High-efficiency passive elution of bacterial lipopolysaccharides from polyacrylamide gels

We recently described a method for recovering polyacrylamide-gel-separated bacterial lipopolysaccharides (LPS) based on the sensitive on-gel LPS detection (1-10 ng/band) with zinc-imidazole followed by passive elution from 32 microm average size gel microparticles into water. With this procedure, the recovery of rough- or semismooth-type LPS after 3 h elution is about 70-80%, while that of smooth LPS is only about 10%. Here we evaluated whether a simple replacement of water with other eluents would enhance the passive diffusion of LPS. We found that solutions of the detergents sodium dodecyl sulfate (SDS), sodium deoxycholate (DOC) and Triton X-100, or mixtures of the organic solvents acetonitrile and triethylamine and water, increased the recovery of a smooth LPS band from Vibrio cholerae O1 in a concentration-dependent manner. Furthermore, a quantitative recovery of rough or smooth LPS from V. cholerae O1, Escherichia coli O111:B4, E. coli K-235, or Serratia marcescens was feasible in 1% SDS or DOC after 3 h or in 5% triethylamine after only 2 min. A simple dilution of SDS or DOC or evaporation of triethylamine rendered the eluted LPS preparations compatible with biochemical activity determination, as tested by Limulus amebocyte lysate assay. Thus, this improved micropurification method may be a suitable interface between analytical gel electrophoresis and further characterization or use of LPS.

A mutant streptokinase lacking the C-terminal 42 amino acids is less reactive with preexisting antibodies in patient sera

Streptokinase (SK) is an efficacious thrombolytic drug for the treatment of myocardial infarction. Because of its immunogenicity, patients receiving SK therapy develop high anti-SK antibody (Ab) titers, which might provoke severe allergic reactions and neutralize SK activity. In this report we studied the reactivity of a synthetic 42-residue peptide resembling SKC-2 C-terminus with patient sera. SKC-2(373-414) peptide was recognized by 39 and 64% of patients, before and after SKC-2 therapy, respectively. An SKC-2 deletion mutant (mut-C42), lacking the same 42 C-terminal residues, was constructed and expressed in Escherichia coli. Recognition of mut-C42 by preexisting Abs from patient sera was 51 and 68% of reactivity to SKC-2, as assessed by direct binding and competition assays, respectively. For most of the patients, mut-C42-neutralizing activity titer (NAT) significantly decreased with respect to SKC-2-NAT. This study opens the possibility of producing a less immunogenic variant of SK, which could constitute a preferred alternative for thrombolytic therapy.

Mice immunization with gel electrophoresis-micropurified bacterial lipopolysaccharides

Some evidence on the possible use of sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) to elicit antibodies against smooth- or rough-type bacterial lipopolysaccharides (LPS) is shown. Gel-separated LPS were negatively stained with zinc-imidazole to precisely localize the bands of interest under fully reversible conditions. Then the bands of interest were excised and the resulting gel slices washed in a solution of a zinc-complexing agent (e.g., 100 mM EDTA), after which they were extruded through a metal sieve of 32 microm average size contained in a 1 mL syringe, to generate homogeneous gel microparticles. The LPS-containing gel slurries were used directly to immunize female BALB/c mice. Using this procedure, positive mouse polyclonal antibody responses against gel-purified smooth- or rough-LPS forms from Escherichia coli K-235 or Bordetella pertussis were elicited, as tested by a dot-immunoblotting assay. Our results may encourage the use of SDS-PAGE-micropurified LPS to develop optimized immunization procedures for the generation of specific antibodies against LPS bands of defined sizes, and therefore they constitute an intermediate step toward that aim.

Elution of lipopolysaccharides from polyacrylamide gels

A simple procedure for elution in water of bacterial lipopolysaccharides (LPS) from sodium dodecyl sulfate-polyacrylamide gels is described. It consists of the combination of three principal steps: first, highly sensitive on-gel LPS detection (1-10 ng/band) with zinc-imidazole (negative or reverse staining); second, washing of the individual LPS band in a solution of a zinc-complexing agent (e.g., 100 mM EDTA); and finally, elution of the LPS (100-200 microliters water for a 0.5-microgram LPS band) from gel microparticles for 3 h at room temperature. Using this procedure, we have successfully eluted a variety of LPS forms from Bordetella pertussis, Escherichia coli 0111:B4, E. coli K-235, Salmonella enteritidis, and Pseudomonas aeruginosa. Elution recovery of rough or semismooth LPS was about 70-80%, while that of smooth LPS was only about 10%. Eluted LPS was biologically active as tested by limulus amebocyte lysate and TNF-alpha assays.

Sensitive reverse staining of bacterial lipopolysaccharides on polyacrylamide gels by using zinc and imidazole salts

We present a new method for visualizing bacterial lipopolysaccharides (LPS)/lipooligosaccharides (LOS) electrophoresed in sodium dodecyl sulfate-polyacrylamide gels. After electrophoresis, gels are washed in boiling water to appreciably remove remaining electrophoresis reagents, then incubated in 10 mM zinc sulfate for 15 min, and subsequently immersed in 0.2 M imidazole for 3 min. As a result, zinc salts precipitate all over the gel surface except in the zones occupied by LPS/LOS, which appear as transparent, colorless bands. Gels can be stored in distilled water for weeks without loss of the negative image. The sensitivity of this stain is similar to that of silver. We believe that zinc-imidazole may be a suitable nontoxic alternative to silver in the rapid analysis of LPS/LOS by polyacrylamide gel electrophoresis.

Negative staining with zinc-imidazole of gel electrophoresis-separated nucleic acids

Zinc and imidazole salts were applied for the detection of nucleic acids on either polyacrylamide of agarose gels. After electrophoresis, polyacrylamide gels are washed in distilled water to remove most of the residual electrophoresis reagents, then incubated in 10 mM zinc sulfate for 10 min, and subsequently immersed in 0.2 M imidazole for 3 min. As a result, zinc salts precipitate on the gel surface, except in the positions occupied by nucleic acids, which appear as transparent, colorless bands. Staining of nucleic acids on agarose gels can be performed by incubation in 40 mM zinc sulfate for 10 min, followed by immersion in 0.2 M imidazole for 5 min to form a deep white-stained background. On soaking in 2 M imidazole for 45 min, the imidazole-induced zinc precipitate is removed from the positions were nucleic acids are located resulting in a negative image of colorless and transparent nucleic acid bands against a white background. The sensitivity of this stain ranges from 5 to 7 ng/band for small (from 1 to 0.2 kbp) DNA, from 7.8 to 13 ng/band for different 22-base oligonucleotides, from 62 to 125 ng/band for large (from 20 to 2 kbp) DNA, and is 1 microgram/band for human peripheral-blood monocyte RNA. After chelation of zinc with EDTA, the nucleic acids can be quantitatively recovered from the gel. The principal advantage of this technique over ethidium bromide staining is evident for preparative purposes. Using zinc-imidazole in the detection of purified pBACIB.1 (2.8 kbp) plasmid DNA and anti-HBsAg single chain Fv antibody fragment (0.7 kbp) DNA, followed by elution from gel slices, ligation and transformation of competent E. coli XL-1 Blue cells, the number of transformants notably increased from 280 (obtained with conventional ethidium bromide staining plus UV-irradiation at 312 nm) to 10,000.

Correspondence between radioactive and functional methods in the quality control of DNA restriction and modifying enzymes

We evaluated the use of two radiolabeled lambda DNA/Hpa II substrates to detect 5'-->3', 3'-->5' single and double stranded DNA dependent exonuclease and phosphatase activities found as contaminants in restriction and modifying enzyme preparations. Looking for the meaning of the radioactive assays results in a real cloning experience, we performed a cloning simulation assay using the same conditions established for the radioactive assay (enzyme units and pmols of DNA ends). As a result, we found that for degradation percentages of the radioactive DNA substrate per enzyme unit below 0.5, the false positives in the cloning stimulation assay were less than 5%. This conditions could ensure a good performance of the enzyme preparations for cloning experiments. Finally, we described the use of the radiolabeled [gamma 33P] ATP lambda Hpa II DNA substrate to detect 5'-->3' single stranded DNA dependent exonuclease and phosphatase contaminating activities in some critical steps of the purification process of the restriction enzyme Kpn I.

Zinc-imidazole positive: a new method for DNA detection after electrophoresis on agarose gels not interfering with DNA biological integrity

Nucleic acids separated by gel electrophoresis are commonly detected within the gel matrix with ethidium bromide staining, followed by gel irradiation with ultraviolet (UV) light. When the separated nucleic acids are to be recovered for further characterization or use, this methodology is unsuitable (i) because a significant number of chemical lesions to the nucleic acid molecules are caused, heavily compromising their biological activity, and (ii) because of health hazards due to accumulative direct contact with ethidium bromide and exposure to UV-light. As an alternative, for preparative purposes, a new nontoxic detection method employing zinc and imidazole salts is described. After electrophoresis, the gel is first washed in distilled water to substantially remove remaining electrophoresis reagents, then incubated in 40 mM zinc sulfate for 10 min to allow binding of Zn2+ to the DNA, and subsequently washed with distilled water to remove unbound Zn2+ from gel regions devoid of DNA. On soaking in 0.2 M imidazole for a few minutes, zinc-DNA complexes are visualized as deep-white (positive) stained bands against a slightly opaque background. The sensitivity is similar to that of ethidium bromide. Gels can be kept in distilled water for months without loss of staining. After zinc chelation, e.g. with EDTA, it is feasible to quantitatively recover chemically intact and biologically active DNA from the gels, as shown by reelectrophoresis and transformation experiments.