Phosphoserine Phosphatase Activity Research Articles

Rationalizing the Influence of the Binding Affinity on the Activity of Phosphoserine Phosphatases

AbstractThe Sabatier principle states that catalytic activity can be maximized when the substrate binding affinity is neither too strong nor too weak. Recent studies have shown that the activity of several hydrolases is maximized at intermediate values of the binding affinity (Michaelis–Menten constant: Km). However, it remains unclear whether this concept of artificial catalysis is applicable to enzymes in general, especially for those which have evolved under different reaction environments. Herein, we show that the activity of phosphoserine phosphatase is also enhanced at an intermediate Km value of approximately 0.5 mM. Within our dataset, the variation of Km by three orders of magnitude accounted for a roughly 18‐fold variation in the activity. Owing to the high phylogenetic and physiological diversity of our dataset, our results support the importance of optimizing Km for enzymes in general. On the other hand, a 77‐fold variation in the activity was attributed to other physicochemical parameters, such as the Arrhenius prefactor of kcat, and could not be explained by the Sabatier principle. Therefore, while tuning the binding affinity according to the Sabatier principle is an important consideration, the Km value is only one of many physicochemical parameters which must be optimized to maximize enzymatic activity.

Rationalizing the Influence of the Binding Affinity on the Activity of Phosphoserine Phosphatases.

The Sabatier principle states that catalytic activity can be maximized when the substrate binding affinity is neither too strong nor too weak. Recent studies have shown that the activity of several hydrolases is maximized at intermediate values of the binding affinity (Michaelis-Menten constant: Km ). However, it remains unclear whether this concept of artificial catalysis is applicable to enzymes in general, especially for those which have evolved under different reaction environments. Herein, we show that the activity of phosphoserine phosphatase is also enhanced at an intermediate Km value of approximately 0.5 mM. Within our dataset, the variation of Km by three orders of magnitude accounted for a roughly 18-fold variation in the activity. Owing to the high phylogenetic and physiological diversity of our dataset, our results support the importance of optimizing Km for enzymes in general. On the other hand, a 77-fold variation in the activity was attributed to other physicochemical parameters, such as the Arrhenius prefactor of kcat , and could not be explained by the Sabatier principle. Therefore, while tuning the binding affinity according to the Sabatier principle is an important consideration, the Km value is only one of many physicochemical parameters which must be optimized to maximize enzymatic activity.

Effect of l-serine and magnesium ions on the functional properties of human phosphoserine phosphatase and its pathogenetic variants

L-Ser supply in the central nervous system of mammals mostly relies on its endogenous biosynthesis by the phosphorylated pathway (PP). Defects in any of the three enzymes operating in the pathway result in a group of neurometabolic diseases collectively known as serine deficiency disorders (SDDs). Phosphoserine phosphatase (PSP) catalyzes the last, irreversible step of the PP. Here we investigated in detail the role of physiological modulators of human PSP activity and the properties of three natural PSP variants (A35T, D32N and M52T) associated with SDDs. Our results, partially contradicting previous reports, indicate that: i. PSP is almost fully saturated with Mg2+ under physiological conditions and fluctuations in Mg2+ and Ca2+ concentrations are unlikely to play a modulatory role on PSP activity; ii. Inhibition by L-Ser, albeit at play on the isolated PSP, does not exert any effect on the flux through the PP unless the enzyme activity is severely impaired by inactivating substitutions; iii. The so-far poorly investigated A35T substitution was the most detrimental, with a 50-fold reduction in catalytic efficiency, and a reduction in thermal stability (as well as an increase in the IC50 for L-Ser). The M52T substitution had similar, but milder effects, while the D32N variant behaved like the wild-type enzyme. iv. Predictions of the structural effects of the A35T and M52T substitutions with ColabFold suggest that they might affect the structure of the flexible helix-loop region.

Characterization of a Non‐Classic Type Phosphoserine Phosphatase Homolog in Thermodesulfidibacter takaii , a Thermophilic, Autotrophic Bacterium in the Phylum Aquificae

Phosphoserine phosphatase (PSP) catalyzes the dephosphorylation of phosphoserine to serine, and is found in all three domains of life. Classic, metal-dependent PSPs (dPSPs) belong to the haloacid dehalogenase-like hydrolase superfamily. Some organisms, however, lack a known PSP even though they exhibit a functional phosphoserine synthetic pathway, and should then possess a genetically new type of PSP. Hydrogenobacter thermophilus, a thermophilic, obligate chemolithoautotroph in the phylum Aquificae, possesses two novel-type, metal-independent PSP (iPSP). These proteins, named HtPspA and HtPspB, belong to the histidine phosphatase superfamily. Despite their high primary structure similarity and the conservation of residues essential for phosphatase activity, HtPspA and HtPspB present notable differences. In H. thermophilus they are expressed as homodimers (A-A) or heterodimers (A-B). However, only HtPspA shows significant PSP activity, and is therefore the catalytic subunit as a PSP. HtPspB does not have substantial PSP activity and is only soluble in the presence of HtPspA, at least when it is expressed in E. coli. Crystal structure analysis revealed that His85 and the C-terminal region, which exist in HtPspA but not in HtPspB, are necessary for PSP activity in this organism. Thermosulfidibacter takaii, which belongs to a distinct family in the phylum Aquificae, possesses only one homolog of iPSP which lacks His85 and the C-terminal region, showing higher identity to HtPspB than to HtPspA. To examine whether this homolog can serve as a serine synthetic enzyme in T. takaii - which should have a serine synthetic pathway - while lacking the structural units of HtPspA essential for PSP activity, we characterized its enzymatic properties. In contrast to HtPspB, TtPsp was expressed in the soluble fraction in E. coli. It is thermostable at 70°C, the optimal growth temperature of T. takaii, and is likely to form a homodimer as suggested by gel chromatography. Enzymatic activity assays revealed that TtPsp possesses only about 1% of the PSP activity (per mole of protein) exhibited by HtPspA, and displays higher activity towards L-phosphothreonine than L-phosphoserine. The phylogenetic characterization of TtPspB showed that this protein forms a clade with PspB homologs of distinct families within the phylum Aquificae. Our study revealed that an ancestor of Aquificae already possessed PspA and PspB, and that the lack of a PspA homolog in T. takaii was caused by gene loss. Whether the common ancestor of PspA and PspB was a functional PSP or not, and the physiological function of PspB, is still unknown.

Investigation of the Structure and Function of Phosphoserine Phosphatases

With the ever increasing availability of new genome sequences every year, there is a growing need for functional annotation both on a structural and cellular level. In the case of large superfamilies of proteins with a great deal of sequence divergence, assignment of function based on sequence alone remains difficult. For instance, although Bacillus subtilis is the model gram positive bacterium, only 30% of the genome has assigned activity. YsaA (BSU28940), a B. subtilis protein discovered in a knockout screen for metabolic mutants, has been confirmed to have phosphoserine phosphatase activity by determining the steady‐state kinetic constants. Despite this, YsaA shares only 12% sequence identity to the known E. coli phosphoserine phosphatase SerB. We are currently working to determine the molecular structure of YsaA, along with a Bacillus halodurans protein BH2155 (60% similarity with YsaA) which will provide better understanding of the structure‐function relationship between these new proteins and SerB. It is very possible that although YsaA and SerB are within the same super family, they may have independently evolved the same function.BH2155 has been over‐expressed in E.coli, purified, and subjected to sparse‐matrix high throughput screens yielding initial crystallization conditions. Additionally, experiments have been performed using a phosphate assay to obtain the steady state kinetic constants for BH2155 and compare them to YsaA and known phosphoserine phosphatase SerB. By determining the structures of these proteins as well as profiling their activity we hope to gain insight into the structure‐function relationship and the possibility of pseudo‐convergent evolution in phosphoserine phosphatases.Support or Funding InformationWe thank the Undergraduate Research Opportunities Program at Boston University for funding supporting this project (to MCB).This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Phosphoserine phosphatase activity is elevated and correlates negatively with plasma d-serine concentration in patients with schizophrenia

The pathophysiology of schizophrenia may involve N-methyl-D-aspartate receptor (NMDAR) hypofunction. D-3serine and glycine are endogenous l-serine-derived NMDAR co-agonists. We hypothesized that the l-serine synthesis pathway could be involved in schizophrenia. We measured the activity of phosphoserine phosphatase (PSP), a rate-limiting enzyme in l-serine synthesis, in peripheral blood mononuclear cells of 54 patients with schizophrenia and 49 normal control subjects. Plasma amino acid (l-serine, d-serine, glycine, glutamine, and glutamate) levels were measured by high performance liquid chromatography. Peripheral blood mRNA expression levels of PHGDH, PSAT1, PSP, and SR, determined by quantitative real-time PCR were compared between patients and controls. PSP activity was higher in patients than in controls, especially in male patients. In male patients, the plasma l-serine concentration was higher than that in controls. In patients, PSP activity was negatively correlated with plasma d-serine and glycine levels. Furthermore, PSP activity was positively correlated with plasma l-serine concentration. These results were statistically significant only in male patients. PSP, PSAT1, and PHGDH mRNA levels were lower in patients than in controls, except when the PHGDH expression level was compared with ACTB expression. In summary, we found the l-serine synthesis system to be altered in patients with schizophrenia, especially in male patients.

Lack of phosphoserine phosphatase activity alters pollen and tapetum development in Arabidopsis thaliana

Formation of mature pollen grain, an essential process for the reproduction of higher plants, is affected in lines that are deficient in the enzymes of the phosphorylated pathway of serine biosynthesis (PPSB). Mutants of phosphoserine phosphatase (PSP), the enzyme that catalyses the last step of PPSB, are embryo-lethal. When they are complemented with a construct carrying PSP1 cDNA under the control of the 35S promoter (psp1.1 35S:PSP1), which is poorly expressed in anther tissues, plants display a wild-type phenotype, but are male-sterile. The pollen from the psp1.1 35S:PSP1 lines are shrunken and unviable. Here we report the morphological alterations that appear in the psp1.1 35S:PSP1 lines during microspore development. We show that the pollen wall from these lines presents a normal exine layer, but a shrunken and collapsed shape. Lack of PSP activity also affects oil bodies formation in the tapetosomes of tapetal cells which, in turn, may influence microspore pollen coat formation. All these results highlight the important role of the PPSB in the normal development of microspores in Arabidopsis thaliana.

Substrate Ambiguous Enzymes within the Escherichia coli Proteome Offer Different Evolutionary Solutions to the Same Problem

Many enzymes exhibit some catalytic promiscuity or substrate ambiguity. These weak activities do not affect the fitness of the organism under ordinary circumstances, but can serve as potential evolutionary precursors of new catalytic functions. We wondered whether different proteins with the same substrate ambiguous activity evolve differently under identical selection conditions. Patrick et al. (Patrick WM, Quandt EM, Swartzlander DB, Matsumura I. 2007. Multicopy suppression underpins metabolic evolvability. Mol Biol Evol. 24:2716-2722.) previously showed that three multicopy suppressors, gph, hisB, and ytjC, rescue ΔserB Escherichia coli cells from starvation on minimal media. We directed the evolution of variants of Gph, histidinol phosphatase (HisB), and YtjC that complemented ΔserB more efficiently, and characterized the effects of the amino acid changes, alone and in combination, upon the evolved phosphoserine phosphatase (PSP) activity. Gph and HisB are members of the HAD superfamily of hydrolases, but they adapted through different, kinetically distinguishable, biochemical mechanisms. All of the selected mutations, except N102T in YtjC, proved to be beneficial in isolation. They exhibited a pattern of antagonistic epistasis, as their effects in combination upon the kinetic parameters of the three proteins in reactions with phosphoserine were nonmultiplicative. The N102T mutation exhibited sign epistasis, as it was deleterious in isolation but beneficial in the context of other mutations. We also showed that the D57N mutation in the chromosomal copy of hisB is sufficient to suppress the ΔserB deletion. These results in combination show that proteomes can offer multiple mechanistic solutions to a molecular recognition problem.

Identification of a novel ligand binding site in phosphoserine phosphatase from the hyperthermophilic archaeon Thermococcus onnurineus

Phosphoserine phosphatase (PSP) catalyzes the final and irreversible step of L-serine synthesis by hydrolyzing phosphoserine to produce L-serine and inorganic phosphate. Developing a therapeutic drug that interferes with serine production is of great interest to regulate the pathogenicity of some bacteria and control D-serine levels in neurological diseases. We determined the crystal structure of PSP from the hyperthermophilic archaeon Thermococcus onnurineus at 1.8 Å resolution, revealing an NDSB ligand bound to a novel site that is located in a fissure between the catalytic domain and the CAP module. The structure shows a half-open conformation of the CAP 1 module with a unique protruding loop of residues 150-155 that possesses a helical conformation in other structures of homologous PSPs. Activity assays indicate that the enzyme exhibits marginal PSP activity at low temperature but a sharp increase in the k(cat)/K(M) value, approximately 22 fold, when the temperature is increased. Structural and biochemical analyses suggest that the protruding loop in the active site might be an essential component for the regulation of the activity of PSP from hyperthermophilic T. onnurineus. Identification of this novel binding site distantly located from the catalytic site may be exploited for the development of effective therapeutic allosteric inhibitors against PSP activity.

Why calcium inhibits magnesium-dependent enzyme phosphoserine phosphatase? A theoretical study

Phosphoserine phosphatase (PSP) utilizes one Mg2+ ion to catalyze the hydrolysis of phospho-l-serine. The displacement of Mg2+ by Ca2+ results in the loss of activity. The reaction mechanisms for the enzyme with both Mg2+ and Ca2+ bound were investigated using hybrid density functional theory. A large quantum chemical model abstracted from the X-ray crystal structure was employed in the calculations. Our calculations shed new insight into the catalytic mechanism of the natural enzyme and its lack of activity by Ca2+ substitution. For the catalytic reaction, our calculations showed that the whole reaction proceeds through two steps, namely dephosphorylation and phosphate hydrolysis. The associated barriers for these two steps are calculated to be 11.9 and 12.0 kcal mol−1, respectively. The Mg-bound Asp11 residue functions as a nucleophile to attack the phosphorus moiety, in concomitant with the departure of the leaving group, which takes a proton from the neutral Asp13 residue. In the subsequent step, the newly formed anionic Asp13 residue activates a water molecule to perform the reverse attack on the phosphoryl intermediate, affording the phosphate product. The substitution of Mg2+ by Ca2+ results in different metal coordination fashion, in which the Asp167 residue changes from bidentate to monodentate and a second water molecule becomes ligated to Ca2+. The calculated barriers for the hydrolysis are ca 8 kcal mol−1 higher than those in the native enzyme, which reconciles with the fact that Ca2+ inhibits the activity of PSP. Several possible reasons are discussed.

Discovery and Analysis of Cofactor-dependent Phosphoglycerate Mutase Homologs as Novel Phosphoserine Phosphatases in Hydrogenobacter thermophilus

Phosphoserine phosphatase (PSP) catalyzes the dephosphorylation of phosphoserine to serine and inorganic phosphate. PSPs, which have been found in all three domains of life, belong to the haloacid dehalogenase-like hydrolase superfamily. However, certain organisms, particularly bacteria, lack a classical PSP gene, although they appear to possess a functional phosphoserine synthetic pathway. The apparent lack of a PSP ortholog in Hydrogenobacter thermophilus, an obligately chemolithoautotrophic and thermophilic bacterium, represented a missing link in serine anabolism because our previous study suggested that serine should be synthesized from phosphoserine. Here, we detected PSP activity in cell-free extracts of H. thermophilus and purified two proteins with PSP activity. Surprisingly, these proteins belonged to the histidine phosphatase superfamily and had been annotated as cofactor-dependent phosphoglycerate mutase (dPGM). However, because they possessed neither mutase activity nor the residues important for the activity, we defined these proteins as novel-type PSPs. Considering the strict substrate specificity toward l-phosphoserine, kinetic parameters, and PSP activity levels in cell-free extracts, these proteins were strongly suggested to function as PSPs in vivo. We also detected PSP activity from "dPGM-like" proteins of Thermus thermophilus and Arabidopsis thaliana, suggesting that PSP activity catalyzed by dPGM-like proteins may be distributed among a broad range of organisms. In fact, a number of bacterial genera, including Firmicutes and Cyanobacteria, were proposed to be strong candidates for possessing this novel type of PSP. These findings will help to identify the missing link in serine anabolism.

Identification of a novel PP2C-type mitochondrial phosphatase

A novel phosphatase has been cloned and partially characterized. It has a mitochondrial leader sequence and its amino acid sequence places it in the PP2C family like two known mitochondrial phosphatases. Western blot analysis of subcellular fractions and confocal microscopy of 3T3L1 preadipocytes expressing the GFP-tagged protein confirm its mitochondrial localization. Western blot analysis indicates that the protein is expressed in several mouse tissues, with highest expression in brain, heart, liver, and kidney. The recombinant protein exhibits Mn 2+-dependent phosphoserine phosphatase activity against the branched-chain α-keto acid dehydrogenase complex, suggesting the enzyme may play a role in regulation of branched chain amino acid catabolism. Whether there are other mitochondrial substrates for the enzyme is not known.

Structure and Mechanism of the Phosphotyrosyl Phosphatase Activator

Phosphotyrosyl phosphatase activator (PTPA), also known as PP2A phosphatase activator, is a conserved protein from yeast to human. Here we report the 1.9 A crystal structure of human PTPA, which reveals a previously unreported fold consisting of three subdomains: core, lid, and linker. Structural analysis uncovers a highly conserved surface patch, which borders the three subdomains, and an associated deep pocket located between the core and the linker subdomains. The conserved surface patch and the deep pocket are responsible for binding to PP2A and ATP, respectively. PTPA and PP2A A-C dimer together constitute a composite ATPase. PTPA binding to PP2A results in a dramatic alteration of substrate specificity, with enhanced phosphotyrosine phosphatase activity and decreased phosphoserine phosphatase activity. This function of PTPA strictly depends on the composite ATPase activity. These observations reveal significant insights into the function and mechanism of PTPA and have important ramifications for understanding PP2A function.

The thrH Gene Product of Pseudomonas aeruginosa Is a Dual Activity Enzyme with a Novel Phosphoserine:Homoserine Phosphotransferase Activity

The thrH gene product of Pseudomonas aeruginosa has been shown to complement both homoserine kinase (thrB gene product) and phosphoserine phosphatase (serB gene product) activities in vivo. Sequence comparison has revealed that ThrH is related to phosphoserine phosphatases (PSP, EC 3.1.3.3) and belongs to the l-2-haloacid dehalogenase-like protein superfamily. We have solved the crystal structures of ThrH in the apoform and in complex with a bound product phosphate. The structure confirms an overall fold similar to that of PSP. Most of the catalytic residues of PSP are also conserved in ThrH, suggesting that similar catalytic mechanisms are used by both enzymes. Spectrophotometry-based in vitro assays show that ThrH is indeed a phosphoserine phosphatase with a K(m) of 0.207 mm and k(cat) of 13.4 min(-1), comparable with those of other PSPs. More interestingly, using high pressure liquid chromatography-based assays, we have demonstrated that ThrH is able to further transfer the phosphoryl group to homoserine using phosphoserine as the phosphoryl group donor, indicating that ThrH has a novel phosphoserine:homoserine phosphotransferase activity.

ThrH, a homoserine kinase isozyme with in vivo phosphoserine phosphatase activity in Pseudomonas aeruginosa.

Homoserine kinase, the product of the thrB gene, catalyses an obligatory step of threonine biosynthesis. In Pseudomonas aeruginosa, unlike Escherichia coli, inactivation of the previously identified thrB gene does not result in threonine auxotrophy. A new gene, named thrH, was isolated that, when expressed in E. coli thrB mutant strains, results in complementation of the mutant phenotype. In P. aeruginosa, threonine auxotrophy is observed only when both thrB and thrH are simultaneously inactivated. Thus, thrH encodes a protein with an in vivo homoserine-kinase-like activity. Surprisingly, thrH overexpression allows complementation of serine auxotrophy of E. coli and P. aeruginosa serB mutants. These mutants are affected in the phosphoserine phosphatase protein, an enzyme involved in serine biosynthesis. Comparison analysis revealed sequence homology between ThrH and the SerB proteins from different organisms. This could explain the in vivo phosphoserine phosphatase activity of ThrH when overproduced. ThrH differs from the protein encoded by the serB gene which was identified in P. aeruginosa. Thus, two SerB-like proteins co-exist in P. aeruginosa, a situation also found in Mycobacterium tuberculosis.

Effect of streptozotocin-induced diabetes on GLUT-4 phosphorylation in rat adipocytes.

We have examined the regulation of GLUT-4 phosphorylation in adipocytes isolated from diabetic rats. Despite progressive (40-70%) reductions in GLUT-4 protein contents on the 2nd, 7th, and 14th day of diabetes, the phosphorylation of GLUT-4 was increased two- to fourfold. These alterations were accompanied by concomitant reductions (40-66%) in the insulin-stimulated 2-deoxyglucose transport. Insulin treatment of diabetic animals for 5 d restored glucose transport activity, GLUT-4 protein, and GLUT-4 phosphorylation to control levels whereas vanadate and phlorizin were ineffective. In control adipocytes, insulin promoted GLUT-4 translocation from the low density microsomal (LDM) pool to the plasma membranes (PM) and decreased the state of GLUT-4 phosphorylation. In adipocytes isolated from the diabetic rats, insulin failed to stimulate GLUT-4 translocation and to decrease GLUT-4 phosphorylation. To explore the mechanism of the diabetes-induced increases in the GLUT-4 phosphorylation, we investigated phosphoserine phosphatase (PSPase) activities using 32P-labeled GLUT-4 and phosphorylase "a" as substrates. Diabetes resulted in 50-60% increase in the particulate PSPase activity and concomitant reductions in cytosolic PSPase activities. Although reduced cytosolic PSPase activity correlated with an inadequate dephosphorylation of LDM GLUT-4, the existence of highly phosphorylated PM GLUT-4 in the presence of increased particulate PSPase activity required additional explanation. To address this problem, we used PM GLUT-4 from diabetic rats as a substrate of particulate PSPase. Highly active diabetic particulate PSPase, which dephosphorylated control GLUT-4 and phosphorylase a, failed to dephosphorylate PM GLUT-4 from diabetic rats. These data suggest that PM GLUT-4 from diabetic rats is unable to interact with PSPase or that its phosphorylation sites are not accessible to PSPase action. In summary, an induction of diabetes with streptozotocin resulted in significant increases in GLUT-4 phosphorylation. In contrast to normal cells, insulin failed to promote GLUT-4 recruitment to the plasma membranes and its dephosphorylation in diabetic adipocytes. At the same time, diabetes appears to induce redistribution of PSPases, resulting in lower cytosolic activity and higher particulate activity. It also appears that the existence of highly phosphorylated GLUT-4 in the plasma membranes of diabetic adipocytes resulted from its inability to interact with particulate PSPases.

Calcium-induced inhibition of phosphoserine phosphatase in insulin target cells is mediated by the phosphorylation and activation of inhibitor 1.

In this study, we examined the mechanism of inhibition of phosphoserine phosphatase (PSPase) activity by elevated [Ca2+]i in insulin target cells. In in vitro studies, isolated rat adipocytes were incubated with either 40 mM K+ or parathyroid hormone (PTH) (20 ng/ml) for 1 h. In in vivo studies, rats were injected with PTH (three hourly injections of 40 micrograms intraperitoneally) prior to isolation of either adipocytes or skeletal muscle. Under these conditions, intracellular [Ca2+]i changed from 100 +/- 8.7 to 263 +/- 10.5 nM. There was a concomitant 30% decrease in adipocyte PSPase activity and a 35% decrease in skeletal muscle PSPase activity, assayed using 32P-labeled phosphorylase "a" as a substrate. The inhibition of PSPase was accompanied by a 60% increase in adipocytes (p less than 0.05) and a 118% increase (p less than 0.01) in skeletal muscle inhibitor 1 (I1) activities, respectively. Since I1 is active only in the phosphorylated state, we studied the effect of [Ca2+]i on I1 phosphorylation. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of heat treated extracts immunoprecipitated with I1 antibody revealed significant increase in 32P incorporation (45-60%, p less than 0.05) into I1 protein in cells with elevated [Ca2+]i. Nitrendipine, a calcium channel blocker, completely prevented increases in I1 phosphorylation and activity in cells exposed to K+ but was only partially effective in the PTH-treated cells. In contrast, a cyclic AMP antagonist, RpcAMP, prevented both the K(+)-and the PTH-induced increases in I1 phosphorylation and activity, even though it failed to block the elevations in [Ca2+]i in these cells. We conclude that [Ca2+]i-induced and cAMP-mediated phosphorylation and activation of I1 results in inhibition of PSPase activity in insulin target cells. The inhibition of PSPases may cause inappropriate serine dephosphorylation of substrates of insulin action resulting in insulin resistance.

Differential effects of diabetes on adipocyte and liver phosphotyrosine and phosphoserine phosphatase activities

Differential effects of diabetes on adipocyte and liver phosphotyrosine and phosphoserine phosphatase activities

Alkaline phosphatase and phosphoamino acid phosphatases in normal and cancerous tissues of the human larynx

The activities of alkaline phosphatase and phosphoamino acid phosphatases were measured in normal and cancerous regions of the human larynx. For each larynx, alkaline phosphatase and phosphotyrosine phosphatase activities were higher in the tumor than in the corresponding normal tissue. Phosphothreonine and phosphoserine phosphatase activities were relatively low and there were no consistent trends. The increased alkaline phosphatase activity in the tumors supports histological observations that ossification of cartilage seems to occur at the site of invasion; the phosphatase acting on phosphotyrosine could serve as a regulator of cell differentiation during tumorigenesis.

Phosphoamino acid phosphatases in human normal and leukaemic lymphocytes

The activities of phosphoamino acid phosphatases were measured in lymphocytes from normal adults. For cells from white individuals the mean value (+/- SD) for phosphotyrosine phosphatase activity was 342 +/- 120 mU/mg protein; mean values for phosphothreonine and phosphoserine phosphatase activities were 35.6 +/- 17.1 and 21.6 +/- 10.2 mU/mg protein, respectively. The corresponding activities were similar to these for both the Indian and Bantu population groups. The activity of phosphotyrosine phosphatase was significantly lower in lymphocytes from white children with acute lymphocytic leukaemia than in cells from normal children or adults. For phosphothreonine and phosphoserine phosphatases, there was no significant difference between activities obtained in lymphocytes from normal and leukaemic children.