Quantitation of related proteins by Western blot analysis

Sensitive electrochemical detection of horseradish peroxidase at disposable screen-printed carbon electrode

A rapid, simple and sensitive electrochemical assay of horseradish peroxidase (HRP) performed on disposable screen-printed carbon electrode was developed. HRP activities were monitored by square-wave voltammetric (SWV) measuring the electroactive enzymatic product in the presence of o-aminophenol and hydrogen peroxide substrate solution. SWV analysis demonstrated a greater sensitivity and shorter analysis time than the widely used amperometric and differential-pulsed voltammetric methods. The voltammetric characteristics of substrate and enzymatic product as well as the parameters of SWV analysis were optimized. Under optimized conditions, a linear response for HRP from 0.003 - 0.1 U/mL and a detection limit of 0.002 U/mL (1.25×10(-15) mol in 25 μL) were obtained with a good precision (RSD = 8%; n = 6). This rapid and sensitive HRP assay with microliters-assay volume could be readily integrated to portable devices and point-of-care (POC) diagnosis applications.

Estimation of serum antibody to subgingival species using checkerboard immunoblotting

Measurement of serum antibody to subgingival bacterial species has been useful in discriminating possible periodontal pathogens and in assessing the host's immune response to subgingival species. An immunoassay system was developed to measure the level of serum antibody to multiple subgingival species in multiple serum samples on a single nitrocellulose membrane. The principle steps of the assay are the following: 1) binding of antigens from bacterial preparations and protein A in parallel lanes on nitrocellulose membranes; 2) incubation of known concentrations of human immunoglobulin as well as various dilutions of serum from patients in lanes perpendicular to the antigen lanes; 3) incubation of the membrane with a peroxidase-conjugated second antibody; 4) detection of positive reactions by enhanced chemiluminescence. Emitted light was captured on a photographic film in which the positive reactions appeared as squares at the intersections of antigens with appropriate antibody. Antibody was quantified using a laser densitometer to compare the signal intensity of unknown samples with the ones generated by known amounts of human immunoglobulin captured on the same membrane. The assay permitted simultaneous screening and/or quantification of antibody in as many as 45 serum samples against up to 45 bacterial species. The mean and standard error of the coefficients of variation for replicates within an assay averaged 7.3 +/- 2.3%. Coefficients of variation of the assay run on different days for serum antibody to a range of subgingival species averaged 10.1 +/- 2.1%. Checkerboard immunoblotting is a simple and rapid immunoassay to permit quantification and/or screening of antibody to multiple subgingival species or antigens in multiple serum samples.

Xerographic paper as a transfer medium for western blots: quantification of bovine alpha S1-casein by western blot

We have found that xerographic paper (regular photocopy or laser printer paper) can be used as a transfer medium for protein blots for the immunodetection of low concentrations (a few nanograms) of bovine alpha S1-casein. With paper blotting, we could detect the protein with three times more sensitivity than with polyvinylidene difluoride. Blotting was achieved by transfer from sodium dodecyl sulfate-polyacrylamide gel electrophoresis to the methanol-wet paper. The blot was incubated with chicken anti-casein antibodies, sequentially peroxidase-labeled rabbit anti-chicken antibodies, and diaminobenzidine substrate. The blots were directly scanned and the pixel intensity of the band areas was integrated. An analysis of the scanned blots showed that the log of protein concentration was linearly related with the square root of an integrated pixel intensity (r2 greater than 0.96). This linear relationship was observed in a wide range of protein concentration from 5 ng to 15 micrograms. The coefficients of variation were 12.4% for intraassay and 15.7% for interassay. This new analytical procedure can be applied to estimate the concentration of proteins by immunological blotting.

Transgenetic studies implicate interactions between homologous PrP isoforms in scrapie prion replication

Transgenic (Tg) mice expressing both Syrian hamster (Ha) and mouse (Mo) prion protein (PrP) genes were used to probe the mechanism of scrapie prion replication. Four Tg lines expressing HaPrP exhibited distinct incubation times ranging from 48 to 277 days, which correlated inversely with HaPrP mRNA and HaPrPC. Bioassays of Tg brain extracts showed that the prion inoculum dictates which prions are synthesized de novo. Tg mice inoculated with Ha prions had approximately 10(9) ID50 units of Ha prions per gram of brain and less than 10 units of Mo prions. Conversely, Tg mice inoculated with Mo prions synthesized Mo prions but not Ha prions. Similarly, Tg mice inoculated with Ha prions exhibited neuropathologic changes characteristic of hamsters with scrapie, while Mo prions produced changes similar to those in non-Tg mice. Our results argue that species specificity of scrapie prions resides in the PrP sequence and prion synthesis is initiated by a species-specific interaction between PrPSc in the inoculum and homologous PrPC.

Identification of interleukin-6 as an autocrine growth factor for Epstein-Barr virus-immortalized B cells

Autocrine growth factors are believed to be important for maintenance of an immortalized state by Epstein-Barr virus (EBV), because cell-free supernatants of EBV-immortalized cell lines promote the proliferation of autologous cells and permit their growth at low cell density. In this study, we provide evidence for the existence of two autocrine growth factor activities produced by EBV-immortalized lines distinguished by size and biological activities. Much of the autocrine growth factor activity in lymphoblastoid cell line supernatants resided in a low-molecular-weight (less than 5,000) fraction. However, up to 20 to 30% of the autocrine growth factor activity resided in the high-molecular-weight (greater than 5,000) fraction. While the nature of the low-molecular-weight growth factor activity remains undefined, the high-molecular-weight growth factor activity was identified as interleukin-6 (IL-6). Culture supernatants from six EBV-induced lymphoblastoid cell lines tested contained IL-6 activity, because they promoted proliferation in the IL-6-dependent hybridoma cell line B9. In addition, a rabbit antibody to human IL-6 neutralized the capacity of the high-molecular-weight (greater than 5,000) fraction of a lymphoblastoid cell line supernatant to promote growth both in autologous EBV-immortalized cells and in B9 cells. Similarly, this high-molecular-weight autocrine growth factor activity was neutralized by a monoclonal antibody to human IL-6. Furthermore, characteristic bands, attributable to IL-6, were visualized in supernatants of each of four EBV-induced lymphoblastoid cell lines after immunoprecipitation with a rabbit antiserum to human IL-6. Thus, in addition to its previously reported properties, IL-6 is an autocrine growth factor for EBV-immortalized B cells cultured under serum-free conditions.

Analysis of antibody reactivities in ELISA using protein blots as antigen substrates: S-ELISA

We describe here a novel immunoassay procedure, designated strip-ELISA (S-ELISA), in which specific antigens are purified by SDS-PAGE, transferred to support membranes, and utilized in situ as substrate in routine ELISA procedures. Using two different lentivirus systems, simian immunodeficiency virus and equine infectious anemia virus, we demonstrate the utility of S-ELISA for screening hybridoma supernatants during production of monoclonal antibodies and for the dissection of polyclonal antibody responses into defined antigen specificities. The data presented here indicates that the S-ELISA combines the sensitivity of conventional ELISA with the specificity afforded by Western blotting procedures, producing a relatively simple and versatile assay for evaluating antibody reactivities.