (DIP) concentration. Activity was also elevated in nutrient-addition experiments in which nitrate amendments were Used to force community drawdown of DIP. These data are consistent with phosphate regulation of APA. A cell-specific APA assay revealed that the percentage of diatoms with APA mimicked the trend in the hydrolytic rate, but such a trend was not observed for the dinoflagellates. Further, the percentage of dinoflagellate cells with APA was routinely higher than the percentage of diatom cells with activity. In nutrient-addition experiments designed to evaluate the regulation of APA, diatom taxa expressed APA less frequently than dinoflagellates, but they displayed a tighter regulation of the activity by DIP than dinoflagellates. The variability observed in the presence and regulation of APA in these eukaryotic phytoplankton indicates that DOP bioavailability is a potential driver of phytoplankton nutrition and species composition in the coastal ocean.

Regulation of ALP Activity by TNF-α on Human Dental Pulp

In this study, we examined the effects of TNF-α on Bone morphogenetic protein (BMP-2), Smads (which play intracellular signaling of BMPs) expression and alkaline phosphatase (ALP) activity of human dental pulp (HDP) cells to clarify the mechanism of tertiary dentin formation. The quantity of RT-PCR product for BMP-2 from the HDP cells stimulated by TNF-α is increased. However, ALP activity was not increased on the cells incubated with TNF-α. On the other hand, ALP activity was significantly increased on HDP cells treated with Ammonium Pyrrolidinedithiocarbamate (PDTC, NF- κB inhibitor) groups and combined supplementation of TNF-α and PDTC groups. Furthermore, we examined the effect of TNF-α and PDTC on Smad7 expression using RT-PCR and western blot analysis. Smad7 expression in HDP cells was increased by TNF-α, but decreased by PDTC treatment. These results suggest that NF- κB and Smad7 play an important role in the down regulation of ALP activity by TNF-α on HDP cells.

HPTH-fragments (53–84) and (28–48) antagonize the stimulation of calcium release and repression of alkaline phosphatase activity by hPTH-(1–34) in vitro

Different C-terminal fragments of parathyroid hormone (PTH)-(1–84) in blood participate in the regulation of calcium homeostasis by PTH-(1–84), and an antagonizing effect for the large carboxyl-terminal parathyroid hormone (C-PTH)-fragment (7–84) on calcium release has been described in vivo and in vitro. In this study the smaller C-PTH-fragment (53–84) and mid-regional PTH fragment (28–48), which represent discrete areas of activity in the PTH-(7–84) molecule, were assayed for their effects on calcium release and alkaline phosphatase (ALP) activity in a chick bone organ culture system. Neither PTH-(28–48) nor PTH-(53–84) had any effect on calcium release into the medium and both fragments stimulated ALP activity in the bone tissue, suggesting that the cAMP/PKA signalling pathway was not affected by these fragments. However they suppressed the calcium release induced by PTH-(1–34) and attenuated the down regulation of ALP activity caused by PTH-(1–34), suggesting that the effect on the cAMP/PKA signalling pathway may be indirectly. In conclusion, the study shows that the PTH-fragments (53–84) and (28–48) antagonize the PTH-(1–34) induced effects on calcium release and inhibition of ALP activity in a chick bone organ culture system.

Temporal and spatial variability of dissolved organic and inorganic phosphorus, and metrics of phosphorus bioavailability in an upwelling‐dominated coastal system

High‐frequency temporal and spatial shifts in the various dissolved P pools (total, inorganic, and organic) are linked to upwelling/relaxation events and to phytoplankton bloom dynamics in the upwelling‐dominated Oregon coastal system. The presence and regulation of alkaline phosphatase activity (APA) is apparent in the bulk phytoplankton population and in studies of cell‐specific APA using Enzyme Labeled Fluorescence (ELF®). Spatial and temporal variability are also evident in phytoplankton community composition and in APA. The spatial pattern of dissolved phosphorus and APA variability can be explained by bottom‐controlled patterns of upwelling, and flushing times of different regions within the study area. The presence of APA in eukaryotic taxa indicates that dissolved organic phosphorus (DOP) may contribute to phytoplankton P nutrition in this system, highlighting the need for a more complete understanding of P cycling and bioavailability in the coastal ocean.

Effects of organic and inorganic nitrogen compounds on the activity of bacterial alkaline phosphatase

The stimulation of bacterial alkaline phosphatase activity (APA) by inorganic and organic nitrogen compounds was investigated in the southern Baltic Sea monthly between February and August 2001, by adding albumin, casein, leucine, ammonium, nitrate, ammonium + glucose or nitrate + glucose to 0.8 µm filtered seawater. The following questions were addressed: (1) Are there seasonal changes in the stimulation of APA by these substances?; (2) Does nitrogen alone stimulate this activity or only in combination with organic carbon?; (3) Is there a relationship between ambient nutrient concentrations and the degree of stimulation? The addition of the mentioned compounds stimulated the APA in bacteria to a high degree, however, there were seasonal variations. Stimulation was low in February and March but high from May to August when the stimulation, e.g., by ammonium + glucose, was up to 6000-fold higher compared with February. In most experiments, the addition of the amino acid leucine and of inorganic nitrogen alone resulted in an inhibition of the bacterial APA. A relationship between ambient nutrient concentrations and the stimulation of the bacterial APA was only observed for albumin, which correlated negatively with dissolved inorganic nitrogen (DIN) and phosphate concentrations and for casein, which correlated only with DIN. The results indicate that the regulation of bacterial APA and the DOP degradation can be significantly influenced by the availability of nitrogen and organic carbon.

Cytoskeletal changes and the system of regulation of alkaline phosphatase activity in human periodontal ligament cells induced by mechanical stress.

The periodontal ligament (PDL) is a specialized, mechanically responsive tissue that adapts via cellular responses to equilibrate the effects of mechanical stress on teeth. However, the mechanism of remodelling by which individual cells in periodontal tissue detect and respond to mechanical stress is not well understood. To identify the cellular mechanisms induced by mechanical stress in the periodontal ligament, we examined the effects of cyclic stretching on periodontal ligament fibroblast-like cells (PDL cells). Furthermore, we investigated the effects of 1alpha,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), and interaction with peripheral blood mononuclear cells (PBMCs) on mechanically-simulated PDL cells. PDL cells were cultured on type I collagen-coated silicon membranes with 10% FBS alpha-MEM, and then subjected to cyclic mechanical stimulation (1 s stretching/1 s relaxation, 15% maximum elongation). Alkaline phosphatase activity was monitored by cytochemical and spectrophotometric methods. Morphologically, the cells assumed a spindle shape, and the cytoskeletal components, including microtubules and F-actin filaments, were aligned perpendicular to the strain force vector. Cyclic stretching decreased ALPase activity in PDL cells. The anabolic systemic hormone 1,25(OH)(2)D(3) increased ALPase activity, but this effect was suppressed by cyclic stretching. ALPase activities were reduced by co-culture with PBMCs, including lymphocytes and monocytes. This PBMC-induced ALPase reduction was synergistically reduced by cyclic stretching. ALPase activity was decreased by co-culture with PBMCs, and ALPase activity was reduced synergistically by treatment with PBMCs and cyclic stretching. We conclude that PDL cells changed their shape and alignment in response to cyclic stretching. Furthermore, local factors, such as mechanical stress and PBMCs, showed synergistic suppressive effects on ALPase activity.

Regulation of alkaline phosphatase activity by p38 MAP kinase in response to activation of Gi protein-coupled receptors by epinephrine in osteoblast-like cells.

The signaling mechanisms responsible for the regulation of alkaline phosphatase (ALP) activity by exogenous factors in osteoblast-like cells remain poorly understood. Among various agents, epinephrine was recently found to increase ALP activity in differentiating MC3T3-E1 cells by stimulating alpha1 adrenergic receptors coupled to Gi proteins. In the present study, we investigated the role of both ERK2 and p38 mitogen-activated protein (MAP) kinases in mediating this response in MC3T3-E1 cells. Our results indicate that both MAP kinases are transiently stimulated by epinephrine in differentiating cells via a pertussis toxin sensitive mechanism. The role of each MAP kinase pathway in mediating the stimulation of ALP activity by epinephrine was investigated using specific inhibitors. The MEK inhibitor PD98059, blocked ERK2 activity induced by epinephrine but had no effect on the stimulation of ALP activity. In contrast, low concentrations of SB203580, a specific inhibitor of the p38 MAP kinase, completely blunted this cellular response. However, this inhibitor had no influence on the stimulation of ALP activity induced by ascorbic acid. In conclusion, the results of this study suggest distinct roles for ERK and p38 MAP kinase pathways in regulating activity of MC3T3-E1 osteoblastic cells. The ERK pathway is likely involved in the control of cell proliferation whereas the p38 MAP kinase pathway regulates ALP activity in response to activation of Gi protein-coupled receptors.

Regulation of Alkaline Phosphatase Activity by p38 MAP Kinase in Response to Activation of Gi Protein-Coupled Receptors by Epinephrine in Osteoblast-Like Cells

Regulation of Alkaline Phosphatase Activity by p38 MAP Kinase in Response to Activation of Gi Protein-Coupled Receptors by Epinephrine in Osteoblast-Like Cells

Dual regulation of alkaline phosphatase activity by calcitonin in porcine kidney cells.

Intracellular signal transduction for regulation of alkaline phosphatase (ALP) activity in renal epithelial cells treated with calcitonin is not yet completely understood, although it is known that calcitonin receptors couple to cyclic AMP-dependent protein kinase (PKA) and calcium/phospholipid-dependent protein kinase (PKC). Salmon calcitonin increased the cyclic AMP content in LLC-PK1 porcine kidney cells in a concentration-dependent manner. When the confluent cells were incubated for 47 h after a 1 h-pulse exposure or continuously exposed to calcitonin and forskolin for 48 h, ALP activity in the cells was increased by calcitonin about 2-fold compared with the basal activity at the maximum level but was not dependent on the exposure time; it was markedly increased by forskolin in parallel with the exposure time. The increase in activity produced by calcitonin was abolished by a PKA inhibitor H-89, and, in contrast, potentiated by a PKC inhibitor, NA-382 to near the forskolin-induced level. These results indicate that calcitonin exerts a dual-regulation of ALP activity in LLC-PK1 cells, positively through the PKA pathway and negatively through PKC.

Direct Involvement of cAMP-Dependent Protein Kinase in the Regulation of Alkaline Phosphatase Activity by Parathyroid Hormone (PTH) and PTH-Related Peptide in Osteoblastic UMR-106 Cells

The present study was performed to characterize the participation of parathyroid hormone (PTH)- and PTH-related peptide (PTHrP)-responsive dual signal transduction systems [cAMP-dependent protein kinase (PKA) and Calcium/protein kinase C (Ca/PKC)] in the regulation of alkaline phosphatase (ALP) activity in osteoblastic osteosarcoma cells (UMR-106). Both human (h) PTH-(1-34) and hPTHrP-(1-34) at 10 −8M stimulated ALP activity to the similar degree. Dibutyryl cAMP (dbcAMP) (10 −5, 10 −4M) and Sp-diastereoisomer of adenosine cyclic 3′, 5′-phosphorothioate (Sp-cAMPS), a direct stimulator of PKA (10 −4M) also stimulated its activity. Phorbor 12-myristate 13-acetate (PMA), an activator of PKC, (10 −7, 10 −6M) did not affect its activity, while calcium ionophores, A23187 and ionomycin (10 −7, 10 −6M) inhibited it. Although Rp-diastereoisomer of adenosine cyclic 3′, 5′-phosphorothioate (Rp-cAMPS), a direct inhibitor of PKA, (10 −4M) did not affect ALP activity by itself, it significantly antagonized not only Sp-cAMPS-induced increase in ALP activity, but also PTH- and PTHrP-induced one. The present study first indicated that the activation of PKA was directly involved and acted as a main pathway in the regulation of ALP activity by PTH and PTHrP in osteoblasts.

Expression and nature of the alkaline phosphatase gene in cultured osteosarcoma cells

The molecular mechanism for the differences in specific activity of alkaline phosphatase in six human osteosarcoma cell lines was investigated. Five of the lines expressed only the tissue-non-specific or liver/bone/kidney isoenzyme of alkaline phosphatase. The sixth line had the lowest levels of alkaline phosphatase and this was determined to be a mixture of liver/ bone/kidney isoenzyme and at least one other form. The mRNA of liver/bone/kidney alkaline phosphatase was identified by Northern analysis in the three cell lines that expressed the largest amount of alkaline phosphatase catalytic activity. This mRNA was indistinguishable in size from that seen in control mRNA from normal kidney (2.5 kb). Southern analysis demonstrated that EcoRI or HindIII restriction fragment patterns and the intensity of the bands, of the liver/bone/kidney alkaline phosphatase gene in the osteosarcoma cell lines were identical to that of the control DNA from normal peripheral blood leukocytes. Thus, the gene coding for liver/bone/kidney alkaline phosphatase appears to be intact in all of these osteosarcoma cells and it is unlikely that rearrangement, deletion or amplification of the gene is responsible for its activation or inactivation. Slot blot analysis revealed varying amounts of the transcripts of the liver/bone/kidney isoenzyme in each of the cell lines. The best fit line of a plot of the log of the level of mRNA of alkaline phosphatase vs. the log of the specific activity of liver/bone/kidney alkaline phosphatase was contructed. This gave a Pearson correlation coefficient of 0.92 ( P < 0.008), demonstrating a significant relationship between the two variables. It is likely that the regulation of alkaline phosphatase activity is at the transcriptional process rather than the translational or post-translational processes and that the specific activity of the enzyme may be controlled by the amount of steady-state mRNA of the liver/ bone/kidney isoenzyme.

1,25-Dihydroxyvitamin D3 exerts opposite effects on the regulation of human embryonic and nonembryonic alkaline phosphatase isoenzymes.

The regulation of alkaline phosphatase activity by steroid hormones was studied in two human breast cancer cell lines, MDA-MB-157 and BT20. MDA-MB-157 cells were shown to express the alkaline phosphatase isoenzyme produced by normal breast tissue, and the activity of this isoenzyme increased 3-fold after a 72-h treatment of these cells with 10(-7) M 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], 2-fold after treatment with 10(-6) M hydrocortisone (HC), and 5-fold after treatment with both hormones. BT20 cells did not express the isoenzyme phenotypic to breast, but ectopically expressed the isoenzyme phenotypic to term placenta and other embryonic tissue. Treatment of BT20 cells with 1,25-(OH)2D3 results in a 30% decrease in alkaline phosphatase activity of the embryonic isoenzyme. There was a 2-fold increase in activity after treatment with HC, and enzyme activity was similar to control values after treatment with both hormones. For both cell lines, changes in alkaline phosphatase activity correlated with changes in nanograms of isoenzyme per mg cellular protein, as measured by RIA. Increases in enzyme activity were inhibited when the cells were incubated simultaneously with the steroids and cycloheximide. Studies with receptors in each cell line showed that both cell lines bound 1,25-(OH)2D3 and that a 1,25-(OH)2D3-binding protein with the same mol wt as the D3 receptor was present in both. The BT20 cells also express a larger mol wt protein which binds 1,25-(OH)2D3 but is not as specific for the 1,25-(OH)2D3 isomer. HC receptors were similar in quantity and binding affinity in both cell lines.

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 γ-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°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°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 K m 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.

Thyroidal regulation of alkaline phosphatase activity in kidney cortex brush border membranes.

In kidney cortex homogenates and brush border membranes of hypothyroid rats undergoing T&lt;sub&gt;3&lt;/sub&gt; substitution with 50 µg/kg BW a day for 1–4 days, a significant decrease of alkaline phosphatase activity was observed. The data indicate that thyroid hormones may affect the luminal membrane.

751 ALKALINE PHOSPHATASE EXPRESSION IN CULTURED HUMAN SKIN FIBROBLASTS AND SOMATIC CELL HYBRIDS

We have developed an in vitro model system for studying the human hypophosphatasias, a group of familial diseases characterized by low serum alkaline phosphatase (ALP) levels and poor bone development. ALP electrophoretic and activity patterns were studied in human fibroblasts, mouse fibroblasts, and mouse-human hybrids. On polyacrylamide gels, human fibroblast ALP is seen as two zones of activity (a fast and a slow band), while the mouse fibroblast ALP appears as only one. Studies in human fibroblasts suggest that the ALP fast band is a product of modification of the slow band. In the hybrids, ALP fast band segregates with the gene for human malate dehydrogenase (MDH) which is located on chromosome 2 (p<0.001). This association has been confirmed by concordant segregation of ALP fast band and MDH in two hybrid subclones. ALP activity profiles were studied on 37 independently isolated hybrid clones. In one group of clones, human chromosome 19 was associated with high ALP activity levels (p=0.04), while in another group, chromosome 19 was missing from clones with repressed ALP activity levels (p=0.005). These observations suggest that a gene on human chromosome 19 may be involved in the regulation of ALP activity. The identification of genes which control ALP expression in cultured cells should help us to understand the genetic mechanism operating in hypophosphatasia.

Regulation of alkaline phosphatase activity in KB cells: Influence of serum

1. 1. KB cells, derived from a human nasopharyngeal carcinoma, have high alkaline phosphatase (orthophosphoric monoester phosphohydrolase, EC 3.1.3.1) activity. Specific activity is 150 times higher than in HeLa S3 cells. Alkaline phosphatase activity in KB cells exhibits heterogeneity consisting of a large heat-labile and a small heat-stable component. 2. 2. Alkaline phosphatase activity of KB cells is modulated by the concentration of human serum in the nutrient medium. Increasing the serum content results in reduction of activity. Cells grown in serum without the basal components of the medium have the lowest activity. The maximally depressed enzyme has the same thermostability as the activity of control cultures. 3. 3. The effect of serum is not due to the removal from the culture medium of components essential for enzyme synthesis and activity. Alkaline phosphatase depression is reversible as demonstrated by the appearance of normal levels of activity following the change of the nutrient environment from serum to regular medium. Serum does not hinder the induction of the heat-stable form of the enzyme by prednisolone and by increased osmolality. 4. 4. The total cell protein content and the activities of acid phosphatase, β-galactosidase and N-acetyl- β-glucosaminidase are not affected by serum.

Regulation of alkaline phosphatase activity in human cell cultures: Role of serum

Alkaline phosphatase activity in certain established cell lines of human origin (HeLa S3, Zimmer liver 4A, and H.Ep II) is controlled by the concentration of human serum in the culture medium. Enzyme activity is inversely proportional to the concentration of serum. The effect of serum is maximal in the absence of the basal components of the medium. During a 7-day growth cycle in serum a sequential depletion in enzyme activity occurs. Serum does not affect total cell protein content nor does it influence the activities of acid phosphatase, β -glucosidase, β -galactosidase, and N -acetyl- β -glucosaminidase. The effect of serum on alkaline phosphatase is reversible as evidenced by the normal levels of activity following the transfer of cells into regular medium and by changing the nutrient environment from serum to regular medium. Serum does not prevent enzyme induction by prednisolone and/or increased osmolality. The alkaline phosphatase of cells grown in serum has the same intracellular distribution and properties as that of control cultures.