Investigation of the subcellular distribution of cyclic-AMP phosphodiesterase in rat hepatocytes, using a rapid immunological procedure for the isolation of plasma membrane

Purification of an insulin-sensitive cyclic AMP phosphodiesterase from rat liver

A low-Km cyclic nucleotide phosphodiesterase solubilised from rat liver membranes by mild proteolysis with chymotrypsin has been purified to apparent homogeneity. The purification included chromatography on cellulose phosphate, Ecteola-cellulose, hydroxyapatite, a theophylline affinity matrix and HPLC on a DEAE-substituted column. The purified enzyme has linear kinetic plots with a Km of 0.24 microM and a Vmax of 6.2 mumol mg-1 min-1 with cyclic AMP as a substrate. It also hydrolyses cyclic GMP with a Km of 0.17 microM and a Vmax which is about a third of that with cyclic AMP. Cyclic GMP is also a competitive inhibitor of cyclic AMP hydrolysis with a Ki of 0.18 microM. The proteolytically solubilised enzyme has a subunit molecular mass of 73 kDa by SDS gel electrophoresis and of 130 kDa by HPLC size-exclusion chromatography, suggesting that it exists as a dimer. A partially purified preparation of this enzyme was used to raise antiserum in a sheep. The antiserum immunoprecipitated activity from liver and adipose tissue of rat and mouse. It had little activity against phosphodiesterase from other rat tissues or other species. Insulin-activated phosphodiesterase from both adipocytes and hepatocytes was immunoprecipitated by the antiserum suggesting that the purified enzyme was an insulin-sensitive phosphodiesterase.

Immunoaffinity purification of subcellular particles and organelles

The application of immunoaffinity techniques to subcellular fractionation is reviewed and the basic principles underlying the various methods that have been successfully employed, identified. The requirement for organelle-specific antigens and high-avidity antibodies is discussed, as is the widespread use of indirect immunoadsorbents. Approaches for the optimization of immunoaffinity-based subcellular fractionation are suggested.

Acute in vivo stimulation of low-Km cyclic AMP phosphodiesterase activity by insulin in rat-liver Golgi fractions

A low-Km phosphodiesterase activity, which is acutely stimulated by insulin in vivo, has been identified in plasma membranes and Golgi fractions prepared from rat liver homogenates in isotonic sucrose. Within seconds after insulin injection (25 micrograms/100 g body weight) cAMP phosphodiesterase activity increases by 30-60% in Golgi fractions and by 25% in plasma membranes; activity in crude particulate and microsomal fractions is unaffected. The increase in activity is short-lived in the light and intermediate Golgi fractions, but persists for at least 10 min in the heavy Golgi fraction. It precedes the translocation of insulin and insulin receptors to these fractions, which is maximal at 5 min. The doses of insulin required for half-maximal and maximal activation are, respectively, 7.5 micrograms/100 g and 25 micrograms/100 g body weight. Golgi-associated cAMP phosphodiesterase activity shows non-linear kinetics; a high-affinity component (Vmax, 13 pmol min-1 mg protein-1; Km, 0.35 microM) is detectable. Insulin treatment increases the Vmax 60-70%, but does not affect the Km. Unlike the low-Km cAMP phosphodiesterase associated with crude particulate fractions, the Golgi-associated activity is not easily extractable by solutions of low or high ionic strength. On analytical sucrose density gradients, low-Km cAMP phosphodiesterase associated with the total particulate fraction equilibrates at lower densities than endoplasmic reticulum and lysosomal markers, but at a higher densities than plasma membrane, Golgi markers and insulin receptors. Insulin treatment increases the specific activity of the enzyme by 20-60% at densities below 1.12 g cm-3, and by 20-40% in the density interval 1.23-1.25 g cm-3. Such treatment also causes a slight, but significant shift in the distribution of phosphodiesterase towards lower densities. It is suggested that Golgi elements or physically similar subcellular structures are a major site of localization of insulin-sensitive cAMP phosphodiesterase in rat liver. However, internalization of the insulin-receptor complex is probably not required for enzyme activation.

Immunoaffinity purification of intact, metabolically active, cholinergic nerve terminals from mammalian brain

A method for the immunoaffinity purification of cholinergic nerve terminals from mammalian brain was developed. A sheep antiserum to Torpedo electric-organ synaptic membranes, previously shown to be specific for cholinergic terminals in mammalian brain, was incubated with crude mitochondrial fractions prepared from rat brain. Cholinergic nerve terminals sensitized by this serum were purified from the mitochondrial fractions on a high-capacity cellulose immunoadsorbent bearing a mouse monoclonal anti-(sheep immunoglobulin G) antibody. Adsorption of nerve terminals on to the immunoadsorbent was assessed by using a variety of enzyme markers and gave a maximum yield of 24% of choline acetyltransferase, whereas non-specific binding was less than 1.0% for all of the enzymes measured. Cholinergic terminals were purified 26-fold from rat caudate nucleus, 30-fold from rat hippocampus and 38-fold from rat cerebral cortex. The terminals were shown to be intact, osmotically sensitive and metabolically active.

Insulin-stimulated high affinity cyclic AMP phosphodiesterase in rat mammary acini

High affinity cyclic AMP phosphodiesterase activity in preparations of acini isolated from mammary tissue of lactating rats is shown to be stimulated by the addition of physiological concentrations of insulin to incubations of acini in vitro. This effect is expressed specifically on membrane-associated phosphodiesterase and occurs in the absence of concurrent protein synthesis. The possible functional role of this aspect of insulin's action on mammary tissue is discussed and compared with the well-known reversal by this hormone of the effects of lipolytic agents in adipose tissue and liver.

The isolation of endosome-derived vesicles from rat hepatocytes

Intracellular 5'-nucleotidase involved in membrane circulation in rat hepatocytes is latent, and is protected from inhibition when whole cells are incubated with inhibiting antiserum at 2 degrees C [Stanley, Edwards & Luzio (1980) Biochem. J. 186, 59-69]. These two criteria were used to identify intracellular membrane vesicles containing 5'-nucleotidase on Ficoll density gradients. A sharply defined turbid band containing intracellular 5'-nucleotidase isolated on density gradients was further fractionated by immunoadsorption of plasma-membrane fragments derived from the cell surface of surface-inhibited cells on to an anti-(immunoglobulin G) immunoadsorbent. The resulting non-adsorbed membrane fraction consisted of vesicles of uniform size (approx. 65 nm diam.), but was not identifiable as any known organelle. This fraction could account for approx. 5% of the total cell 5'-nucleotidase activity, and the enzyme activity measured was 55% latent. The fraction had a restricted polypeptide composition but similar phospholipid composition compared with plasma membrane. We suggest that the vesicles observed in this fraction were derived from the endocytic pathway.

Presynaptic distribution of the cholinergic-specific antigen Chol-1 and 5'-nucleotidase in rat brain, as determined by complement-mediated release of neurotransmitters

Nerve terminals prepared from rat cortex and hippocampus were loaded with seven radioactive putative neurotransmitters (serotonin, noradrenaline, dopamine, gamma-aminobutyric acid, aspartate, glutamate, and taurine). The release of these transmitters, choline acetyltransferase, 3,4-dihydroxyphenylalanine decarboxylase, enolase, and lactate dehydrogenase was monitored during complement-mediated lysis. Three antisera were used: anti-5'-nucleotidase, anti-Chol-1, and anti-rat cerebrum. Anti-5'-nucleotidase serum did not cause the release of any labelled transmitter or of any of the enzymes studied. Anti-Chol-1 serum released choline acetyltransferase and small amounts of enolase and lactate dehydrogenase. Anti-rat cerebrum caused the release of all seven transmitters, choline acetyltransferase, and small amounts of the other three enzymes. It was concluded that 5'-nucleotidase was not present on any of the terminals studied, and that Chol-1 is only present on cholinergic terminals.

Detergent solubilisation of rat liver particulate cyclic AMP phosphodiesterase

Detergent solubilisation of particulate rat liver low Km cyclic AMP phosphodiesterase in the presence of protease inhibitors yields a form of the enzyme with a larger molecular weight than the form solubilised by protease treatment. The detergent solubilised enzyme could be partially purified by anion exchange chromatography. It displayed a marked tendency to precipitate from solution when detergent was removed.

Simulations of the roles of multiple cyclic nucleotide phosphodiesterases

1. Simulations were performed using a model for cellular cyclic AMP metabolism involving a hormone-activated adenylate cyclase and two cyclic nucleotide phosphodiesterases with different Michaelis constants. 2. The response curves of cyclic AMP concentration as a function of hormone concentration were affected by regulating the phosphodiesterases. The maximum velocity of the high-affinity phosphodiesterase (V1) was important in determining the position of the response curve; when v1 was less than the maximal activity of adenylate cyclase (Vc), sigmoid response curves were readily produced. The maximum attainable concentration of cyclic AMP was determined primarily by V1 when Vc less than V1, and primarily by the activity of the low-affinity enzyme when Vc greater than V1 (V2 much greater than Vc in all cases). 3. The glucagon-stimulated adenylate cyclase and insulin-stimulated phosphodiesterase of the rat liver plasma membrane were simulated using experimentally determined values for the enzyme-kinetic parameters, and a considerable potential for regulation of the system by insulin was demonstrated. 4. Other possible functions for the regulation of phosphodiesterases are considered, in particular the value of increasing the speed of response to decreases in hormone concentration.

Characterization of the phosphorylated form of the insulin-stimulated cyclic AMP phosphodiesterase from rat liver plasma membranes

Incubation of intact purified rat liver plasma membranes with insulin, cyclic AMP and ATP led to the activation of the peripheral "low-Km" cyclic AMP phosphodiesterase. When (gamma-32P]ATP was included in the incubation mixture, after purification of this enzyme to homogeneity it was found to contain 1 mol of alkali-labile 32P/mol of enzyme. Treatment of the homogeneous phosphorylated enzyme with alkaline phosphatase released all of the 32P from the protein while restoring its activity to the native state. The reversibility of the activation that is achieved by the phosphorylation of this enzyme could also be demonstrated with a high-speed supernatant from rat liver. This restored the activity of the activated membrane-bound enzyme to its native state. The Ka for the cyclic AMP-dependence of this process (1.6 micrometer) was unaffected by a range of ATP concentrations (1-10 mM) and by a range of membrane protein concentrations (0.2-2 mg/ml). Adenylyl imidodiphosphate could not substitute for ATP, and concanavalin A could not substitute for insulin, as essential ligands in the activation process. The purified activated enzyme exhibited Km 0.6 microM, Vmax 10.9 units/mg of protein and Hill coefficient (h) 0.47. The Vmax. for this activated enzyme was much higher than that of the native enzyme, yet h was much lower.

Insulin trigger, cyclic AMP-dependent activation and phosphorylation of a plasma membrane cyclic AMP phosphodiesterase

Regulation of blood glucose levels by the liver is primarily achieved by the action of two peptide hormones, insulin and glucagon, which bind to specific receptors associated with the hepatocyte plasma membrane. Whilst the molecular action of glucagon at the level of the cell plasma membrane in activating adenylate cyclase is relatively well understood, we know little, if anything, of the molecular consequences of insulin occupying its receptor. We demonstrate here that insulin, at physiologically relevant concentrations, can trigger the cyclic AMP-dependent activation and phosphorylation of a low Km cyclic AMP phosphodiesterase attached to the liver plasma membrane. Such an effect may in part explain the ability of insulin to inhibit the increase in cellular cyclic AMP content that glucagon alone produces by activation of adenylate cyclase. Our observation that basal, intracellular cyclic AMP levels are insufficient to allow insulin to activate the cyclic AMP phosphodiesterase, yet those cyclic AMP levels achieved after exposure of the cells to glucagon are sufficient, gives a molecular rationale to Butcher and Sutherland's proposal that it is necessary to first elevate cellular cyclic AMP levels before they can be depressed by insulin.

Cyclic AMP phosphodiesterase activities in Xenopus laevis oocytes

Cyclic AMP phosphodiesterase activity has been identified in full-grown Xenopus oocytes in vivo and in vitro. About 50% of the in vitro phosphodiesterase activity was present in the solution fraction and 35% in a partially purified membrane fraction. Both activities exhibited high substrate affinity (Km about 10(-6) M). Sucrose gradient fractionation revealed two forms of phosphodiesterase: a 5 S form (peak I) and a 6.5 S form (peak II). Treatment with trypsin led to the activation of the soluble enzyme with the transformation of peak II into peak I. Ethylene glycol bis (beta-aminoethyl ether)-N,N'-tetraacetic acid, calcium dependent regulator, and Fluphenazine did not influence the enzyme activities suggesting that the oocyte phosphodiesterases were not Ca2+-dependent. Intact oocytes were induced to mature by exposure to progesterone; their phosphodiesterase activities and distribution tested in vitro were comparable to those of untreated oocytes.

Subcellular distribution and movement of 5'-nucleotidase in rat cells

1. Cell-surface 5'-nucleotidase was assayed by incubating whole-cell suspensions with 5'[3H]-AMP in iso-osmotic buffer and measuring [3H]adenosine production. The activity of cell-surface 5'-nucleotidase in hepatocytes, adipocytes and lymphocytes isolated from the rat was 15.0, 0.5 and 0.8pmol/min per cell at 37 degrees C respectively. 2. Disruption of the cells by vigorous mechanical homogenization or detergent treatment exposed additional 5'-nucleotidase activity, which represented 52%, 25% and 21% of the total activity in the three cell types respectively. This increase in 5'-nucleotidase activity which occurred when the cells were homogenized was due to a second pool of 5'-nucleotidase within the cell, rather than activation of the cell-surface enzyme. 3. In hepatocytes the intracellular 5'-nucleotidase activity was membrane-bound, indistinguishable from cell-surface 5'-nucleotidase in its inhibition by rabbit anti-(rat liver 5'-nucleotidase) serum and its kinetics with AMP, and was located on the extracytoplasmic face of vesicles within the cell. 4. The cell-surface 5'-nucleotidase of rat hepatocytes was rapidly inhibited when rabbit anti-(rat liver 5'-nucleotidase) serum or concanavalin A was added to the medium at 37 degrees C. Incubation with antiserum for 5 min at 37 degrees C inhibited 83 +/- 3% of the cell-surface enzyme. 5. Incubation of hepatocytes with exogenous antiserum or concanavalin A for 30 min at 37 degrees C resulted in over 50% inhibition of the intracellular enzyme. This inhibition was not prevented by disruption of the cytoskeleton or by ATP depletion. 6. Incubation of hepatocytes with exogenous antiserum or concanavalin A for up to 2h at 0 degrees C caused little or no inhibition of the intracellular enzyme, but over 75% inhibition of the cell-surface enzyme. 7. When surface-inhibited hepatocytes were washed and resuspended in buffer at 37 degrees C, 5'-nucleotidase was observed to redistribute from the intracellular pool to the cell surface.