Enzymatic Iodination of Sindbis Virus Proteins

Sindbis virus was iodinated by using the enzyme lactoperoxidase, an iodination technique which labels only surface proteins. By this technique, the two viral glycoproteins are labeled, and the internal viral protein is not. The two glycoproteins are iodinated to strikingly different extents. This difference in susceptibility to iodination apparently is due to the position or conformation of the glycoproteins in the envelope spikes of the virion and not to differing contents of tyrosine, the amino acid substrate of lactoperoxidase. Both viral glycoproteins are iodinated by lactoperoxidase on the surface of Sindbis-infected chicken cells. Here, as in the virion, the glycoproteins are iodinated unequally, with the smaller glycoprotein again being preferentially iodinated. Another virus-specific protein found in large amounts in infected cells, and from which the preferentially iodinated virion glycoprotein is produced by a proteolytic cleavage, is not iodinated by lactoperoxidase. Thus it appears that the viral glycoproteins are present on the cell surface and that the precursor protein is not.     

Iodination (125I) of the apical plasma membrane of toad bladder epithelium: Electron-microscopic autoradiography and physiological effects

Summary The apical plasma membrane of toad bladder epithelial cells has been enzymatically iodinated, using lactoperoxidase, H₂O₂ (generated by a glucose-glucose oxidase system) and NaI. The site of labeling was demonstrated by electron-microscopic autoradiography; the silver grains (¹²⁵I) were found exclusively overlying the luminal plasma membranes of the epithelium. The iodination reaction reached completion in less than 5 min. The dependence of the degree of iodination on NaI concentrations (range=6.3×10^–8 to 6.3×10^–2 m) in the mucosal medium was determined. The results suggest that three classes of sites are iodinated within this concentration range. At concentrations of NaI of 6.3×10^–6 m or less, iodination of the apical membrane had no significant effect on either the fine structure of the epithelium or on electrophysiological properties. The baseline short-circuit current (SCC) remained steady and the response to vasopressin was unimpaired. At concentrations of 6.3×10^–5 m NaI and greater, the baseline SCC was depressed and the response to vasopressin was partially inhibited. The results indicate that¹²⁵I may serve as a covalent marker (specific for tyrosine and histidine residues) of the apical plasma membrane of epithelia.     

Lactoperoxidase-catalyzed iodination of surface membrane lipids

Lactoperoxidase-catalyzed iodination of intact cells, is known to label predominantly, if not exclusively, the exposed tyrosine residues of cell surface proteins. The present study demonstrates that during this iodination process surface membrane lipids are also iodinated through an enzyme-dependent step. Phosphatidylcholine, cholesterol-phosphatidylcholine liposomes and confluent secondary cultures of chick embryo cells were iodinated by the lactoperoxidase-glucose oxidase-glucose [¹²⁵I] procedure. Liposomes were efficiently labeled. In the cells, 20–30% of the radioactivity was found in proteins and 20–30% in the lipids. Both neutral and polar lipids were found to bind [¹²⁵I] covalently. Controls in which lactoperoxidase was omitted showed < 6% of the radioactivity found in liposomes or cells labeled with the enzyme.     

Studies on the mechanism of the iodination of tyrosine by lactoperoxidase

Studies with lactoperoxidase showed that a highly reactive intermediate is produced (on the enzyme) from I- and H2O2 which then diffuses from the enzyme and very rapidly and indiscriminately iodinates any Tyr or peptides containing Tyr which are in the same solution. The evidence supporting these conclusions follows. 1) The rate followed the Michaelis-Menten pattern with I- and H2O2 while the concentration of Tyr peptides had no measurable effect on the rate; 2) the rates of reaction were independent of the type of peptide in which Tyr was located; 3) the amount of iodination which had occurred after the reaction had gone to completion and the amounts of monoiodination and diiodination after completion of the reaction were independent of the peptide type, the pH, the solvent polarity, or the ionic strength; 4) competition for reaction by two very different Tyr peptides depended only on their initial concentrations; and 5) iodination of a large protein occurred through a dialysis membrane. Free Tyr was iodinated at the same rate as Tyr peptides by lactoperoxidase, but monoiodotyrosine and m-fluorotyrosine were iodinated at one-half that rate. The results also showed that one can choose ratios of [peptide] to [H2O2] such that monoiodination is maximized relative to diiodination. It was also found that the iodination capacity of a mixture of I- and H2O2 with lactoperoxidase (when Tyr was absent) was only slowly dissipated. Finally, the results showed that lactoperoxidase can be used to brominate and chlorinate Tyr peptides at a slow rate.     

Lactoperoxidase-Catalyzed Iodination of Plasmamembrane Lipids and Proteins in Arabidopsis Protoplasts

Lactoperoxidase-catalyzed iodination was evaluated as a techniqueto identify plasma membrane components in protoplasts isolatedfrom leaf tissue of Arabidopsis thaliana (L.) Heynh. Restrictionof the probe to the plasma membrane was assessed by determiningradioactivity in lipids characteristic of intracellular organellesafter labeling intact versus broken protoplasts. Intact protoplastswere purified on a sucrose-sorbitol gradient and iodinated usingNa[¹²⁵I] with lactoperoxidase and H₂O₂. Lipids as well as proteinswere shown to be iodinated in a lactoperoxidase-catalyzed reaction.Labeling of intracellular lipids was low in the intact protoplastswhich indicated that the iodination reaction was restrictedto the plasma membrane. (Received March 18, 1988; Accepted May 25, 1988)     

The pheparation and characterization of mono-iodinated photoreactive analogs of insulin

Photoreactive derivatives of insulin (B29-(p-azidobenzoyl-insulin) iodinated primarily in either the B₂₆ or A₁₄ tyrosine of insulin were prepared by lactoperoxidase catalyzed iodination followed by separation on reverse-phase high-performance liquid chromatography. The binding affinities and photoaffinity labeling characteristics of these derivatives were studied in isolated rat adipocytes. Under nonreducing conditions, three forms of the insulin receptor were labeled equally by the B₂₆-derivative, the A₁₄-derivative, and the mixture of the iodinated derivatives. When dithiothreitol was used to reduce the radiolabeled receptors, the radioactivity associated with the binding subunit was much less than that in the intact receptor and the magnitude of the decrease was proportional to the amount of iodine in the A chain of the photoderivatives. Use of the photoreactive derivative iodinated primarily in the B₂₆ position resulted in greater labeling of insulin receptor subunits since most of the radioactivity (80%) remained associated with the receptor upon reduction.     

HPLC–MS analysis of iodotyrosines produced by sample hydrolysis: A simple method for monitoring iodinated casein in feed premixes

A new and simple HPLC–MS method was developed for monitoring iodinated casein in feed premixes. In this method, feed premixes were hydrolyzed, and the iodotyrosines thus released were analyzed. Sample pretreatment included precipitation of transition metals ions with Na₂S, hydrolysis with sodium hydroxide, and cleaning up with an Oasis SAX cartridge. Gradient elution was carried out on a C₁₈ column with water (containing 0.1% formic acid) and acetonitrile. Ion detection was performed using ESI positive SIM at m/z 262, 308, 388, and 434. Iodinated casein levels were monitored by qualitative analysis of the iodotyrosines released upon sample hydrolysis and by quantifying the 3,5-diiodotyrosine released. The validation data demonstrated that the method was selective and sensitive (≤0.2 mg g^?1) for iodinated casein and had acceptable accuracy (recoveries: 81.3–106.7%) and precision (RSD: 1.7–16.0%).     

The effect of ruthenium red on the assembly and disassembly of microtubules and on rapid axonal transport

The assembly of microtubules was found to decrease in proportion to the amount of added ruthenium red, indicating a high affinity of ruthenium red for the microtubule system. An equimolar amount of ruthenium red per tubulin dimer inhibited the microtubule assembly completely and disassembled existing microtubules. Binding of ruthenium red to tubulin is accompanied by a shift in the absorption maximum from 535 to 538 nm. The binding is very strong, as shown by the finding that ruthenium red could not be displaced from tubulin by gel chromatography on Sephadex, or by the addition of Ca²⁺ or Mg²⁺. The binding of ruthenium red to tubulin did not affect the single colchicine site, nor the Mg²⁺ site(s), as shown by use of Mn²⁺ as an EPR probe. Ruthenium red also interfered with microtubules in an intact cell system, as it inhibited rapid axonal transport in the frog sciatic nerve, measured by the accumulation of [³H]leucine-labelled proteins in front of a ligature.     

Polymerization of tubulin in the presence of colchicine or podophyllotoxin. Formation of a ribbon structure induced by guanylyl-5'-methylene diphosphonate

GTP-dependent in vitro polymerization of rat brain microtubular protein is inhibited to 50% by substoichiometric concentrations of the antimitotic drugs colchicine (0.12 mol/mol of tubulin) and podophyllotoxin (0.14 mol/mol of tubulin). Substitution of pp(CH₂)pG² for GTP, however, results in an extensive microtubular protein polymerization at such concentrations. In the presence of pp(CH₂)pG, suprastoichiometric concentrations of podophyllotoxin (19 mol/mol of tubulin) are required to inhibit the polymerization process by 50%. Colchicine is very ineffective since 3 × 10⁵ moles/mole of tubulin are required to give a 50% inhibition. Electron microscopical analysis shows that the polymers formed by microtubular protein in the presence of suprastoichiometric concentrations of drugs are not the normal short microtubules typical of pp(CH₂)pG-driven polymerization, but are ribbons with three or four protofilaments. The colchicine content of the harvested ribbons has been measured directly and found to be approximately 0.8 moles colchicine/mole of tubulin. Treatment of microtubular protein with substoichiometric concentrations of drugs results in an increase in the number of protofilaments forming the ribbons. Many of the ribbons can close into morphologically normal microtubules when microtubular protein is treated with only 0.05 moles of either colchicine or podophyllotoxin per mole of tubulin.     

Visualization of lactoperoxidase binding to microtubule and tubulin

The binding of lactoperoxidase to microtubules and tubulin was shown in both electron micrography and polyacrylamide gel electrophoresis by tracing the enzymatic activity of lactoperoxidase. Lactoperoxidase bound to purified microtubules appeared to distribute evenly on the surface without forming special structures. Both alpha and beta-tubulin separated by sodium dodecylsulfate-polyacrylamide gel electrophoresis bound lactoperoxidase, and could be detected by the use of lactoperoxidase reaction. Electrophoretic study revealed that the interaction between lactoperoxidase and tubulin were not strictly specific and a variety of proteins other than alpha- and beta-tubulin, including actin and neurofilament subunits, bound lactoperoxidase.     

Some features of the vinblastine-induced assembly of porcine tubulin.

Porcine tubulin precipitated by 10^?3, m vinblastine (VLB) contains approximately 0.50 molecule of VLB bound per 110,000-molecular-weight tubulin dimer. The amount of precipitate, followed by turbidity, is a linear function of the initial tubulin concentration. The rate of precipitation is roughly first order in protein concentration. Vindoline and velbanamine halves of VLB are ineffective separately or together in producing the tubular aggregates observed for VLB precipitates by electron microscopy. At 10^?3, m concentrations no turbidity is observed nor is there any competition with VLB-induced turbidity. Removal of GTP from tubulin by dialysis or incubation of tubulin in the absence of added GTP blocks VLB-induced assembly. Readdition of GTP at room temperature or above restores sensitivity to VLB precipitation. The β,γ methylene analog of GTP cannot substitute for GTP in this process. About 0.7 mol of added GTP is found bound per mole of tubulin dimer. During the course of VLB-induced assembly, roughly half of this GTP is displaced. These results show interesting similarities and differences in the VLB-induced assembly of tubulin and the normal in vitro assembly of microtubules. Further comparisons between both assembly processes should be useful.     

ON THE SPATIAL ARRANGEMENT OF PROTEINS IN MICROSOMAL MEMBRANES FROM RAT LIVER

Rat liver rough microsomes were labeled enzymatically with ¹²⁵I using lactoperoxidase and glucose oxidase. In intact microsomes only proteins exposed on the outside face of the microsomal membrane were iodinated. Low concentrations of detergent (0.049% deoxycholate) were used to allow entrance of the iodination system into the vesicles without disassembling the membranes. This led to iodination of the soluble content proteins and to an increased labeling of the membrane proteins. The distribution of radioactivity in microsomal proteins was analyzed after separation by sodium dodecyl sulfate acrylamide gel electrophoresis. Most membrane proteins were labeled when intact microsomes were iodinated. No major membrane proteins were exclusively labeled in the presence of low detergent concentrations or after complete membrane disassembly. Therefore it is unlikely that there are major membrane proteins, other than glycoproteins, present only on the inner membrane face or completely embedded within the microsomal membrane. Microsomal proteins were also labeled by incubating rough microsomes with [³H]-NaBH₄ after reaction with pyridoxal phosphate. Microsomal membranes were permeable to these small molecular weight reagents as shown by the fact that proteins in the vesicular cavity as well as membrane proteins were labeled with this system.     

Radioiodination of brain tubulin with Bolton-Hunter reagent

Radio-iodination of tubulin can be achieved by Bolton-Hunter reagent both in the absence and presence of microtubule associated proteins. Specific radioactivities as high as 400 C_i/mmole tubulin dimer can be obtained, i.e. an average of 0.2 molecule of reagent is bound per molecule of tubulin. About 80 % of the [¹²⁵I]- labelled tubulin keeps its ability to assemble in microtubules and polymerizes with the same critical concentration as the native tubulin, which makes the method adequate for preparing tracer tubulin useful for in vivo and in vitro studies. Both α and β subunits are labelled, 60 % of the radiolabel being bound to the β subunit.     

Iodination of H-2 and TL with a coupled insoluble enzyme system: kinetics and effects on antigenic activity.

The murine alloantigen TL and H-2^a were iodinated using a coupled insolubilized enzyme system of lactoperoxidase and glucose oxidase. The kinetics of iodination were followed and it was found that maximum specific activity was achieved between 15 and 20 minutes. Antigenic activities were followed by inhibition of cytotoxicity. Under the conditions described there was little effect on antigenic activities, yet high specific radioactivities were obtainable.     

Subcellular localization of iodinated thyroid tubulin

Subcellular fractions enriched in mitochondria, plasma membranes, microsomes and Golgi apparatus were obtained from thyroid glands of rats injected with I¹²⁵. Autoradiography of SDS-polyacrylamide gels revealed the presence of a number of radiolabelled proteins in each membrane fraction. One polypeptide, with the same electrophoretic mobility as brain tubulin, was found in all fractions except the plasma membranes and was immunoprecipitated with commercial anti-tubulin monoclonal antibodies. Hydrolysis of Asp-Pro linkages of I¹²⁵ labelled tubulin with formic acid indicated that there were iodination sites in both the carboxy terminal one third and the amino terminal two thirds of the molecule. These results, together with the absence of iodinated tubulin from the cytosolic fraction, are consistent with the idea that a population of thyroid membrane tubulin is iodinated at multiple sites either just before or after insertion into intracellular membranes where it may act as an anchorage point for microtubule-membrane interactions.     

Involvement of tryptophan residues in colchicine binding and the assembly of tubulin

Chemical modification of tubulin with 2-hydroxy-5-nitrobenzyl bromide, a reagent selective for tryptophan, inhibits tubulin's colchicine binding and in vitro assembly activities. Loss of colchicine binding shows a linear relationship with the modification of tryptophan residues, and is complete when not more than five residues are modified. GTP affords partial protection against this loss of colchicine binding. The in vitro assembly of tubulin is somewhat less sensitive, since microtubules are formed from tubulin dimers possessing 3–4 but not five modified residues. Furthermore, two of the eight tryptophans per dimer are reactive when tubulin is assembled into microtubules.     

Lactoperoxidase-catalyzed iodinaton of cell cultures infected with mycoplasma

Summary Hamster BHK cells or secondary cultures of mouse embryo fibroblasts are not iodinated by lactoperoxidase in the absence of hydrogen peroxide. When such cell cultures are infected with a noncultivable strain ofM. hyorhinis, endogenous peroxide generation is sufficient to permit nearly maximal iodination. The SDS-polyacrylamide gel pattern of iodinated cell surface polypeptides is essentially the same regardless of the source of peroxide and whether or not the cultures are infected with mycoplasma.     

Binding of glycerol by microtubule protein.

When microtubules are purified by polymerization and depolymerization in a buffer containing glycerol, some glycerol becomes bound to the microtubule protein and is not removable by gel filtration or by prolonged dialysis. Both 6s tubulin and larger aggregates containing tubulin and accessory proteins bind glycerol. The 6s fraction has associated with it about 5 moles of glycerol per mole of tubulin dimer; 3 moles are exchangeable upon polymerization-depolymerization and 2 moles are not. The aggregate fraction has associated with it about 22 moles of glycerol per mole of tubulin dimer; approximately 11 moles are exchangeable and 11 moles are not.     

An enzymatic solid-phase method for trace iodination of proteins and peptides with 125-iodine.

Solid-phase methodology has previously been applied to labeling of proteins and peptide hormones used in immunoassay with the aid of enzyme sorbent. In this publication a method based on the use of a new carrier-copolymer of maleic anhydride and butanediol divinylether is introduced. As a model, bovine serum albumin (BSA) was labeled using three different procedures: Chemical, with chloramine-T as oxidizing agent: enzymatic, in a liquid phase with lactoperoxidase (LP) and horseradish peroxidase (HRP); and enzymatic, in a solid phase with maleic anhydride butanediol divinylether-copolymer as the carrier of lactoperoxidase and horseradish peroxidase.The lactoperoxidase-mediated iodinating activity in both the liquid and solid phases was similar (incorporating 47 and 39% for the total ¹²⁵iodine added, 1 $\text{mCi}\text{10 μ}\text{g BSA}$), while HRP was more efficient in a liquid (11%) than in a solid phase (3%).Although the specific activity of the BSA labeled with chloramine-T was highest, this ¹²⁵I-labeled BSA was badly degraded during iodination. However, in either liquid or solid phase enzymatic iodinations, no degradation of the protein could be observed.Peptide hormones, luteinizing hormone, follicle-stimulating hormone and angiotensin II, iodinated with lactoperoxidase or lactoperoxidase sorbent for radioimmunoassays reacted better than peptide hormones iodinated with chemical oxidants and remained unaltered during storage.