Activation of adenylate cyclase in hepatic membranes involves interactions of the catalytic unit with multimeric complexes of regulatory proteins

Regulation of neuronal adenylate cyclase

It appears that several components function in a spirit of integrated cooperation toward the intracellular regulation of neurotransmitter responsiveness. We have demonstrated that cytoskeletal proteins might interact with GNs and that GNs and GNi might interact with one another. At this juncture, it appears that both of these phenomena might occur only in cells of neural origin. Calmodulin and antidepressants may also affect adenylate cyclase in nervous tissue alone. The effects of AAGTP are different in nervous tissue from other tissues, and experiments with that nucleotide have led to the discovery of a new, 32 kDa GTP-binding protein which appears only in neural crest cells. Appreciation of the intricacies of signal transduction through the adenylate cyclase system are developing along with our understanding of that system. When combined with the complexity of neurotransmitter responsiveness, comprehension of the combined systems remains in its infancy, destined to grow as well as to surprise and delight all who are interested.

Characteristics of ethacrynic acid highly sensitive Mg2+-ATPase in microsomal fractions of the rat brain: functional molecular size, inhibition by SITS and stimulation by Cl-

Studies were performed to characterize ethacrynic acid (EA) highly sensitive Mg2+-ATPase isolated from microsomal fractions of the rat brain. The functional molecular sizes of the EA highly sensitive and EA less sensitive Mg2+-ATPases, estimated by a radiation inactivation method, were 480 and 80 kDa, respectively. An anion transport inhibitor, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS) inhibited the EA highly sensitive Mg2+-ATPase activity. The type of inhibition was uncompetitive with respect to ATP, and the inhibition was suppressed by anions such as Cl-, Br- and I-. Chloride ions stimulated enzyme activity with an increase in Vmax, but not in Km, for ATP. Anions tested also increased the enzyme activity in the following order of decreasing potency: Cl- greater than Br- greater than CH3COO- = I- greater than SO4(2-) = HCO3- greater than SO3(2-). These results suggest that EA highly sensitive Mg2+-ATPase is a relatively large molecule with anion-sensitive sites that affect the ATP hydrolyzing activity and the SITS binding capacity through anions, with Cl- being the most potent.

Radiation inactivation of multimeric enzymes: application to subunit interactions of adenylate cyclase

Radiation inactivation has been applied extensively to determine the molecular weight of soluble enzyme and receptor systems from the slope of a linear ln (activity) vs. dose curve. Complex nonlinear inactivation curves are predicted for multimeric enzyme systems, composed of distinct subunits in equilibrium with multimeric complexes. For the system A1 + A2----A1A2, with an active A1A2 complex (associative model), the ln (activity) vs. dose curve is linear for high dissociation constant, K. If a monomer, A1, has all the enzyme activity (dissociative model), the ln (activity) vs. dose curve has an activation hump at low radiation dose if the inactive subunit, A2, has a higher molecular weight than A1 and has upward concavity when A2 is smaller than A1. In general, a radiation inactivation model for a multistep mechanism for enzyme activation fulfills the characteristics of an associative or dissociative model if the reaction step forming active enzyme is an associative or dissociative reaction. Target theory gives the molecular weight of the active enzyme subunit or complex from the limiting slope of the ln (activity) vs. dose curve at high radiation dose. If energy transfer occurs among subunits in the multimer, the ln (activity) vs. dose curve is linear for a single active component and is concave upward for two or more active components. The use of radiation inactivation as a method to determine enzyme size and multimeric subunit assembly is discussed with specific application to the hormone-sensitive adenylate cyclase system. It is shown that the complex inactivation curves presented in the accompanying paper can be used select the best mechanism out of a series of seven proposed mechanisms for the activation of adenylate cyclase by hormone.

Molecular size of histamine H-1 receptor determined by target size analysis

Target size analysis (radiation inactivation) was used to study the molecular size of the histamine H-1 receptor of bovine and human cerebral cortex in the intact membrane-bound state. The H-1 receptor in bovine and human cerebral cortex was found to exist in the membrane as a homogeneous population of the same size (160,000 daltons) in each case. Thus no evidence for the existence of multiple forms of the receptor has been found.

Glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides is a reliable internal standard for radiation-inactivation studies of membranes in the frozen state

The target size of four soluble enzymes (beta-galactosidase, pyruvate kinase, alcohol dehydrogenase, and glucose-6-phosphate dehydrogenase) in the presence or absence of subcellular membrane fractions has been determined by the radiation-inactivation method using samples in the frozen state. For each of the four enzymes, full activity was recovered after freezing and thawing in the absence of radiation. We found minimal (less than 20%) binding of the enzymes to either submitochondrial vesicles or sarcoplasmic reticulum vesicles. Under the conditions tested, beta-galactosidase, pyruvate kinase, and alcohol dehydrogenase exhibited target sizes which varied according to the experimental conditions, i.e., the buffer selected and also the presence or absence of membrane preparations. For these tetrameric enzymes, the target sizes were generally comparable to either a monomer or a dimer. By contrast, the target size of glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides was found to be essentially invariant when frozen in a variety of buffers and in the presence or absence of either cryoprotectant (sucrose or glycerol) or different membrane preparations. The target size from 19 separate determinations gave an average value of 104 +/- 16 kDa, which is comparable to the molecular weight of the enzyme (104 kDa). We conclude that glucose-6-phosphate dehydrogenase from L. mesenteroides is a reliable internal standard for radiation-inactivation studies of membrane preparations in the frozen state.

Target sizes of human erythrocyte membrane Ca2+-ATPase and Mg2+-ATPase activities in the presence and absence of calmodulin

We have investigated the subunit structure of Ca2+-transport ATPase in human erythrocyte membranes using radiation inactivation analysis. All inactivation data were linear on a semilog plot down to at least 20% of the control activity. We found a target size for the calmodulin-dependent Ca2+-ATPase activity of 331 kDa, consistent with the presence of this enzyme as a dimer in calmodulin-depleted ghosts. Membranes which had been saturated with calmodulin before irradiation yield a a similar size of 317 kDa, implying that activation of Ca2+-transport ATPase by calmodulin does not involve significant change in oligomeric structure. Basal (calmodulin-independent) Ca2+-ATPase activity corresponded to a size of 290 kDa, suggesting that this activity resides in the same, or similar-sized, complex as the calmodulin-dependent activity. Mg2+-ATPase activity, however, was found to reside in a smaller complex of 224 kDa, which proved to be statistically distinct from the target size of Ca2+-ATPase activity. It would appear that Mg2+-ATPase is a distinct entity whose function is likely unrelated to the Ca2+-transport ATPase.

Aggregation state of the gonadotropin receptor

The aggregation state of the gonadotropin receptor has been examined by coupling fluorescence energy transfer donor and acceptor fluorophores to hCG and LH. Energy transfer is observed at low (4 degrees C) but not at high (37 degrees C) temperature. Energy transfer could also be detected with receptor solubilized in the presence of hormone at the lower temperature only. Solubilization of receptor in the absence of hormone and subsequent addition of hormone conjugates revealed no energy transfer. These results are consistent with stabilization of receptor complexes at low temperatures, but presumptive hormone induced receptor dissociation under physiological conditions.

The activation of adenylate cyclase from small intestinal epithelium by cholera toxin

ADP-ribosylation of membrane proteins from rabbit small intestinal epithelium was investigated following incubation of membranes with [32P]NAD and cholera toxin. Cholera toxin catalyzes incorporation of 32P into three proteins of 40 kDA, 45 kDa and 47 kDa located in the brush-border membrane. In contrast, basal lateral membranes do not contain any protein which becomes labeled in a toxin-dependent manner when incubated with cholera toxin and [32P]NAD. The modification of membrane proteins from brush border occurred in spite of the virtual absence in these membranes of adenylate cyclase activatable either by cholera toxin, vasoactive intestinal peptide (VIP) or fluoride. The three agents activated adenylate cyclase when crude plasma membrane were used. Cholera toxin activated fivefold at 10 micrograms/ml. Vasoactive intestinal peptide activated at concentrations from 10-300 nM, the maximal stimulation being sixfold. Fluoride activated 10-fold at 10 mM. When basal lateral membranes were assayed for adenylate cyclase it was found that, with respect to the crude membranes, the specific activity of fluoride-activated enzyme was 3.3-fold higher, VIP stimulated enzyme was maintained while cholera-toxin-stimulated enzyme showed half specific activity. Moreover, while fluoride stimulated ninefold and VIP stimulated fivefold, cholera toxin only stimulated twofold at the highest concentration. The results suggest that the activation by cholera toxin of adenylate cyclase located at the basal lateral membrane requires ADPribosylation of proteins in the brush border membrane.

Thrombin binds to a high-affinity approximately 900 000-dalton site on human platelets

The functional sizes of the binding sites for thrombin on human platelets and isolated membranes have been determined by the technique of radiation inactivation: similar results were obtained. Independent studies using different radiation doses (0, 3, and 48 Mrad) and different thrombin concentrations (10(-10), 10(-8), and 10(-6) M) confirmed the presence of three binding sites with functional sizes of 900 000, 30 000, and 4000 daltons. The binding site of lowest apparent size (4000 daltons) probably corresponds to what has been termed nonspecific binding since its dissociation constant (2900 nM) is well outside the physiological range. The site of intermediate size (30 000 daltons) is also probably not involved in platelet activation since its dissociation constant (11 nM) is also beyond the concentration range required for activation, although it may be involved in other aspects of platelet-thrombin interaction. The sites with the largest functional size are probably important in platelet function since their dissociation constant (0.3 nM) is in the range required for platelet activation. The functional size of these sites (900 000 daltons) suggests that the high-affinity site for thrombin binding to platelets may involve a multimolecular complex of membrane components.

Desensitization of beta-adrenergic receptor-coupled adenylate cyclase activity. Differences following exposure of cells to two full agonists

The effects on the beta-adrenergic receptors of intact L6 muscle cells of exposure to agonists were investigated. Treatment of cells with isoproterenol decreased GTP-, isoproterenol-, and zinterol-stimulated adenylate cyclase activities, whereas exposure of cells to zinterol decreased only isoproterenol- and zinterol-stimulated activities. Although the effects of these two full agonists on GTP-stimulated adenylate cyclase activity were different, the time courses for development and reversal of the effects were similar. The decrease in agonist-stimulated adenylate cyclase activity observed in membranes prepared from cells previously exposed to isoproterenol or zinterol was due to both an increase in the concentration of agonist required for half-maximal activation and a decrease in the maximum level of activation. No effects on fluoride- or manganese-stimulated activities were observed following exposure of cells to either agonist. Decreases in GTP-stimulated adenylate cyclase activity were also observed following exposure of cells to epinephrine, Cc-25, fenoterol, norepinephrine, terbutaline, or metaproterenol, but not following incubation of cells with dobutamine or salmefamol. The results suggest that full agonists interacting with the same receptor can induce different changes in the components of the adenylate cyclase system. Desensitization following exposure to zinterol may result from modification of the receptor, whereas that following exposure to isoproterenol may be due to alterations in both the guanine nucleotide-binding protein and the receptor.

Minimal functional unit for transport and enzyme activities of (Na+ + K+)-ATPase as determined by radiation inactivation

Frozen aqueous suspensions of partially purified membrane-bound renal (Na+ + K+)-ATPase have been irradiated at -135 degrees C with high-energy electrons. (Na+ + K+)-ATPase and K+-phosphatase activities are inactivated exponentially with apparent target sizes of 184 +/- 4 kDa and 125 +/- 3 kDa, respectively. These values are significantly lower then found previously from irradiation of lyophilized membranes. After reconstitution of irradiated (Na+ + K+)-ATPase into phospholipid vesicles the following transport functions have been measured and target sizes calculated from the exponential inactivation curves: ATP-dependent Na+-K+ exchange, 201 +/- 4 kDa; (ATP + Pi)-activated Rb+-Rb+ exchange, 206 +/- 7 kDa and ATP-independent Rb+-Rb+ exchange, 117 +/- 4 kDa. The apparent size of the alpha-chain, judged by disappearance of Coomassie stain on SDS-gels, lies between 115 and 141 kDa. That for the beta-glycoprotein, though clearly smaller, could not be estimated. We draw the following conclusions: (1) The simplest interpretation of the results is that the minimal functional unit for (Na+ + K+)-ATPase is alpha beta. (2) The inactivation target size for (Na+ + K+)-dependent ATP hydrolysis is the same as for ATP-dependent pumping of Na+ and K+. (3) The target sizes, for K+-phosphatase (125 kDa) and ATP-independent Rb+-Rb+ exchange (117 kDa) are indistinguishable from that of the alpha-chain itself, suggesting that cation binding sites and transport pathways, and the p-nitrophenyl phosphate binding site are located exclusively on the alpha-chain. (4) ATP-dependent activities appear to depend on the integrity of an alpha beta complex.

Nucleoside transport in human erythrocytes. Nitrobenzylthioinosine binding and uridine transport activities have similar radiation target sizes

Intact human erythrocytes were irradiated in the frozen state with a high-energy electron beam. Nitrobenzylthioinosine-sensitive uridine influx, equilibrium exchange uridine influx and high-affinity nitrobenzylthioinosine binding were inactivated as a simple exponential function of the radiation dose, indicating an in situ target size of 122 000. The results suggest that the nitrobenzylthioinosine-binding site(s) and the permeation site(s) of the transporter are present on the same transporter element.

Heterogeneity of glucagon receptors of rat hepatocytes: a synthetic peptide probe for the high affinity site

A glucagon analog with the following sequence has been synthesized: His- Ser-Gln-Gly-Thr-Phe-Thr-Ser-Asp-Tyr-Ser-Lys-Tyr-Leu-Asp-Ser-Arg-Arg -Leu-Gln-Glu-Phe-Leu-Gln-Trp-Ala-Leu-Gln-Thr. When interacting with rat hepatocytes, the analog mimics, in part, the activities of glucagon in receptor binding and inhibition of carbohydrate incorporation into glycogen. Comparison of the binding of the analog with that of glucagon demonstrates the existence of two distinct homogeneous populations of glucagon receptors. The synthetic analog acts as a specific probe for those receptors that have a high affinity for glucagon.

Decreased adenylate cyclase responsiveness of transformed cells correlates with the presence of a viral transforming protein

The adenylate cyclase responsiveness of transformed fibroblastic and epithelial cell lines to forskolin, fluoride, guanine nucleotides and cholera toxin was reduced compared to their parental counterparts. This phenomenon was observed in lines transformed by either RNA or DNA tumor viruses, and in the case of polyoma virus, coincided with the expression of middle T antigen. The data suggest that decreased responsiveness of adenylate cyclase to non-hormone activators is a general consequence of viral transformation and may be related to viral regulation of protein kinase activity.

Correspondency between different affinity states and target size of the bovine striatal D2 dopamine receptor

Target size analysis of the D2 dopamine receptor in the bovine striatum revealed the presence of two populations of this receptor, in terms of apparent molecular size. The size of large target was approximately 150 X 10(4) daltons, while that of small target was 11 X 10(4) daltons. The antagonist [3H]spiperone labeled both large and small sized D2 receptors, while agonist [3H]n-propylapomorphine (NPA) labeled only the former. In addition, the apparent molecular size of a functional unit for the GTP effect was calculated to be 150 X 10(4) daltons, such appearing to be identical to that of large target sized D2 dopamine receptors. Therefore, the large sized D2 receptor, probably an oligomeric complex consisting of D2 receptor recognition protein and guanine nucleotide regulatory protein, has a high affinity for both agonist and antagonist, while the small sized receptor, probably a monomeric or dimeric receptor recognition protein, has a high affinity for only the antagonist.

Radiation inactivation of glutamate dehydrogenase hexamer: lack of energy transfer between subunits

The effects of ionizing radiation on glutamate dehydrogenase and on fluorescein isothiocyanate--tagged glutamate dehydrogenase were analyzed by target theory. Enzymatic activity, fluorescence, and the survival of the 56,000-dalton monomer subunit were determined on frozen samples irradiated at -135 degrees C and on lyophilized samples irradiated at either -135 degrees C or +30 degrees C. The effects of temperature were the same for all three parameters. Enzymatic activity was lost after small doses of high-energy electrons, whereas fluorescence and monomer subunits survived much larger doses of radiation. Target analysis revealed that the functional unit size for enzymatic activity was the hexamer, confirming both the earlier radiation study and conventional biochemical analyses. Target sizes obtained from fluorescence and subunit structure measurements were close to that of the monomer. These results indicate that the primary ionization caused by electron bombardment results in damage to a single polypeptide strand and that there is no massive transfer of radiation energy to other units in the hexamer. The large target size observed for enzymatic activity appears to be a structural requirement for the simultaneous presence of six intact subunits rather than the result of the spread of energy from the initial site to adjacent chains with consequent damage to other subunits.

Cell cycle-dependent expression of specific opiate binding with variable coupling to adenylate cyclase in a neurotumor hybrid cell line NG108-15

Monolayer cultures of neuroblastoma X glioma hybrid (clonal) cell line NG108-15, synchronized by the isoleucine/glutamine deprivation method, showed maximal expression of opiate binding sites at the same point in the cell cycle at which prostaglandin E1 (PGE1) had a maximum stimulatory effect on cyclic AMP synthesis. However, the capacity of enkephalin [D-Ala2D-Leu5] to block the stimulation of cyclic AMP synthesis by PGE1 was not related to the number of opiate receptors expressed. The Ki for the inhibition of cyclic AMP synthesis by opioid peptides increased substantially during the period of the cell cycle at which maximal expression of opiate binding sites occurred, making the effective level of inhibition of adenylate cyclase activity by 0.1 microM enkephalin [D-Ala2D-Leu5] the same through the cell cycle. Data are presented to suggest that enkephalin receptor coupling to adenylate cyclase, via a GTP-binding protein, is maximal during G1 phase (which may approximate the state of the differentiated neuron) and minimal during S + G2 phase, just prior to cell division, when many receptors are uncoupled.

Structure of membrane-associated and solubilized uterine adenylate cyclase. Evidence against activation through dissociable subunit interactions in the lipid bilayer

Adenylate cyclase was extracted from the rat uterus with Lubrol PX in a form which remained soluble following centrifugation for 60 min at 100,000g. The soluble enzyme was stimulated by both Mn+2 and by guanyl-5'-yl-imidodiphosphate (Gpp(NH)p), indicating that both the catalytic subunit (C) and the guanyl nucleotide-binding coupling factor (N) had been extracted. Catalytic activity was bound by a GTP-affinity resin only under conditions which resulted in irreversible activation of the native (particulate) form of the enzyme and could be eluted under acidic conditions shown to reverse the activated state. The S020,w of the soluble enzyme in both its activated and unactivated state was determined by linear sucrose gradient centrifugation. Activation by prolonged treatment with Gpp(NH)p did not alter the S020,w of the enzyme whether treatment was carried out before or after solubilization. The chaotrope LiBr (0.4 M) reduced the S020,w of the soluble enzyme but its smaller size was still not altered by activation with Gpp(NH)p. These results indicate that most adenylate cyclase activity in uterine membranes exists as a preformed complex between the catalytic subunit and the coupling factor: NC. The existence of this complex explains some of the temperature-dependent properties previously described for this form of the enzyme and suggests that dissociable interactions between the subunits do not play a role in the activation of C by guanyl nucleotides.

The molecular size of multiple opiate receptors

The functional size of different types of opiate receptors was determined in situ by the radiation inactivation technique. The Mr of opiate binding sites was estimated to be about 90,000 daltons; the data revealed no significant differences between the molecular sizes of either mu-, delta- or kappa-sites in rat brain membranes. An identical molecular size was found also for the delta-sites in the neuroblastoma X glioma hybrid cell clone, NG 108-15.

Putative calcium channel molecular weight determination by target size analysis

The molecular weight of the putative calcium channel in guinea-pig brain membranes, labelled with [3H]-nimodipine, has been determined in situ by target size analysis. Irradiation of guinea-pig brain membranes at 153-173 K with 10 MeV electrons reduced the density of [3H]-nimodipine binding sites without affecting the equilibrium dissociation constant. The decay of the calcium channel blocker binding site density as a function of radiation dose fits to a monoexponential function. Application of the target size theory gives a molecular weight of 185,000. [3H]-Flunitrazepam labelled benzodiazepine receptor target size was measured as an internal control. The molecular weight of the benzodiazepine receptors was 76,000, which is in good agreement with published results.

Radiation inactivation of alpha 1-adrenoceptors

Radiation inactivation of alpha 1-adrenoceptors in rat cerebral cortex membranes has been performed with 10 MeV electrons from a linear accelerator at temperatures less than or equal to -100 degrees C. Alpha 1-adrenoceptor inactivation was monitored with [ 3H ]-prazosin and [( 125I ]-2-(beta-4-hydroxylphenyl)ethylaminomethyl)tetralone [( 125I ]-HEAT). Saturation analysis of irradiated membranes with both ligands indicated that a decrease in alpha-adrenoceptor density occurred with increasing radiation dose. The dissociation constants of [ 3H ]-prazosin and [ 125I ]-HEAT were not markedly changed by the irradiation. Application of the target volume theory gave molecular weights of 91,500 +/- 1,700 (S.D.) (D37: 19,6 +/- 0.36 Mrad) with [ 125I ]-HEAT as ligand, and 77,000 +/- 18,000 (S.D.) (D37: 23.3 +/- 4.6 Mrad) with [ 3H ]-prazosin, respectively, when an empirical temperature correction factor of 2.8 was used. [ 3H ]-flunitrazepam-labelled benzodiazepine receptor target size was used as an internal control. The molecular weight of the alpha 1-adrenoceptors, corrected for this internal control, was 85,000 +/- 1.600 [( 125I ]-HEAT) and 71,500 +/- 17,000 [( 3H ]-prazosin).

Structure-function relationships for hormone receptors and adenylate cyclase: the contribution of target size analysis

Target size analysis of radiation inactivation has been used to determine structural features of hormone receptors and adenylate cyclase. This method allows the determination of molecular sizes of components of enzyme- or receptor-systems in impure preparations and in intact membranes. Principles of application of target size analysis and basic concepts for interpretation are discussed. Reviewing both biochemical and target size data on insulin receptors and adenylate cyclase it is attempted to outline the potential and the limitations of this biophysical approach.