Spectrophotometric assay of 2',3'-cyclic nucleotide 3'-phosphohydrolase: application to the enzyme in bovine brain

Crystallographic analysis of the reaction cycle of 2',3'-cyclic nucleotide 3'-phosphodiesterase, a unique member of the 2H phosphoesterase family

2H phosphoesterases catalyze reactions on nucleotide substrates and contain two conserved histidine residues in the active site. Very limited information is currently available on the details of the active site and substrate/product binding during the catalytic cycle of these enzymes. We performed a comprehensive X-ray crystallographic study of mouse 2′,3′-cyclic nucleotide 3′-phosphodiesterase (CNPase), a membrane-associated enzyme present at high levels in the tetrapod myelin sheath. We determined crystal structures of the CNPase phosphodiesterase domain complexed with substrate, product, and phosphorothioate analogues. The data provide detailed information on the CNPase reaction mechanism, including substrate binding mode and coordination of the nucleophilic water molecule. Linked to the reaction, an open/close motion of the β5–α7 loop is observed. The role of the N terminus of helix α7—unique for CNPase in the 2H family—during the reaction indicates that 2H phosphoesterases differ in their respective reaction mechanisms despite the conserved catalytic residues. Furthermore, based on small-angle X-ray scattering, we present a model for the full-length enzyme, indicating that the two domains of CNPase form an elongated molecule. Finally, based on our structural data and a comprehensive bioinformatics study, we discuss the conservation of CNPase in various organisms.

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A detailed structural analysis of the CNPase catalytic cycle was carried out.

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Complexes with substrates, products, and analogues highlight roles for a nearby helix and loop in the reaction mechanism.

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The full-length CNPase adopts an elongated conformation in solution.

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CNPase is a unique member of the 2H family, and the results will help understand its physiological significance.

Identification of essential residues in 2',3'-cyclic nucleotide 3'-phosphodiesterase. Chemical modification and site-directed mutagenesis to investigate the role of cysteine and histidine residues in enzymatic activity

2',3'-Cyclic nucleotide 3'-phosphodiesterase (CNP; EC ) catalyzes in vitro hydrolysis of 3'-phosphodiester bonds in 2',3'-cyclic nucleotides to produce 2'-nucleotides exclusively. N-terminal deletion mapping of the C-terminal two-thirds of recombinant rat CNP1 identified a region that possesses the catalytic domain, with further truncations abolishing activity. Proteolysis and kinetic analysis indicated that this domain forms a compact globular structure and contains all of the catalytically essential features. Subsequently, this catalytic fragment of CNP1 (CNP-CF) was used for chemical modification studies to identify amino acid residues essential for activity. 5,5'-Dithiobis-(2-nitrobenzoic acid) modification studies and kinetic analysis of cysteine CNP-CF mutants revealed the nonessential role of cysteines for enzymatic activity. On the other hand, modification studies with diethyl pyrocarbonate indicated that two histidines are essential for CNPase activity. Consequently, the only two conserved histidines, His-230 and His-309, were mutated to phenylalanine and leucine. All four histidine mutants had k(cat) values 1000-fold lower than wild-type CNP-CF, but K(m) values were similar. Circular dichroism studies demonstrated that the low catalytic activities of the histidine mutants were not due to gross changes in secondary structure. Taken together, these results demonstrate that both histidines assume critical roles for catalysis.

Diastereomeric specificity of 2',3'-cyclic nucleotide 3'-phosphodiesterase

The diastereomers of adenosine and uridine 2',3'-cyclic phosphorothioates were tested as substrates for 2',3'-cyclic nucleotide 3'-phosphodiesterase from bovine brain. The enzyme cleaves the Sp (or exo) diastereomers efficiently, whereas the Rp (or endo) diastereomers are resistant to hydrolysis, even after long incubation. As the enzyme exhibits strong substrate inhibition the precise determination of kinetic parameters posed problems, particularly with phosphorothioates. The stereoselectivity of this enzyme is opposite to that of RNase T1 and RNase A and thus could be a useful complement in determination of the configuration of nucleoside 2',3'-cyclic phosphorothioates resulting from hydrolysis reactions of unknown stereochemical course.

Monoclonal antibody production to human and bovine 2':3'-cyclic nucleotide 3'-phosphodiesterase (CNPase): high-specificity recognition in whole brain acetone powders and conservation of sequence between CNP1 and CNP2

Monoclonal antibodies against human and bovine 2':3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) were generated by fusing FOX-NY myeloma cells with spleen cells from RBF/Dn mice previously immunized with the purified brain antigens. The enzyme isolated from bovine brain was quite basic, with an isoelectric point of 9.71 and both the bovine and human enzymes consisted of a closely spaced doublet at approximately 44 and 46 kDa on SDS-PAGE. Six monoclonals were were identified as strongly recognizing the enzyme on both ELISA plates and on immunoblots of whole brain protein. Four monoclonals very weakly cross-reacted with guinea pig myelin basic protein. In contrast with two previous reports, some of our monoclonal antibodies did immunostain 2 or 3 protein bands in peripheral nerve, two bands closely corresponding to those immunostained in central nervous system (CNS) myelin, the Wolfgram protein fraction and in acetone powders of whole brain. Each of the 6 monoclonals reacting strongly on immunoblots recognized the enzyme in from 2 to 5 of the species examined (human, bovine, rat, mouse and rabbit). In addition, all 6 monoclonals that immunostained the enzyme in whole brain, myelin and Wolfgram protein immunoblots recognized both CNP1 (44 kDa) and CNP2 (46 kDa). The two closely spaced protein bands observed on SDS-PAGE and previously stained on immunoblots of CNS CNPase using polyvalent rabbit anti-bovine CNPase antisera, and now different monoclonal antibodies, appear to be immunologically related and to contain highly conserved sequences.

Inhibition of bovine and human brain 2':3'-cyclic nucleotide 3'-phosphodiesterase by heparin and polyribonucleotides and evidence for an associated 5'-polynucleotide kinase activity

The present paper establishes a 5'-polynucleotide kinase activity associated with the bovine and human brain enzyme 2':3'-cyclic nucleotide 3'-phosphodiesterase (EC 3.1.4.37) in addition to known extremely high hydrolysis rates against 2':3'-cyclic nucleotides. Modulation of the enzyme activity by the addition of polyadenylate (5') and polyuridylate (5'), histone F3, myelin basic protein (MBP), and other basic molecules suggest that RNA may be the natural substrate for both enzymes. These enzymes, isolated from brain and present in very high activities in oligodendrocytes and in isolated myelin, probably have complex functions.

Studies of the in vitro effect of methylmercury chloride on rat brain neurotransmitter enzymes

The in vitro effect of methylmercury (MM) on the enzymatic activities of brain cell specific marker enzymes, choline acetyltransferase (CAT), glutamic acid decarboxylase (GAD), 2',3'-cyclic nucleotide phosphohydrolase (CNP), glutamine synthetase (GS) and enolase was examined. The results demonstrate that at 100 microM MM, GS activity was not affected whereas a small decrease in the activity of both GAD (20%) and enolase (10%) was observed. CNP and CAT activity appeared to be more sensitive toward MM with 100 microM MM producing inhibition of 50% and 30%, respectively. The addition of sulfhydryl protecting reagents such as DTT or sodium thioglycolate can restore the enzyme activities to normal control levels despite prior exposure of the enzymes to MM.

Defective biosynthesis of proteolipid protein in Pelizaeus-Merzbacher disease

The brain of an 18-year-old patient with Pelizaeus-Merzbacher disease was examined by standard neuropathological and biochemical methods and by immunocytochemical and immunochemical techniques. Analysis revealed a lack of myelin-specific lipids, but showed a residual immunoreactivity for myelin basic protein, myelin-associated glycoprotein, and 2',3'-cyclic nucleotide-3'-phosphodiesterase. Examination by immunocytochemistry and enzyme-linked immunosorbent assay showed an absence of proteolipid apoprotein (lipophilin). The peripheral nervous system was normal. Pelizaeus-Merzbacher disease in humans shares many neuropathological and biochemical features with X-linked mutations in animals, e.g., the jimpy mouse and myelin-deficient rat. The specificity of this protein deficiency in Pelizaeus-Merzbacher disease gains additional support from the recent mapping of the lipophilin gene to the human X chromosome.

The response of optic tract glia during regeneration of the goldfish visual system. I. Biosynthetic activity within different glial populations after transection of retinal ganglion cell axons

We monitored biosynthetic activity of optic tract glia during regeneration of retinal ganglion cell axons in the goldfish and found that the greatest level of incorporated [3H]thymidine and [3H]leucine occurred in glia by 10-15 days after axotomy. During this period there was a marked increase in the number of oligodendroglia and multipotential glia near the site of injury with no change occurring in the astroglial population. Electron microscopic autoradiography showed that oligodendroglia and multipotential cells incorporated 5-7-fold more thymidine than did cells of intact control preparations. Though all glial cell types incorporated more [3H]leucine during axonal regeneration, oligodendroglia and multipotential cells together accounted for more than 90% of measured radioactivity. In order to characterize glial-stimulating events specific to axonal regeneration, we produced axonal degeneration in the optic tract by removal of the retina. Optic tract glia during axonal degeneration incorporated less amino acid when compared to glia associated with regenerating axons. The degenerating optic tract also had less 2',3'-cyclic nucleotide 3'-phosphohydrolase, an enzyme produced by oligodendroglia, than that found in the regenerating visual system. Our results suggest that in response to ganglion cell axotomy oligodendroglia and multipotential glia of the goldfish optic tract proliferate. Moreover, regenerating axons provide one type of stimulant for glial protein biosynthesis.

Differential expression of 2':3'-cyclic nucleotide 3'-phosphodiesterase in cultured central, peripheral, and extraneural cells

The relative levels of the central nervous system myelin marker enzyme 2':3'-cyclic nucleotide 3'-phosphodiesterase (EC 3.1.4.37, CNPase) were determined in neuroblastoma, astrocyte, oligodendrocyte and Schwann cell cultures and in freshly isolated human lymphocytes and platelets. The highest specific activities were associated with the cells that elaborate myelin membrane in the central and peripheral nervous system, oligodendrocytes and Schwann cells, respectively. Antiserum to bovine CNPase recognized both CNP1 and CNP2 in CNS myelin and human oligodendroglioma. In addition, a 53,000 dalton protein was evident on autoradiographs of immunoblotted PNS myelin and human oligodendroglioma proteins. Cultured rat oligodendrocyte, C6 and mouse NA neuroblastoma CNPase appear to share common determinants with the corresponding normal rat CNS enzyme.

Simultaneous measurement of 2':3' cyclic-nucleotide 3' phosphodiesterase and RNase activities in sera and spinal fluids of multiple sclerosis patients

The enzymes 2':3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) and RNase were simultaneously measured in the sera and CSF of multiple sclerosis (MS) and non-MS patients. No evidence of increased activity for these enzymes could be found regardless of pathology in either fluid source. Discrepancies between the present results and those from two previous studies that reported significant increases in CNPase activity in the CSF of patients with MS were carefully analyzed. It was concluded that the apparent increased CNPase activity correlated with MS in both previous studies was most probably the result of methodological and computational difficulties.

Immunochemical identification of 2', 3'-cyclic nucleotide 3'-phosphodiesterase in central and peripheral nervous system myelin, the Wolfgram protein fraction, and bovine oligodendrocytes

Myelin proteins and the total Wolfgram protein fraction were isolated from the CNS of several mammalian species and characterized with rabbit anti-bovine 2', 3'-cyclic nucleotide 3'-phosphodiesterase (CNP) antisera after sodium dodecyl sulfate-polyacrylamide gel electrophoresis and electrophoretic transfer to nitrocellulose membranes. The corresponding CNP proteins cross-reacted across all species examined, suggesting that the CNP amino acid sequence was fairly well conserved in all six species. The same corresponding proteins were also identified immunochemically in the crude total Wolfgram protein fraction in the region of the W1 myelin protein, thus further supporting and extending two different previous reports indicating a relationship between CNP and the W1 protein. In addition to these CNS enzyme sources, peripheral nervous system CNP (rabbit and rat sciatic nerve) was also recognized by these same rabbit anti-bovine (CNS) CNP antisera. CNP was also detected in freshly isolated delipidated bovine oligodendrocyte membranes. These results suggest that rabbit anti-bovine CNP antisera may be of use in localization and structural studies of this enzyme in several different species and will permit clear identification of CNP in oligodendrocytes and their isolated membrane fractions.

Localization of 2',3'-cyclic nucleotide-3'-phosphohydrolase within the vertebrate retina

Histochemical and immunochemical techniques are used to locate 2',3'-cyclic nucleotide-3'-phosphohydrolase (2',3'-cNMP-3'ase) within cells of the vertebrate retina. A new histochemical method is described which links the hydrolysis of 2',3'-cNADP to the formation of a reduced, insoluble tetrazolium formazan. Photoreceptors from fish, bovine, and rat retinas are stained by this procedure. The reaction is blocked by 2'-AMP, a known inhibitor of 2',3'-cNMP-3'ase. Rabbit antibodies prepared against 2',3'-cNMP-3'ase from bovine brain are found to cross-react with bovine and rat retinal enzymes. Peroxidase-labeled antibody shows by light microscopy the greatest staining along the inner segments of the photoreceptors. Electron microscopy of the same preparations confirms binding to the plasma membrane of the inner segments of both rods and cones. Retinal 2',3'-cNMP-3'ase is thus predominantly associated with the photoreceptors, suggesting some role for 2',3'-cyclic nucleotides as substrates in visual function.

A histochemical method for demonstration of 2',3'-cyclic 3'-phosphohydrolase activity

A histochemical method for demonstration of 2',3'-cyclic nucleotide 3'-phosphohydrolase activity is proposed. Cryostat sections, fixed in chlorophorm-methanol mixture, are incubated in a solution containing cyclic adenosine 2',3'-monophosphate, acid phosphatase, lead nitrate in acetate buffer, pH = 6,4 or 6.5. This method is based on conversion by the 2',3'-cyclic nucleotide 3'-phosphohydrolase of cyclic adenosine 2',3'-monophosphate to nucleotide-2'-monophosphate. Under the action of exogenous acid phosphatase, inorganic phosphate is released. It precipitates as lead phosphate if lead ions are present. By adding yellow ammonium sulphide, the lead phosphate is converted to lead sulphide which is the visible reaction product.

Inhibition of 2',3'-cyclic nucleotide 3'-phosphodiesterase activity from bovine, human and guinea pig brain

The purified oligodendrocyte enzyme 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) obtained from bovine, human and guinea pig brain was inhibited by theophylline, caffeine, cupric chloride and organomercurials using a spectrophotometric assay and 1 mM 2',3'- cyclic cytidylate as the substrate. The observed inhibition by theophylline, caffeine and organomercurials was reversible.

Characterization of 2':3'-cyclic nucleotide 3'-phosphodiesterase: limited proteolytic digestion, plant lectin affinity chromatography, and immunological identification

Purified bovine brain 2':3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) migrates as a protein double band in SDS-polyacrylamide gel electrophoresis. The positions of the two protein bands correspond to approximate molecular weights (MW) of 56,000 and 53,000. Limited protease treatment of isolated CNPase leads to subsequent degradation of the enzyme into smaller polypeptides having MWs of approximately 40,000, 30,000, and 20,000. During proteolytic digestion CNPase remains enzymatically active. Binding studies with several immobilized plant lectins as well as periodic acid-Schiff reagent (PAS) staining of SDS gels indicate that CNPase is a glycoprotein. An antiserum against purified CNPase, prepared in rabbits, was used to confirm the immunological identity of various CNPase preparations obtained in our laboratory.

Characterization of 2':3'-cyclic nucleotide 3'-phosphodiesterase: rapid isolation, native enzyme analysis, identification of a serum-soluble activity, and kinetics

The enzyme 2':3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) was isolated from bovine brain white matter by a rapid (72 h) procedure. The minimum molecular weight (MW) of the enzyme was approximately 52,500 as estimated by sucrose density gradient analysis. When this isolated enzyme was stimulated with bovine serum albumin (BSA), the peak of activity was shifted to approximately 90,000 MW. Prior treatment by trypsin blocked the expression of the higher MW form of CNPase, but not the BSA activation of the enzyme. If the trypsin digestion was allowed to progress, the MW was gradually lowered to a broad peak sedimenting between 20,000 and 50,000 MW. An apparently soluble form of CNPase found in serum is described. Kinetic and MW comparisons between the serum soluble enzyme and CNPase isolated from bovine brain, as well as an analysis of substrate specificity, were made and it was concluded that the two enzymes were identical.

Purification of rat 2',3'-cyclic nucleotide 3'-phosphodiesterase

2',3'-Cyclic nucleotide 3'-phosphodiesterase (CNP, EC 3.1.4.37) has been isolated from rat brain myelin by chromatography on successive columns of phenyl-Sepharose CL-4B, CM-Sepharose CL-6B, and 8-(6-aminohexyl) amino-2'AMP-Sepharose 4B. From 15 g of rat brain, approximately 400 micrograms of pure CNP was obtained, with a specific activity of 1,200 (2',3'-cyclic AMP) units/mg protein. The Km of the rat enzyme was 3.7 mM, using 2',3'-cAMP as the substrate. Isoelectric focusing of the enzyme indicated a broad isoelectric range of 8.5-9.0. On SDS polyacrylamide gels, rat CNP appears as two protein bands of approximately 48,000 and 50,000 M.W., with an upper band intensity of about 1/10 that of the lower band. The relative intensities of the bands for CNP and the molecular weights correspond to the Wolfgram proteins W1 and W2 described by other investigators. The amino acid analysis of the purified rat enzyme compared favorably with reported determinations for the bovine enzyme and also with reported values for the rat Wolfgram proteins W1 and W2.

Evidence for the association of 2',3'-cyclic-nucleotide 3'-phosphodiesterase with myelin-related membranes in peripheral nervous system

In PNS, the specific activity of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) in myelin was not enriched over the starting homogenate. Nevertheless, most of the total activity was recovered in myelin. In myelin-deficient mutants, low CNP activities were measured in sciatic nerves. CNP specific activities were similar in myelinated and non-myelinated nerves but in non-nervous tissues, they were significantly lower than in nervous tissue. There was no indication for the presence of an isoenzyme of CNP in peripheral nerves. These results indicate that CNP is present in PNS myelin and preferentially localized in Schwann cell plasma membranes.

Comparison of 2',3'-cyclic nucleotide 3'-phosphodiesterase and the major component of Wolfgram protein W1

Wolfgram protein preparations from myelin are heterogeneous mixtures having major components with molecular weights in the 43,000 to 60,000 range. Through the use of an SDS-slab gel electrophoretic system of high resolving power, the Wolfgram 1 (W1) component described by Nussbaum et al. (1977) has been resolved into two components; bovine W1a and W1b proteins are found to have mobilities identical to those of the two 2',3'-cyclic nucleotide 3'-phosphodiesterase (2'3'-CN 3'-ase) components, CNa and CNb (Drummond, 1979). Crossed immunoelectrophoresis with bovine 2',3'-CN 3'-ase demonstrates that CNa and CNb are antigenically indistinguishable. Rat and bovine Wolfgram proteins W1a and W1b, but not the W2 proteins, display cross reactivity with antiserum directed against bovine 2',3'-CN 3'-ase. Chymotryptic and Staphylococcus aureus V8 protease peptide profiles indicate structural similarities between proteins CNa-W1a and CNb-W1b. It is concluded that an inactivated form of 2',3'-Cn 3'-ase is a major component of Wolfgram protein W1. These results are consistent with the studies on the histochemical localization of rat W1 protein by Roussel et al. (1977, 1978).

Studies on the Wolfgram high molecular weight CNS myelin proteins: relationship to 2',3'-cyclic nucleotide 3'-phosphodiesterase

Evidence is presented that the major protein components of the high molecular weight CNS myelin proteins designated as the Wolfgram protein doublet (W1 and W2) contain the enzyme 2',3'-cyclic nucleotide 3'-phosphodiesterase (EC 3.1.4.37, CNP). CNP is a basic hydrophobic protein containing about 830 to 840 amino acid residues. When electrophoresed on SDS polyacrylamide gels, CNP appears as a protein doublet, separated by a molecular weight difference of about 2500-3000 in bovine, human, rat, guinea pig, and rabbit. A similar protein doublet has been identified as the Wolfgram proteins W2 and W1 in myelin and in the chloroform-methanol-insoluble pellet obtained from myelin. Moreover, the relative Coomassie blue staining intensity of the CNP2 plus CNP1 protein doublet among the species examined was remarkably similar to that observed for electrophoresed myelin and chloroform-methanol-insoluble pellet derived from myelin. Antisera raised against purified bovine CNP recognized the W1 and W2 proteins isolated from bovine and human brain. The amino acid composition of pure bovine CNP is presented and compared with the compositions of several rat and bovine Wolfgram proteins obtained by other investigators. Our electrophoretic, compositional, and immunological data support the contention that the enzyme CNP is a major component of the Wolfgram protein doublet.

Solubilization of 2',3'-cyclic nucleotide 3'-phosphohydrolase from bovine brain without detergents

The enzyme in brain that hydrolyzes 2',3'-cyclic nucleotides to the 2'-mononucleotides has been found by several authors to be concentrated in the myelin fraction. To facilitate further study of the enzyme, one of our objectives has been to develop a method of solubilizing the enzyme without the use of detergents. When an acetone powder from brain white matter is homogenized with 1 M guanidinium chloride in 0.2 M buffer at pH 6, 10(-3) M in EDTA and in dithiothreitol, the enzyme is solubilized. If the guanidinium chloride is removed by dialysis in a single step, the enzyme reprecipitates, but if a fractional precipitation is performed by reducing the guanidinium chloride concentration by dilution, the enzyme remains in solution at 0.2 M guanidinium chloride. The precipitates obtained in this fractionation probably contain constituents which, at low salt concentration, formed a part of an insoluble aggregate, since after removal of the pellet the supernatant solution can be dialyzed free of guanidinium chloride without precipitating the enzymic activity. The enzyme thus prepared remains in the supernatant when centrifuged at 108,000 X g for 3 h and can be submitted to (NH4)2SO4 fractionation and chromatography on carboxymethyl-Sephadex and on hydroxylapatite. The enzyme has thereby been purified 200-fold in about 20% yield.