Ca2+-dependent modulator proteins from Tetrahymena pyriformis, sea anemone, and scallop and guanylate cyclase activation

Emerging therapeutic targets for treatment of leishmaniasis

INTRODUCTION

Parasitic diseases that pose a threat to human life include leishmaniasis - caused by protozoan parasite Leishmania species. Existing drugs have limitations due to deleterious side effects like teratogenicity, high cost and drug resistance. This calls for the need to have an insight into therapeutic aspects of disease. Areas covered: We have identified different drug targets via. molecular, imuunological, metabolic as well as by system biology approaches. We bring these promising drug targets into light so that they can be explored to their maximum. In an effort to bridge the gaps between existing knowledge and prospects of drug discovery, we have compiled interesting studies on drug targets, thereby paving the way for establishment of better therapeutic aspects. Expert opinion: Advancements in technology shed light on many unexplored pathways. Further probing of well established pathways led to the discovery of new drug targets. This review is a comprehensive report on current and emerging drug targets, with emphasis on several metabolic targets, organellar biochemistry, salvage pathways, epigenetics, kinome and more. Identification of new targets can contribute significantly towards strengthening the pipeline for disease elimination.

Calcium-dependent activation of guanylate cyclase by S100b

Calcium concentration in the dark-adapted retinal rod outer segment is in the 200 to 600 nM range, and the guanylate cyclase of rod outer segments is thought to be activated in response to a fall in calcium concentration triggered by light. Calcium-binding proteins that mediate such activation, i.e., activation in the absence of or presence of low nanomolar calcium concentrations, have been identified and termed GCAPs (Guanaylate Cyclase Activating Proteins). In the course of our search for GCAP-like proteins in bovine retina, we isolated a protein fraction that stimulated rod outer segment cyclase activity at calcium concentrations higher than those in dark-adapted rod outer segments. We purified the protein responsible for this calcium-dependent stimulation of cyclase activity and found it to be of 6-7 kDa molecular weight as judged by electrophoresis under denaturing conditions and about 40 kDa by gel filtration analysis. Maximum stimulation of cyclase activity was observed at 3-4 micromolar concentration of the protein. It required about 1.5 micromolar free calcium concentration for half-maximal activation of the enzyme. Partial amino acid sequencing of peptide fragments of the activator suggested that the protein was identical with S100b, a previously described calcium-binding protein. Further characterization with antibody specific for S100b supported this possibility. However, the protein isolated in our laboratory and termed CD-GCAP (Calcium-Dependent Guanylate Cyclase Activator Protein) was found to differ significantly from commercially available S100b in the magnitude and calcium dependence of cyclase activation. It was also found to be inactivated by hydroxylamine while S100b was resistant. Investigation into these differences showed that purification methods had a significant influence on the properties of the activator, producing a less active (S100b) or more active (CD-GCAP) protein, but that it was, otherwise, one and the same protein. We conclude from this study that rod outer segment guanylate cyclase, unlike any cyclase known so far, is capable of activation by two different types of calcium-binding proteins, one that activates in response to a decrease in calcium concentration, and the other, described here, which activates in response to an increase in calcium-concentration. We hypothesize that this cyclase and others like it will be colocalized with one or the other type of activator depending upon the physiological requirement, i.e., activation in response to decreasing or increasing calcium concentration.

Structural and functional characterization of retinal calcium-dependent guanylate cyclase activator protein (CD-GCAP): identity with S100beta protein

Calcium-dependent guanylate cyclase activator protein (CD-GCAP) is a low-molecular-weight retinal calcium-binding protein which activates rod outer segment guanylate cyclase (ROS-GC) in a calcium-dependent manner. This investigation was undertaken to determine the protein's structure and identity. Partial amino acid sequencing (72% of the protein), mass spectral analysis, cloning, and immunological studies revealed that CD-GCAP is identical to S100beta, another low-molecular-weight calcium-binding protein whose structure was known. We had shown earlier that the latter protein, which is usually called S100b (S100betabeta or dimer of S100beta), also activates ROS-GC but that the Vmax of activated cyclase was about 50% lower than when stimulated by CD-GCAP. S100b also required about 15 times more calcium (3.2 x 10(-)5 vs 1.5 x 10(-)6 M) for half-maximal stimulation of cyclase. To investigate the possibility that CD-GCAP is a post-translationally modified form of S100b, both proteins were treated with 1 M hydroxylamine which is known to deacylate proteins. After the treatment, CD-GCAP did not activate cyclase while S100b activation remained unaffected suggesting that CD-GCAP could not be a modified form of S100b. Hydroxylamine also broke down CD-GCAP into smaller fragments while leaving S100b intact. It therefore appeared that in spite of identical primary structures, the conformations of the two proteins were different. We then investigated the possibility that the purification procedures of the two proteins, which were quite different, could have contributed to such conformational differences: CD-GCAP purification included a step of heating at 75 degrees C in 5 mM Ca, while S100b purification included zinc affinity chromatography. To test the influence of these treatments on the properties of the proteins, CD-GCAP was subjected to zinc affinity chromatography and purified as S100b (CD-GCAP-->S100b) and S100b was heated in Ca and purified as CD-GCAP (S100b-->CD-GCAP). Cyclase activation, calcium-sensitivity, and hydroxylamine-lability measurements revealed that CD-GCAP-->S100b is identical to S100b and that S100b-->CD-GCAP is identical to CD-GCAP. Taken together the results demonstrate that CD-GCAP and S100b are one and the same protein and that their functional differences are due to different interconvertible conformational states.

A calcium-dependent protein kinase is present in tetrahymena

A Ca(2+)-dependent protein kinase of Tetrahymena thermophila has been partially purified and characterized. The molecular mass of the enzyme is less than that of similar enzymes (for example protein kinase C), being about 55 kDa. After purification and in the presence of Ca2+ the enzyme activity increased. The promoter of protein kinase C (PKC) activity, phorbol myristate acetate (PMA), increased the activity while the protein kinase inhibitor H-7 decreased the activity of the enzyme. The experiments demonstrate the presence, activity and similarity to vertebrate enzymes of a protein kinase at a low level of phylogeny.

Study of the glycogenolytic action of platelet activating factor in Tetrahymena pyriformis

1. A novel action of AGEPC on non-inflammatory cells was revealed, namely the ability to stimulate glycogenolysis in Tetrahymena pyriformis cells. 2. The glycogenolytic effect of AGEPC seems to be dependent on Ca2+ transport and regulation, thus the effects are completely inhibited by Verapamil and partially by EGTA. 3. The influence of Propranolol, Labetalol, Atenolol and Theophylline in the glycogenolytic effect of AGEPC are also studied. 4. Our findings suggest that the AGEPC promoted glycogenolysis in Tetrahymena through a mechanism distinct from that of catecholamines.

Increased calmodulin levels in fibroblasts from progressive systemic sclerosis

Calmodulin levels in cultured skin fibroblasts from patients with progressive systemic sclerosis (PSS) and healthy controls were measured by their ability to activate cyclic AMP-phosphodiesterase. Calmodulin levels were significantly increased in PSS fibroblasts compared with normal control fibroblasts. The changes in calmodulin content of PSS fibroblasts were also assessed by a radioimmunoassay. These findings suggest that an elevated level of calmodulin may play a role in the pathogenesis of PSS.

Identification of testis specific calcineurin beta subunit isoform by a monoclonal antibody and detection of a specific six amino acid sequence

Two isoforms of calcineurin beta subunit(beta 1 and beta 2) were identified in rat testis by a monoclonal antibody Va1. Both beta 1 and beta 2 were recovered in calmodulin binding protein fraction and showed calcium shift on SDS-polyacrylamide gel electrophoresis which is the specific character for EF-hand calcium binding protein. beta 2 showed same apparent molecular weight on SDS-PAGE as that of brain calcineurin beta and was found in wide variety of tissues. beta 1 was shown to have six amino acid polypepeptide sequence and it showed higher molecular weight than brain beta and was specific for testis.

Binding shift assay of parvalbumin, calmodulin and carbonic anhydrase by high-performance capillary electrophoresis

Shifts in mobility caused by binding of Ca2+ to calmodulin and parvalbumin were studied using high-performance capillary electrophoresis in a Tris-glycine buffer, rather than conventional polyacrylamide gel electrophoresis which requires larger amounts of sample and longer assay time. A Zn(2+)-binding protein, carbonic anhydrase, also showed a partial shift in mobility following Zn(2+)-binding.

Two affinity states for [3H]imipramine binding to the human platelet 5-hydroxytryptamine carrier: an explanation for the allosteric interaction between 5-hydroxytryptamine and imipramine

5-Hydroxytryptamine (5-HT) showed a biphasic effect on the dissociation rate of [3H]imipramine from human platelet membranes: At low concentrations (EC50, approximately 2.5 microM), 5-HT stimulated the rate, as expected for mutually exclusive binding of 5-HT and imipramine; at higher concentrations (EC50, approximately 40 microM), 5-HT reduced this stimulated rate, a result consistent with 5-HT binding at a site that is physically distinct from both the imipramine binding site and the 5-HT transport recognition site of the 5-HT carrier. This modulatory effect could be mimicked by tryptamine, was saturable and independent of Na+ concentration, and could also be demonstrated for detergent-solubilized carriers. Monophasic association kinetics for [3H]imipramine binding were found. Heat stability experiments showed biphasic thermal inactivation curves. These results are consistent with [3H]imipramine binding to two classes of binding sites at the 5-HT carrier on human platelet membranes, with affinities three- to fivefold different. 5-HT can convert the lower-affinity state into the higher-affinity state.

Cyclical differentiation of Trypanosoma brucei involves changes in the cellular complement of calmodulin-binding proteins

The present study was undertaken to evaluate changes in the complement of calmodulin-binding proteins which accompany cyclical differentiation in Trypanosoma brucei. An [125I]trypanosome calmodulin overlay procedure was used to detect calmodulin-binding proteins with Mr of 126,000 and 106,000 that were present in homogenates of slender bloodstream froms but were absent in procyclic culture forms. Competition assays with unlabeled bovine brain or trypanosome calmodulins indicated that the developmentally regulated proteins associated with calmodulins from either source. Moreover, [125I]bovine brain calmodulin associated with the same proteins as trypanosome calmodulin. Homogenates of T. evansi exhibited the same pattern of calmodulin-binding activity as T. brucei slender bloodstream forms; however, T. cruzi and Leishmania tarentolae contained distinct patterns of calmodulin-binding activity. Mouse serum contained no detectable binding proteins while mouse brain contained predominantly proteins of Mr 210,000, 60,000, and 49,000 which were associated with the trypanosome calmodulin probe. The developmentally regulated calmodulin-binding proteins from T. brucei were in the 10,000g pellet. We conclude that the cellular complement of calmodulin-binding proteins varies during the trypanosome life cycle.

Antiproliferative action of benzodiazepines in cultured brain cells is not mediated through the peripheral-type benzodiazepine acceptor

The benzodiazepines, Ro 5-4864, diazepam, clonazepam, and also PK-11195, inhibited, at micromolar concentrations, the proliferation of rat C6 glioma and mouse neuro-2A neuroblastoma cells in culture. The cells possessed high levels of "peripheral-type" high-affinity benzodiazepine binding sites as judged by binding assays and displacement potencies. However, the different potencies and specificities of compounds for the antiproliferative actions and binding affinities for the binding site suggest that the antiproliferative actions were not mediated through the peripheral-type binding site. In support of this, these compounds have also been shown to inhibit proliferation of some nonneuronal cultured cell lines, e.g., mouse SP2/O-Ag 14 hybridoma and rat NCTC epithelial cells, which have no detectable high-affinity peripheral-type benzodiazepine binding sites.

Spectral studies of the Ca2+-dependent interaction of trifluoperazine with S100b

S100b is a calcium-binding protein that will bind to many calmodulin target molecules in a Ca2+-dependent manner. In order to study the Ca2+-dependent binding properties of S100b, its interaction with a calmodulin antagonist, trifluoperazine (TFP), was investigated using [19F]- and [1H]-NMR and UV-difference spectroscopy. It was estimated from [19F]-NMR that in the absence of Ca2+, the k1/2 value of TFP was 130 microM, while its k1/2 value decreased to 28 microM in the presence of Ca2+. The addition of KCl was not antagonistic to the Ca2+-dependent interaction of TFP to S100b. The chemical exchange rate of TFP with Ca2+-bound S100b was estimated to be 9 x 10(2) sec-1. By comparison with TFP-calmodulin exchange rates, it is suggested that the TFP-binding site on S100b is structurally different from its binding sites on calmodulin. Proton NMR resonance broadening in the range 6.8-7.2 ppm, corresponding to phenylalanine nuclei of S100b, indicates that these residues may be involved in TFP binding. Addition of Ca2+ to a 1:1 mixture of S100b and TFP resulted in a red-shifted UV-difference spectrum, while no significant difference spectrum was detected when Mg2+ was added to a S100b-TFP solution. Thus, we suggest that Ca2+ induces the exposure of a hydrophobic domain on S100b containing one or more phenylalanine residues that will bind TFP but that this domain is different from the hydrophobic domain on calmodulin.

Calmodulin-stimulated particulate guanylate cyclase in crayfish hepatopancreas

1. A particulate guanylate cyclase from crayfish hepatopancreas membranes was investigated with respect to its dependence on Ca2+ and calmodulin. Addition of Ca2+ to EGTA-treated membranes increased cyclase activity by 100%. 2. Calmodulin stimulated the activity about 5-fold. 3. This effect could be abolished by the calmodulin antagonist compound 48/80. 4. These results present evidence that the particulate guanylate cyclase of crayfish hepatopancreas is a Ca2+/calmodulin-dependent enzyme. 5. The implications of this observation upon glycogen metabolism of crustaceans are discussed.

Purification and properties of the age-related protein (ARPB) detected in human brain: comparison with human brain calmodulin

ARPB is an age-related protein in the human brain which is present in individuals younger than approximately 40 years of age, but disappears or becomes remarkably decreased in individuals above this age (Yasuda, T. and Kishi, K. (1985) Proc. Japan Acad. 61B, 273-276). ARPB was isolated from four different cerebra obtained from subjects 0, 15, 35 and 37 years old, and purified to an electrophoretically homogeneous state. Its molecular weight was estimated to be 17,000 and 36,000-38,000 by SDS-polyacrylamide gel electrophoresis and gel filtration, respectively. The amino acid composition of the purified ARPB, containing an excess of acidic amino acid residues (about 33%), proved to be very similar to that of calmodulin. ARPB exhibited some of the properties characteristic of calmodulin, such as a Ca2+-dependent mobility change upon electrophoresis, and activation of calmodulin-deficient phosphodiesterase activity in a Ca2+-dependent manner. However, it was possible to distinguish ARPB and human calmodulin with regard to their kinetic parameters for the activation reaction of phosphodiesterase activity, despite a close similarity in their reaction mode. It can be concluded that ARPB belongs to one of the calmodulin group proteins, although it remains unknown whether ARPB is identical to calmodulin.

Effect of tryptic calmodulin fragments on guanylate cyclase activity from Paramecium tetraurelia

Tryptic bovine brain calmodulin fragments 1-77 or 1-106 reactivated La-inactivated ciliary guanylate cyclase from Paramecium dose-dependently up to 60%. They were 20-fold less potent compared to bovine brain calmodulin. Fragment 78-148 was even less active. Concomitant addition of fragments 1-77 and 78-148 had no additive effect. Genetically engineered calmodulin lacking a blocked amino terminus and trimethyllysine at position 115 reactivated La-treated guanylate cyclase as good as bovine brain calmodulin. After detergent solubilization of La-inactivated guanylate cyclase intact bovine brain calmodulin and calmodulin fragments 1-77 and 78-148 were equipotent. 80% Reactivation was obtained with 40 microM of either fragment.

Properties of digitonin-solubilized calmodulin-dependent guanylate cyclase from the plasma membranes of Tetrahymena pyriformis NT-1 cells

Calmodulin-dependent guanylate cyclase from Tetrahymena plasma membranes was solubilized in about a 22% yield by using digitonin in the presence of 0.2 mM CaCl2 and 20% glycerol. The detergent, when present in the assay at concentrations above 0.05%, diminished the basal and calmodulin-stimulated activity of the enzyme. Guanylate cyclase solubilized with digitonin was eluted from DEAE-cellulose with 200 mM KCl in a yield of 50%. Properties of the solubilized enzyme were similar to those of the native membrane-bound enzyme. The Kms for Mg-GTP and Mn-GTP were 140 and 30 microM, respectively. The enzyme required Mn2+ for maximum activity, the relative activity in the presence of Mg2+ being 30% of the activity with Mn2+. The solubilized enzyme retained the ability to be activated by calmodulin, with its extent being reduced as compared to the membrane-bound enzyme. The presence of a Ca2+-dependent calmodulin-binding site on the solubilized enzyme was shown by the Ca2+-dependent retention of the enzyme on a calmodulin-Sepharose-4B column.

Immunochemical and immunohistochemical localization of parvalbumin in rat nervous tissues

The contents of parvalbumin in various nervous tissues of the rat were measured by radioimmunoassay (RIA) and its cellular distribution was immunohistochemically examined by peroxidase-antiperoxidase methods. The antibody, raised in rabbits using rat skeletal muscle parvalbumin, did not cross-react with other Ca2+-binding proteins such as calmodulin or S-100 proteins. The RIA demonstrated the wide distribution of the antigen, with very high levels in the cerebellum (3,217 +/- 519 ng/mg protein). The immunohistochemical description by Celio and Heizmann [Nature 293, 300-302 (1981)] was confirmed concerning the existence of the antigen in Purkinje cells of the cerebellum; nonpyramidal neurons of the cerebral cortex; and medium-sized cells of the olfactory bulb, hippocampus, and reticular nucleus of the thalamus. In addition to these neurons, we found the parvalbumin-like immunoreactivity in the large neurons of the superior vestibular nucleus and the neurons of the medial superior olive nucleus. In the gamma-aminobutyric acid (GABA)-containing nuclei such as substantia nigra, caudatoputamen, and globus pallidus, parvalbumin-positive cells and fibers were rare. In the medial lemniscus of the midbrain which contains no GABA, parvalbumin-immunoreactive fibers were prominent. The possibility was discussed that parvalbumin exists in a specific population of neurons that differ from those containing GABA.

Comparative structural analysis of calmodulins from Trypanosoma brucei, T. congolense, T. vivax, Tetrahymena thermophila and bovine brain

Calmodulin is an intracellular calcium receptor protein utilized extensively by eukaryotic cells to mediate responsiveness to calcium signals. The present study evaluates the effects on protein structure of amino acid substitutions in trypanosome calmodulin. Calmodulin conformation, hydrophobicity and antigenic determinants are compared among Trypanosoma brucei, Trypanosoma congolense, Trypanosoma vivax, Tetrahymena thermophila and bovine brain. Trypanosome calmodulin differs from brain and Tetrahymena calmodulins based upon isoelectric point, retention time on a C-2/C-18 reverse phase column and interaction with polyclonal antibodies against trypanosome calmodulin by radioimmunoassay or Western procedures. These same analyses do not distinguish trypanosome calmodulins from each other. Polyclonal antibodies against Tetrahymena calmodulin are equally specific and do not recognize the trypanosome or brain calmodulins. Calcium-induced exposure of hydrophobic binding sites are quantitated using the fluorescent probe, N-phenyl-1-naphthylamine. All calmodulins, regardless of source, enhance the fluorescence of N-phenyl-1-naphthylamine 3-4 fold in the presence of calcium. These data demonstrate the extent to which functional calmodulins vary in their structures. We conclude that African trypanosomes share a common calmodulin that is structurally distinct from calmodulin of vertebrates or Tetrahymena.

Calmodulin and Ca2+-dependent cyclic AMP phosphodiesterase activity in Trypanosoma cruzi

Calmodulin has been purified from Trypanosoma cruzi epimastigote forms by ion-exchange chromatography, gel filtration and affinity chromatography on 2-chloro-10-(3-aminopropyl)phenotiazine-Sepharose. Upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the factor showed a polypeptide band with an apparent molecular weight of 16 000. In addition, cyclic AMP phosphodiesterase activity from T. cruzi epimastigote forms was purified by ion-exchange chromatography and affinity chromatography on a brain calmodulin-Sepharose column. The enzyme was activated by homologous calmodulin as well as by bovine brain and Neurospora crassa calmodulins. The activation required micromolar concentrations of Ca2+ and it was blocked by EGTA and by some neuroleptic drugs such as chlorpromazine, fluphenazine and compound 48/80. Activations were observed at micromolar concentrations of cyclic AMP as substrate. In addition, T. cruzi calmodulin was also active in bringing about the stimulation of brain phosphodiesterase.

Growth-dependent changes of guanylate and adenylate cyclase activities in cilia and cell bodies of Tetrahymena pyriformis

The calmodulin-dependent guanylate cyclase of Tetrahymena pyriformis was shown previously to be localized in surface membranes (ciliary and pellicular membranes) (Kudo, S, Nakazawa, K, Nagao, S & Nozawa, Y, Japan j exp med 52 (1952) 193) [21], whereas in a recent report Schultz et al, (Schultz, J E, Schonefeld, U & Klumpp, S, Eur j biochem 137 (1983) 89) [12] demonstrated the localization of this enzyme in ciliary membrane, arguing against its presence in pellicular membrane. To examine the discrepancy, the activities of guanylate and adenylate cyclases were examined in cilia and cell bodies of Tetrahymena pyriformis during transition from early log to stationary growth phase. The guanylate cyclase activity in the cell bodies increased significantly with growth of age, while in cilia the activity was rather consistent. In contrast, adenylate cyclase did not show any growth-dependent activity changes in both cilia and cell bodies. The increase of guanylate cyclase activity was not related to the increase of its activator calmodulin, because the change in enzyme activity could not be negated by addition of a saturating amount of calmodulin. These results suggest that the content of guanylate cyclase itself would be increased in the cell bodies during growth.

Interaction of calcium-binding proteins with calmodulin-dependent guanylate cyclase in Tetrahymena plasma membrane

Addition of bovine brain calmodulin and S-100 inhibited Tetrahymena calmodulin-induced stimulation of guanylate cyclase, but they did not affect enzymatic activity in the presence of calcium alone. Troponin C shows little effect on the cyclase activity regardless of the presence or absence of Tetrahymena calmodulin. The inhibitory effects of brain calmodulin and S-100 were overcome by the addition of Tetrahymena calmodulin, but not by calcium. Both calmodulins from Tetrahymena and bovine brain elicited stimulation of heart phosphodiesterase, while troponin C and S-100 did not affect the phosphodiesterase activity in the presence and absence of Tetrahymena calmodulin.

Regulation by calcium of hormone-insensitive adenylate cyclase and calmodulin-dependent guanylate cyclase in Tetrahymena plasma membrane

Agents such as 5'-guanylyl-imidodiphosphate(GppNHp), fluoride and forskolin did not activate adenylate cyclase from Tetrahymena. In addition, the cyclase was not stimulated by hormones including catecholamines and glucagon when assayed with or without GppNHp at conditions where they increased adenylate cyclase activity from rat heart. Sodium azide, NaNO2 or N-methyl-N'-nitro-N-nitroguanidine (MNNG) failed to activate Tetrahymena guanylate cyclase. Adenylate cyclase activity was activated at low free Ca2+ level and inhibited at high levels, while guanylate cyclase activity was activated by Tetrahymena calmodulin only at high physiological concn of Ca2+.

Calmodulin-binding proteins of Tetrahymena microsomal membranes

Tetrahymena calmodulin radioiodinated with a lactoperoxidase method retained full ability to activate Tetrahymena guanylate cyclase. Binding of [125I]calmodulin to Tetrahymena microsomal membranes was Ca2+-dependent and inhibited by excess unlabeled calmodulin or trifluoperazine. When Triton X-100-solubilized microsomes were chromatographed on calmodulin Sepharose, several proteins were found to interact with calmodulin in a Ca2+-dependent manner.

The effect of trifluoperazine (Stellazine) on Hymenolepis diminuta in vitro

The influence of the phenothiazine trifluoperazine (Stellazine) on the rat tapeworm Hymenolepis diminuta was examined. The parasite was incubated in glucose-containing Krebs-Ringer media (pH 7.4) at 37 degrees C which included Ca2+ or EGTA and a range of trifluoperazine concentrations (0-2 mM). Release of soluble protein and lactate dehydrogenase activity were taken as measures of release of cytosolic components. The release of lactate dehydrogenase depended on drug concentration, maximum levels occurring at 2 mM trifluoperazine, this corresponded to 2% of the total lactate dehydrogenase present in the cestode. The effect of phenothiazines of differing lipophilicity were compared, and for trifluoperazine sulfoxide only minimal amounts of lactate dehydrogenase activity and protein were released. These values were similar to those obtained when H. diminuta was incubated in drug-free media. Our findings suggest that the integrity of the parasite is related to its calmodulin content. The potential cestocidal properties of trifluoperazine are considered.

Evidence for a Ca2+-dependent activator protein in the rat tapeworm Hymenolepis diminuta

A low molecular weight, acidic, heat stable protein has been characterised from the rat tapeworm Hymenolepis diminuta. This protein was found to activate cyclic 3', 5'-nucleotide phosphodiesterase in a Ca2+-dependent manner. The activation process was inhibited by the phenothiazine drug trifluoperazine. The biochemical properties of this protein clearly resemble those of ovine brain calmodulin. Our investigation thus concludes that there is a calmodulin-like activator protein in this cestode.

An enzymatic assay for calmodulins based on plant NAD kinase activity

NAD kinase with increased sensitivity to calmodulin was purified from pea seedlings (Pisum sativum L., Willet Wonder). Assays for calmodulin based on the activities of NAD kinase, bovine brain cyclic nucleotide phosphodiesterase, and human erythrocyte Ca2+-ATPase were compared for their sensitivities to calmodulin and for their abilities to discriminate between calmodulins from different sources. The activities of the three enzymes were determined in the presence of various concentrations of calmodulins from human erythrocyte, bovine brain, sea pansy (Renilla reniformis), mung bean seed (Vigna radiata L. Wilczek), mushroom (Agaricus bisporus), and Tetrahymena pyriformis. The concentrations of calmodulin required for 50% activation of the NAD kinase (K0.5) ranged from 0.520 ng/ml for Tetrahymena to 2.20 ng/ml for bovine brain. The K0.5's ranged from 19.6 ng/ml for bovine brain calmodulin to 73.5 ng/ml for mushroom calmodulin for phosphodiesterase activation. The K0.5's for the activation of Ca2+-ATPase ranged from 36.3 ng/ml for erythrocyte calmodulin to 61.7 ng/ml for mushroom calmodulin. NAD kinase was not stimulated by phosphatidylcholine, phosphatidylserine, cardiolipin, or palmitoleic acid in the absence or presence of Ca2+. Palmitic acid had a slightly stimulatory effect in the presence of Ca2+ (10% of maximum), but no effect in the absence of Ca2+. Palmitoleic acid inhibited the calmodulin-stimulated activity by 50%. Both the NAD kinase assay and radioimmunoassay were able to detect calmodulin in extracts containing low concentrations of calmodulin. Estimates of calmodulin contents of crude homogenates determined by the NAD kinase assay were consistent with amounts obtained by various purification procedures.

Binding of [3H]imipramine to human platelet membranes with compensation for saturable binding to filters and its implication for binding studies with brain membranes

Apparent specific binding of [3H]imipramine to human platelet membranes at high concentrations of imipramine showed deviation from that expected of a single binding site, a result consistent with a low-affinity binding site. The deviation was due to displaceable, saturable binding to the glass fibre filters used in the assays. Imipramine, chloripramine, desipramine, and fluoxetine inhibited binding to filters whereas 5-hydroxytryptamine and ethanol were ineffective. Experimental conditions were developed that eliminated filter binding, allowing assay of high- and low-affinity binding to membranes. Failure to correct for filter binding may lead to overestimation of binding parameters, Bmax and KD for high-affinity binding to membranes, and may also be misinterpreted as indicating a low-affinity binding component in both platelet and brain membranes. Low-affinity binding (KD less than 2 microM) of imipramine to human platelet membranes was demonstrated and its significance discussed.