Saturation mutagenesis of the plasminogen activator inhibitor-1 reactive center

Plasminogen activator inhibitor-1 (PAI-1) is a specific inhibitor of the serine proteases tissue-type plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA). To systematically investigate the roles of the reactive center P1 and P1' residues in PAI-1 function, saturation mutagenesis was utilized to construct a library of PAI-1 variants. Examination of 177 unique recombinant proteins indicated that a basic residue was required at P1 for significant inhibitory activity toward uPA, whereas all substitutions except proline were tolerated at P1'. P1Lys variants exhibited lower inhibition rate constants and greater sensitivity to P1' substitutions than P1Arg variants. Alterations at either P1 or P1' generally had a larger effect on the inhibition of tPA. A number of variants that were relatively specific for either uPA or tPA were identified. P1Lys-P1'Ala reacted 40-fold more rapidly with uPA than tPA, whereas P1Lys-P1'Trp showed a 6.5-fold preference for tPA. P1-P1' variants containing additional mutations near the reactive center demonstrated only minor changes in activity, suggesting that specific amino acids in this region do not contribute significantly to PAI-1 function. These findings have important implications for the role of reactive center residues in determining serine protease inhibitor (serpin) function and target specificity.

Determination of the physical structure of biological materials at biosensor interfaces by techniques of increasing magnification from microscopic to molecular scale

Chemical selectivity of biosensors is derived from biological materials interfaced to the surface of transducing devices. Molecular recognition events lead to macroscopic function suitable for analytical measurements. The structure-function relationships of biochemical species at interfaces must be established to characterize and optimize biosensor operation. The techniques of ellipsometry, fluorescence microscopy, electron microscopy, and scanning tunneling microscopy are used to investigate the structure of monolayers and multilayers of proteins and lipids at interfaces that are prepared by Langmuir-Blodgett techniques and by self-assembly from bulk solution. The relative merits and limitations of the measurement techniques in the determination of aspects of interfacial structure are considered.

The prevention of adsorption of interferents to radiolabelled protein by Tween 20

Radiolabels are often used to quantitatively determine the amount of protein immobilized on chromatographic supports, immunochemical plates and biosensor surfaces. Bovine serum albumin (BSA) was chosen as a model protein for quantitative deposition studies. BSA was radioiodinated (125I-) or fluorescently labelled (fluorescein), then incubated with the following surfaces: quartz, quartz derivatized by 3-aminopropyltriethoxysilane (Qz-APTES), and Qz-APTES reacted with glutaraldehyde or tresyl chloride. The amounts of BSA immobilized to the different surfaces were compared using data from radioactivity and fluorescence assays. Irreproducible results were obtained with radioiodinated BSA due to adsorption/desorption behaviour of an unidentified radioactive species. When the non-ionic detergent Tween 20 was added to the protein/surface incubation mixture, radiolabelled BSA gave reproducible protein binding results which agreed with fluorescent protein binding patterns. The effect of Tween 20 was due to either the binding to BSA displacing the interferent and/or the solubilization of the interferent.

Chemical transduction with fluorescent lipid membranes using selective interactions of acetylcholine receptor with agonist/antagonist and acetylcholine with substrate

Alterations in the physical structure of vesicles and monolayers of phospholipids and soybean lecithin were monitored by measurement on the average fluorescence intensity changes from N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)dipalmitoyl-L-a-phosphatidyl ethanolamine (NBD-PE) located in the lipid matrices. This probe was intimately dispersed at a concentration of 1-2 mol-% in lipid membranes and had an emission sensitive to local environmental structure. Alterations in the structure of soybean lecithin vesicles were induced by the selective interaction of acetylcholine receptor with the agonist carbamylcholine and the antagonist alpha-bungarotoxin. Structural changes in vesicles with a 7:3 mole ratio of dipalmitoylphosphatidyl choline to dipalmitoylphosphatidic acid were observed for selective interactions between acetylcholinesterase and acetylcholine. Enhancement of fluorescence emission from the lipid membranes provided transduction of the selective binding events of the receptor and enzyme. A maximum sensitivity of about a 30% enhancement per micromole of carbamylcholine and a detection limit for the toxin of 10 nM were observed for the receptor. Fluorescence microscopy was used to establish that protein could be incorporated in monolayer lipid membranes and to provide information about potential mechanisms of fluorescence enhancement. These studies show that lipid membranes containing NBD-PE can be used as generic transducers of protein-ligand interactions.

Selective electrochemical biosensors from state-switching of bilayer and monolayer lipid membranes by lectin-polysaccharide complexes

Interaction of the lectin concanavalin A with the polysaccharide glycogen can provide rapid spontaneous transients of the surface potential at bilayer and monolayer lipid membranes. The selective binding process can cause large, rapid potassium ion current fluctuations across bilayer membranes in a manner that is periodic and reproducible. The frequency of these transient ion current signals was shown to be related to sub-nanomolar concentrations of the reactive agents in aqueous solution. The physical mechanism responsible for ion current modulation was investigated by fluorescence methods using lipid vesicles, by the thermal dependence of the potassium ion current across planar bilayers and by pressure-area and dipolar potential measurements of lipid monolayers at an air-water interface. The mechanism is primarily associated with physical perturbations of lipid membranes by lectin-polysaccharide aggregates, resulting in the formation of localised domains of variable electrostatic potential and conductivity.

In vivo probes: problems and perspectives

Devices constructed for potential use as invasive bioprobes incorporate a selective receiving site for molecular or ionic recognition, and a transducer which is capable of translating a perturbation of physical chemistry of the determinant-site reaction (interaction) into a usable signal. Four types are envisioned--implants for general hospital use, transient-use probes to replace classical blood tests, short-term implantable probes and the long-term variety. Performance criteria are selectivity, sensitivity, fast response, site-reversible, small, rugged, inexpensive, biocompatible, calibratible, facile use by non-expert personnel and ease of telemetry. These demands, not surprisingly, create enormous challenges to the sensor specialist. With respect to biocompatibility the sensor must not be involved in infection, clot formation or antigenic response, and, furthermore, protein adsorption, etc., which can affect the sensor response should be avoided. Calibration remains a problem of monumental proportions. Many devices drift from calibrated levels even in in vitro experiments, let alone in the implanted milieu. One solution has been to carry out on-line switching between patient blood and standard solutions. However, this type of approach leaves a lot to be desired with respect to portability. Another method which is attracting increasing attention is the chemometric or artificial intelligence system involving compensation by multi-sensor array configurations. Sensitivity and limit-of-detection have attracted little research due to the overwhelming nature of other difficulties. In the present paper we evaluate a number of these technical problems and discuss the architecture of devices that are currently available. Finally, some thoughts as to priorities for re-directing sensor research in the bioprobe area are presented.