The activation of actin:DNase I complex with rat liver plasma membranes. The possible role of 5'-nucleotidase

Effects of growth factors on the enzymes of purine metabolism in culture of regenerating rat liver cells

Hepatocytes are affected by many cytokines and growth factors during liver regeneration. In regenerating rat liver cells cultures, liver cell growth factor (LCGF), hepatic stimulator substance (HSS), interleukin-1 beta (IL-1 beta), as well as their combination, were tested for their ability to activate the enzymes involved in purine metabolism. The enzymes tested were 5' nucleotidase, AMP deaminase, adenosine deaminase and xanthine oxidase. The cytokines alone or in combination, activated 5' nucleotidase and adenosine deaminase. Activity of AMP deaminase was stimulated by IL-1 beta associated with LCGF, HSS and IL-1 beta. Xanthine oxidase was stimulated by IL-1 beta but not with HSS and LCGF. Associated with IL-1 beta these two substances decreased its activity. A novel approach to the understanding of the mechanisms involved in the regulation of purine metabolism during liver regeneration, is proposed.

Membrane-bound 5'-nucleotidase inhibitor in Ehrlich ascites tumour cells and newborn mouse liver

5'-Nucleotidase activity in Ehrlich ascites tumour cells was undetectable. The cell homogenate, when mixed with adult mouse liver homogenate, inhibited the 5'-nucleotidase activity of the latter, without affecting its p-nitrophenyl phosphate-hydrolysing activity. The inhibitor activity was enriched (6.8-fold) in a membrane fraction which was enriched in (Na+ + K+)-ATPase (14-fold) and alkaline phosphatase (8-fold). 5'-Nucleotidase activity in this membrane fraction could be detected only after separating the inhibitor activity from the enzyme on Sephadex G-50. The inhibitor activity was decreased by 27% when heat-treated, 33% when treated with 6 M urea and was almost completely lost when treated with trypsin. It was dialysable from a tubing with a molecular exclusion limit of 10,000, but was retained in a tubing with an exclusion limit of 3000. From these results we conclude that a small molecular weight protein inhibitor(s) of 5'-nucleotidase is present in the plasma membrane of Ehrlich ascites tumour cells. Also, the presence of such an inhibitor in the newborn mouse liver but not in the adult liver suggests that it may have some role in cellular ageing and cancer.

The membrane topography of ecto-5'-nucleotidase in rat hepatocytes

The transmembrane topography of the rat hepatocyte ectoenzyme 5'-nucleotidase was studied by the use of glycoprotein labelling and limited-proteolysis techniques. Comparison, by one-dimensional peptide mapping, of enzyme iodinated from outside the cell with that iodinated in the solubilized state showed that no additional iodination sites were revealed on solubilization. Incubation of newly synthesized enzyme in a microsomal membrane fraction with proteinase showed that the entire molecule of 5'-nucleotidase was protected from proteolysis. These data suggest that little, if any, of the 5'-nucleotidase molecule is present on the cytoplasmic side of the plasma membrane. No evidence was found for a previously proposed interaction between 5'-nucleotidase and actin, although the ability of preparations of 5'-nucleotidase to prevent inhibition of deoxyribonuclease I by actin was explained by minute traces of ATPase activity. Comparison of peptide maps of enzyme labelled by iodination or by methods specific for carbohydrate showed that in both cases predominantly one section of the molecule was labelled. It is proposed that the enzyme is a short-stalked integral membrane protein without a cytoplasmic domain in which about one-third of the molecule forms the accessible molecular surface.

Concanavalin A induced inhibition of 5'-nucleotidase from guinea pig skeletal muscle and bull seminal plasma: a comparative study

Both purified and membrane-bound 5'-nucleotidases (EC 3.1,3.5) from guinea pig skeletal muscle and bull seminal plasma are inhibited by Concanavalin A (Con A). 5'-Nucleotidase purified from skeletal muscle is inhibited by Con A by an apparent uncompetitive process (K'i = 160 nM), while the lectin inhibits the particulate enzyme by an apparent non-competitive process (Ki = K'i = 50 nM). 5'-Nucleotidase purified from bull seminal plasma is inhibited by Con A by an apparent non-competitive process (K'i = Ki = 270 nM), while the membrane-bound enzyme is subjected to a mixed type inhibition by the lectin (K'i greater than Ki; 30 and 14 nM, respectively). The enzyme purified from skeletal muscle exhibits a significant cooperativity in the interaction with Con A. The inhibition of bull seminal plasma particulate 5'-nucleotidase brought about by Con A is not completely reversed by addition of alpha-methyl-D-mannoside.

The interaction of 5'-nucleotidase purified from chicken gizzard and actin, and the reversible loss of the inhibitory capacity of actin on deoxyribonuclease I

Evidence is presented for a direct interaction of the intrinsic membrane protein 5'-nucleotidase (5'-ribonucleotide phosphohydrolase, EC 3.1.3.5) purified from avian smooth muscle (chicken gizzard) and the cytoskeletal component actin. Two different modes of interaction can be discerned: firstly, an immediate inhibitory effect of preferentially filamentous actin (F-actin) on the enzymic (i.e., AMPase) activity of 5'-nucleotidase and a direct binding of this enzyme to immobilized F-actin. Since these effects are suppressed by the addition of myosin subfragment 1, binding of 5'-nucleotidase appears to occur along the F-actin filament axis. Secondly, a time- and 5'-nucleotidase concentration-dependent transformation of also preferentially F-actin into a form unable to inhibit the enzymic activity of deoxyribonuclease I (DNAase I). This desensitization of actin versus DNAase I is not due to a denaturation process and was found to be reversible after addition of ATP. Furthermore, it does not seem to effect the ability of actin to bind to DNAase I. The transformation is accompanied by the hydrolysis of actin-bound nucleotide into adenosine, which remains bound to actin. Therefore, the desensitization of actin versus DNAase I appears to be due to a nucleotide-dependent conformational change of actin. An unidentified contamination of the 5'-nucleotidase preparations to a varying degree with ADPase and ATPase activities appears to be responsible for the desensitization process, although a synergistic role of these activities and 5'-nucleotidase cannot be excluded.

Isolation and kinetic properties of 5'-nucleotidase from guinea-pig skeletal muscle

5'-Nucleotidase (EC 3.1.3.5) has been solubilized and purified 1200-fold from guinea-pig skeletal muscle, to a specific activity of 40 U/mg protein. The purified enzyme yields a single protein band on polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Guinea-pig skeletal muscle 5'-nucleotidase is extremely sensitive to inhibition by nucleoside di- and triphosphates. The inhibition is of the competitive type, and can be reversed only by strong excess of Mg2+. Nucleoside diphosphates are more powerful inhibitors than nucleoside triphosphates. The Ki values for ADP and ATP are 0.036 and 0.28 microM, respectively. The purified enzyme does not require exogenous cations for maximal activity and is inhibited by EDTA. This inhibition is reversed by divalent cations. This indicates that the enzyme contains a tightly bound metal cation.

Complex glycoproteins associated with the detergent-resistant membrane matrix of the rhabdomeral microvilli of crayfish photoreceptors

The lectin-binding properties of crayfish photoreceptor membrane were examined using isolated rhabdoms and frozen sections of whole retinas after permeabilization. Fluorescently-labelled concanavalin A (Con A) and wheat germ agglutinin (WGA) bind specifically to the rhabdoms, the accessory pigment cells, and the basement membrane of the retina in permeabilized frozen sections. Con A also binds to the clear zone in sectioned material. Peanut agglutinin (PA) binds strongly to domains associated with the periphery of rhabdoms. Intact isolated rhabdoms also bind Con A and WGA with high affinity. The major lectin-binding proteins of the photoreceptor membrane were characterized on nitrocellulose blots after SDS electrophoresis of purified photoreceptor membrane. Two major glycoproteins were found to bind a number of lectins. A 130 000 molecular weight peptide (GPII) specifically bound Con A, WGA, Dolichos biflorus agglutinin, (DBA) Ulex europaeus agglutinin (UEA) and PA, whereas the major 142 000 d protein (GPI) of the photoreceptor membrane bound only Con A and WGA implying a rather simple hexose chain consisting mainly of mannose and N-acetylglucosamine. Photoreceptor membrane vesicles, extracted with a high concentration of non-ionic detergent and centrifuged, show a specific enrichment of both proteins as well as actin and a 40 000 d component. Significant amounts of both glycoproteins are removed from the membrane residue by 0.6 M KI, even under conditions known to stabilize actin-based cytoskeletons. Both the stoichiometric amounts of these peptides in relation to the actin complement and their physicochemical properties are consonant with the hypothesis that one or both may be involved in linking a submembranous, actin-based cytoskeleton to the lipid bilayer. Rhodopsin, the major component of photoreceptor membrane, appears to be a glycoprotein by periodic acid-Schiff (PAS) staining methods but fails to bind significant amounts of lectins on Western blots. These findings are discussed in the light of post-translational processing and intracellular pathways for photoreceptor membrane components in the compound eye.

Evidence for the presence of DNase-actin complex in L1210 leukemia cells

L1210 leukemia cell cytosol was analysed for the presence of DNase I activity. No free activity was determined in crude cytosol. DNase I enzyme was found to occur in a latent form bound to cytoplasmic actin. DNase-actin complex was partially isolated by Sephadex filtration and DNase I-like activity was demonstrated after SDS gel electrophoresis of the complex and enzyme renaturation. The results were compared with those for synthetic complex of pancreatic bovine DNase I and chicken muscle actin.

Immunological evidence that plasma-membrane 5'-nucleotidase is a transmembrane protein

Antibodies inhibiting specifically plasma membrane-bound 5'nucleotidase were used to determine the disposition of the enzyme in various lymphoma cell lines. Fluorescent Fab fragments of the inhibiting antibodies bound to MF2s and MOPC 173 plasmacytoma cells, whereas no fluorescence was observed with P 1798 thymoma cells in which the enzyme was absent. The relative potency of the antiserum in inhibiting the enzyme in right-side-out and inside-out plasma membrane vesicles prepared from the above cell lines indicated that various group of antigenic determinants were present. One group of antigenic determinants was present at the external face of the cell, and a second was associated with the inner surface of the membrane. A third group of antigenic determinants was located in the vicinity of the active site of the enzyme and it is the group that varied in the various plasmacytoma cells studied. The results are interpreted as immunological evidence that 5'nucleotidase is a transmembrane glycoprotein.

Localization of 5'-nucleotidase activity in the parotid acinar cells of a rat treated with isoproterenol

Cytochemical localization of 5'-nucleotidase (AMPase) has been investigated in the parotid acinar cells of rats at various stages of exocytic secretion induced by an administration of isoproterenol (IPR). In the resting stage, the acinar cells show AMPase activity located on the baso-lateral and luminal plasmalemma, and in the earliest secretory stage the luminal plasma membranes are devoid of the enzymatic activity. However, these particular regions exhibit AMPase activity during the advanced stages of secretion, and the AMPase positive membranes become absorbed into the cytoplasm by endocytic activity. The absorbed membrane components then seem to be degraded by the action of lysosomes. The intracellular fate of the endocytic vacuoles has been examined by the aid of ferritin particles introduced retrogradely through ductal lumina. Ferritin containing vacuoles are distributed in the cytoplasm, and these droplets change into secondary lysosomes. No tracer particles are recognized in the internal space of the Golgi lamella and its associated vesicles. The results suggested that in the exocytic secretion of parotid acinar cells, AMPase originating from plasma membrane intermingles with the membranes derived from secretion granules, and is translocated into cytoplasm by an endocytic mechanism. The internalized membrane components are, at least partly, degraded by lysosome action.

Biochemical analysis of actin in crane-fly gonial cells: evidence for actin in spermatocytes and spermatids--but not sperm

A biochemical assay employing DNase-I affinity chromatography, two-dimensional peptide analysis and SDS polyacrylamide gel electrophoresis was used to isolate, identify, and assess the amount of actin from gonial cells of the crane fly, Nephrotoma suturalis. Based on the analysis of cell homogenates under conditions in which all cellular actin is converted to the monomeric DNase-binding form, actin comprises approximately 1% of the total protein in homogenates of spermatocytes and spermatids. SDS gel analysis of mature sperm reveals no polypeptides with a molecular weight similar to that of actin. Under conditions that preserve native supramolecular states of actin, approximately 80% of the spermatocyte actin is in a sedimentable form whereas only approximately 30% of the spermatid actin is sedimentable. These differences could be meaningful with regard to strutural changes that occur during spermiogenesis. A comparative analysis of two-dimensional peptide maps of several radioiodinated actins reveals similarities among spermatocyte, spermatid, and human erythrocyte actins. The results suggest the general applicability of this approach to other cell types that contain limited amounts of actin.