Phosphodiesterase induction in Dictyostelium discoideum by inhibition of extracellular phosphodiesterase activity

Adenosine 3′,5′-monophosphate (cAMP) is a chemoattractant in Dictyostelium discoideum; it also induces phosphodiesterase activity. Recently it was shown (M. H. Juliani, J. Brusca, and C. Klein, (1981)Develop. Biol.83, 114–121) that N⁶-(aminohexyl)adenosine 3′,5′-monophosphate (hexyl-cAMP) effectively induced phosphodiesterase activity, while this compound was chemotactically inactive and did not effectively bind to the cell surface receptor for cAMP. It was suggested that hexyl-cAMP and cAMP induce phosphodiesterase activity via a chemoreceptor-independent mechanism. In another recent report (P. J. M. Van Haastert, R. C. Van der Meer, and T. M. Konijn (1981)J. Bacteriol.147, 170–175) investigation of induction of phosphodiesterase by several cAMP derivatives revealed that phosphodiesterase induction and chemotaxis had similar cyclic nucleotide specificity. Based on this result it was suggested that cAMP induces phosphodiesterase activity via activation of the chemotactic receptor. In this report we show that hexyl-cAMP transiently inhibits extracellular and cell surface phosphodiesterase. This transient inhibition of the inactivating enzyme and the permanent release of small amounts of cAMP by the cells leads to a transient increase of extracellular cAMP levels. Hexyl-cAMP does not inhibit beef heart phosphodiesterase, and is not degraded by this enzyme. Addition of hexyl-cAMP to a cell suspension containing beef heart phosphodiesterase does not result in an accumulation of extracellular cAMP, and phosphodiesterase induction is absent. We conclude that hexyl-cAMP inhibits phosphodiesterase activity which leads to the accumulation of cAMP; consequently cAMP binds to the chemotactic cAMP receptor resulting in the induction of phosphodiesterase activity.     

In vitro inhibition of the helper activity of GAT-specific T-cell lines by a syngeneic anti-idiotypic serum: preferential effect on the IgG response

We described in this paper the characteristics of a syngeneic anti-idiotypic serum made in BALB/c against BALB/c anti-poly (Glu⁶⁰ Ala³⁰ Tyr¹⁰) (GAT) antibodies. This serum recognizes idiotypic determinants present in all anti-GAT sera whatever the allotypic markers of the mice used to prepare the sera. The functional effect of this serum on two helper cell lines is also described. Cell line BDF₁/52 was obtained from GAT immunized lymph node cells (LNC). Cell line BDF₁/E₃ was selected from splenic T-cells educated in vitro on GAT-pulsed adherent cells. Both lines were propagated in presence of filler cells, antigen, and medium containing T-cell growth factor(s) from splenic cells activated with concanavalin A. Both cell lines exhibit a helper activity as measured by the plaque-forming cell (PFC) response they induce in vitro in the presence of DNP-GAT and DNP sensitized B cells. Their helper activity is specific and they require a hapten-carrier bridge to activate B cells. These lines are able to induce IgG₁, IgG_2a and IgG_2b anti-TNP PFC. Syngeneic anti-idiotypic serum B 658 inhibits specifically the function of these two lines but does not affect the helper activity of an OVA-specific T-cell line. The blocking activity of the serum can be adsorbed on a hybridoma protein with anti-GAT activity. This inhibition affects more dramatically the IgG₁ response than the IgG_2a and IgG_2b responses.     

The inhibition of acetylcholinesterase by arsenite and fluoride

The effect of fluoride on the rate of reaction of acetylcholinesterase with arsenite, on the rate of dissociation of the enzyme-arsenite complex, and on the equilibrium between enzyme and arsenite was studied. Fluoride decreases the rate of the reaction between acetylcholinesterase and arsenite and changes the apparent equilibrium dissociation constant between the enzyme and arsenite, but even at concentrations as high as 0.2 M has no effect on the rate of dissociation of the enzyme-arsenite complex. The binding of fluoride and arsenite with the enzyme is highly anticooperative and may well be mutually exclusive. These results are consistent with a model in which the binding sites overlap and in which the same functional groups are involved.     

Modification of specific lysine residues in E. coli methionyl-tRNA synthetase by crosslinking to E. coli formylmethionine tRNA

A protein affinity labeling derivative of E. coli tRNAfMet has been prepared which carries an average of one reactive side chain per molecule, distributed over four structural regions. Each side chain contains a disulfide bond capable of reaction with cysteine residues and an N-hydroxysuccinimide ester group capable of coupling to lysine epsilon-amino groups in proteins. Reaction of the modified tRNA with E. coli methionyl-tRNA synthetase leads to crosslinking only by reaction with lysine residues in the protein. Examination of the tRNA present in the crosslinked complex reveals that the enzyme is coupled to side chains attached to the 5' terminal nucleotide, the dihydrouridine loop, the anticodon and the CCA sequence. Digestion of the crosslinked enzyme with trypsin followed by peptide mapping reveals that the major crosslinking reactions occur at four specific lysine residues, with minor reaction at two additional sites. Native methionyl-tRNA synthetase contains 90 lysine residues, 45 in unique sequences of the dimeric alpha 2 enzyme. Crosslinking of the protein to different regions in tRNAfMet thus occurs with the high degree of selectivity necessary for use in determining the peptide sequences which are near specific nucleotide sequences of tRNA bound to the protein.     

An equilibrium and kinetic study of 1,25-dihydroxyvitamin D3 binding to chicken intestinal cytosol employing high specific activity 1,25-dehydroxy[3H-26, 27] vitamin D3

A reexamination of the equilibrium and the kinetics of 1,25-dihydroxy vitamin D₃ binding with its receptor in chick intestinal cytosol was performed because of the recent availability in our laboratory of high specific activity 1,25-dihydroxy[${}^3\text{H-26,27}$]vitamin D₃ (160 Ci/mmol). Under saturating conditions at 25 °C, Scatchard analysis revealed an equilibrium dissociation constant (K_d) of 7.1 × 10^?11m which is several fold lower than previously reported for this binding reaction. Furthermore, an estimate of 1.8 × 10³ receptor sites per cell was obtained from the intercept of the line with the abscissa of the Scatchard plot. From a kinetic analysis of 1,25-dihydroxy vitamin D₃ binding with chick intestinal cytosol, association and dissociation rate constants were determined. Values that were obtained at 25 °C for these processes were 9.5 × 10⁸m^? min^? and 7.1 × 10^?3 min^?, respectively. Although these studies, such as for other steroid hormones, were carried out using a crude native cytosol preparation, we have been able to demonstrate unequivocally through the use of high specific activity 1,25-dihydroxy[${}^3\text{H-26,27}$] vitamin D₃ a truly high affinity binding site.     

Differential inhibition of globin chain synthesis by tosyl-L-lysyl chloromethyl ketone

Tosyl-L-lysyl chloromethyl ketone (TLCK), added to an incubation of rabbit reticulocytes, inhibits the synthesis of α globin chain more than that of β chain. TLCK has been previously shown to inhibit initiation of translation in a variety of cells. Thus this drug could be used to test for such a differential effect at the level of peptide initiation on the various mRNAs in these cells.     

Biphasic saturations of binding proteins can be the result of a competitive inhibition of substrate fixation.

A mathematical model is proposed which explains some biphasic saturations of Binding Proteins by their substrates through an effect of a competitive inhibition. The inhibitor can be the substrate itself especially when the retention phenomenon is occuring. This model has been verified with two periplasmic Binding Proteins of Escherichia coli: the Glutamine Binding Protein and the Leucine-Isoleucine-Valine Binding Protein. A significant connection is found between experimental results and the hypothesis.     

A specific DNA orientation in the filamentous bacteriophage fd as probed by psoralen crosslinking and electron microscopy.

The molecular structure of the single-stranded fd DNA inside its filamentous virion has been stabilized by the photochemical reaction with a psoralen derivative and examined in the electron microscope. The results support the notion that the 6389 nucleotide-long DNA molecule is folded back on itself inside the 1 μm-long protein coat. At one end of the virion, there exists a DNA hairpin region 200±50 base-pairs long. This “end hairpin” is mapped on the fd genome to the site of the replication origin. The most stable in vitro hairpin of fd DNA has been mapped previously to this same site. This unique duplex region of fd DNA may play an important role in the formation of specific protein-DNA complexes which are crucial to stages of the fd life cycle: the adsorption of the phage to the bacteria, the initiation of replication of the single-stranded DNA, and the assembly of newly synthesized DNA strands into the filamentous virions.     

Relative importance of pyruvate dehydrogenase interconversion and feed-back inhibition in the effect of fatty acids on pyruvate oxidation by rat heart mitochondria.

Rat heart mitochondria have been incubated with concentrations of pyruvate from 50 to 500 μm as substrate in the presence or absence of an optimal concentration of palmitoylcarnitine and with respiration limited by phosphate acceptor. The rate of pyruvate utilization has been determined and compared with the amount of active (dephosphorylated) pyruvate dehydrogenase measured in samples of mitochondria taken throughout the experiments and assayed under V_max conditions. At a given pyruvate concentration, differences in pyruvate utilization as a proportion of the content of active pyruvate dehydrogenase are attributed to differences in feed-back inhibition and interpreted in terms of the ratios of mitochondrial $\text{NAD}{}^{\mathrm{+}}\text{NADH}$ and CoA/acetyl-CoA. Under most conditions, differences in inhibition can be attributed to differences in the CoA/acetyl-CoA ratio. Feed-back inhibition is very pronounced in the presence of palmitoylcarnitine. These parameters are also examined in the presence of dichloroacetate, used to raise the steady-state levels of active pyruvate dehydrogenase in the absence of changing the pyruvate concentration, and in the presence of various ratios of carnitine/acetylcarnitine, which predominantly change the mitochondrial CoA/acetyl-CoA ratio. The latter experiment provides evidence that a decrease in mitochondrial $\text{NAD}{}^{\mathrm{+}}\text{NADH}$ ratio from 3.5 to 2.2 essentially balances an increase in CoA/acetyl-CoA ratio from 0.67 to 12 in modulating enzyme interconversion, whereas the change in CoA/acetyl-CoA ratio is preponderant in effecting feed-back inhibition. Increasing the free Ca²⁺ concentration of incubation media from 10^?7 to 10^?6m using CaCl₂-ethylene glycol bis(β-aminoethyl ether)-N,N′-tetraacetic acid buffers is shown to increase the steady-state level of active pyruvate dehydrogenase in intact mitochondria, in the absence of added ionophores. Moreover, this activation is reversible. Effects of Ca²⁺ ions are dependent upon the poise of the enzyme interconversion system and were only seen in these experiments in the presence of palmitoylcarnitine.     

Lectin- and ionophore-stimulated Ca2+ influx in murine lymphocytes: inhibition by disialoganglioside

Gangliosides suppress lymphocyte mitogenesis when added exogenously to the cells. On the premise that the mechanism of ganglioside action may be an interference with primary induction events, mitogen-induced 45Ca2+ influx in murine lymphocytes was studied. Disialoganglioside (GD1a) at physiopathological concentrations inhibits concanavalin A-induced 45Ca2+ uptake as well as blast transformation. The suppressive action of GD1a is both concentration dependent (50% suppression at 13 microM) and very rapid (within 1 min). GD1a is not cytotoxic nor does it significantly alter the rate of Ca2+ efflux. The uptake studies were extended to A23187, a compound with mitogenic and specific divalent cation ionophore activities. Ca2+ uptake by lymphoid cells from AKR/J, Swiss, and CBA mice is stimulated by A23187; and GD1a, in a dose-dependent manner, inhibits the ionophore-induced 45Ca2+ influx. Pretreatment of thymocytes with GD1a renders the cells greatly insensitive to the subsequent ionophore activity of A23187. The results suggest that exogenous gangliosides may function as an inhibitor of some of the mitogen-triggered early events, including Ca2+ metabolism, and thus influence the immunological behavior of intact lymphoid cells.     

Identification of N5,N10-methylene tetrahydrofolate reductase as the enzyme involved in the 5-methyl tetrahydrofolate-dependent formation of a beta-carboline derivative of 5-hydroxytryptamine in human platelets.

An enzyme in human platelets or rat brain incubated with 5-methyl tetrahydrofolate (5MeH₄folate) yields formaldehyde (4, 13), which will combine with biogenic amines to form β-carbolines (5) or tetrahydroisoquinolines. This activity was purified 500-fold from human platelets which are the main storage site for 5-hydroxytryptamine in man. This enzyme was identical to N⁵, N¹⁰-methylene tetrahydrofolate (N⁵,N¹⁰-methylene H₄folate) reductase by the following criteria: (i) co-purification, (ii) heat denaturation, (iii) pH response, (iv) molecular weight, (5) cofactor requirements. A mechanism involving the enzymatic generation of formaldehyde followed by adduct formation with a biogenic amine is proposed.     

Selective,reversible inhibition of the lactose phosphotransferase system of Staphylococcus aureus by dodecyl sulfate and deoxycholate

Low concentrations of sodium dodecyl sulfate (0.015%) and sodium deoxycholate (0.33%) completely inhibit phosphorylation of β-galactosides by the lactose phosphotransferase system of Staphylococcus aureus. Inhibition is reversible, even after prolonged detergent treatment. Phosphorylation of methyl-α-glucoside by the same preparations is only slightly inhibited by 0.015% dodecyl sulfate. The membrane-bound component, Enzyme IF^lac, is not solubilized by 0.015% dodecyl sulfate, nor is its ability to bind [¹⁴C]lactose affected. The results are consistent with hypotheses of selective binding of anionic detergent to Enzyme II^lac or to Factor III^lac, the detergent serving in the latter case as a membrane analog.     

Control of glycogen synthase activity in developing rat liver.

Fasting newborn and growing young rats, though capable of synthesizing liver glycogen when fed, are, unlike adult fasted animals, insensitive to glucocorticoid stimulation of the rate of glucose and lactate incorporation into glycogen. Hormone resistance parallels a decreased liver capability for the synthase b to a conversion reaction up to 2 days after birth, after which the b to a transformation becomes adult type in nature. A comparison of the level of glucose 6-phosphate in liver to the effect of the activator on the synthase activity from newborn rat shows that the enzyme has a greater affinity toward the activator than comparable enzyme from the adult, suggesting the presence of an intermediate metabolite-regulated form of synthase in neonatal liver.     

Biological activity of 1 alpha-hydroxyvitamin D2 in the rat.

The biological activity of 1α-hydroxyvitamin D₂ has been determined in vitamin D-deficient rats. In the calcification of the rachitic epiphyseal plate, 1α-hydroxyvitamin D₂ is more active than 25-hydroxyvitamin D₃, while it is equally active in stimulating intestinal calcium absorption. On the other hand, it is much less active (one-third to one-fifth) than 25-hydroxyvitamin D₃ in the mobilization of calcium from bone. In both the intestinal and bone responses, 1α-hydroxyvitamin D₂ (312 pmol) is active in nephrectomized rats while 25-hydroxyvitamin D₃ is not.     

In vitro synthesis of simian virus 40 DNA. I. Synthesis by a soluble extract from infected CV1 cells.

Simian Virus 40 (SV40) DNA replication was studied in vitro using cell free extracts prepared from SV40 infected CV1 cells. The cells were fractionated into a soluble cytoplasmic fraction and nuclei. The nuclei were lysed with high salt and used to prepare a soluble nuclear fraction. Both fractions displayed DNA polymerase activity as measured with activated calf thymus DNA. However, only the cytoplasmic fraction was active when SV40 DNA comonent I molecules were used as template. Under these conditions, the cytoplasmic extract was shown to catalyse the SV40 DNA dependent, in vitro incorporation of the four deoxyribonucleotides into DNA molecules which had, at both neutral and alkaline pH, the same sedimentation behavior as authentic SV40 DNA component I and component II molecules. Optimal Mg++ concentration was 5-8 mM. Incorporation of label into DNA component I molecules showed an initial lag of about 15 min., after which it was linear with time for up to 5 hrs at 32 degrees. Incorporation into DNA component II molecules proceeded without obvious lag and reached a plateau after approximately 2 hrs of incubation. It is concluded that the cytoplasmic extract supports the in vitro synthesis of SV40 DNA and that DNA component II molecules appear to be a precursor to DNA component I molecules in the reaction. Labeling of viral DNA molecules was highly dependent on ATP and on an ATP generating system. In the absence of ATP and of the energy generating system, incorporation occurred but both template and newly synthesized DNA molecules were extensively degraded.     

Optimization of kinetic proofreading: A general method for derivation of the constraint relations and an exploration of a specific case

We have previously presented a general cost-accuracy relationship for a broad class of kinetic proofreading mechanisms. In this paper we present a general matrix method, based upon classical enzyme kinetics, for the derivation of the constraint relation that characterizes specific proofreading mechanisms. For purpose of illustration we present the method in the context of a conventional Michaelis-Menten mechanism with side reactions. We then explore optimization of the general cost function under a variety of different constraints that may exist for such a mechanism. In this way we are able to contrast different perspectives on the optimization of enzyme design.