Immunoglobulin-lipid interaction. A model membrane study

Seventy-Five Years of Research on Protein Binding

This review summarizes evidence that the impact of protein binding of the activity of antibiotics is multifaceted and more complex than indicated by the numerical value of protein binding alone. A plethora of studies has proven that protein binding of antibiotics matters, as the free fraction only is antibacterially active and governs pharmacokinetics. Several studies have indicated that independent from protein binding of immunoglobulin G, albumin, α1-acid-glycoprotein, and pulmonary surfactant acted synergistically with antibacterial agents, thus suggesting that some intrinsic properties of serum proteins may have mediated serum-antibiotic synergisms. It has been demonstrated that IgG and albumin permeabilized Gram-negative and Gram-positive bacteria and facilitated the uptake of poorly penetrating antibiotics. Alpha-1-acid-glycoprotein and pulmonary surfactant also exerted a permeabilizing activity, but proof that this property results in a sensitizing effect is missing. The permeabilizing effect of serum proteins may explain why serum-antibiotic synergisms do not represent a general phenomenon but are limited to specific drug-bug associations only. Although evidence has been generated to support the hypothesis that native serum proteins interact synergistically with antibiotics, systematic and well-controlled studies have to be performed to substantiate this phenomenon. The interactions between serum proteins and bacterial surfaces are driven by physicochemical forces. However, preparative techniques, storage conditions, and incubation methods have a significant impact on the intrinsic activities of these serum proteins affecting serum-antibiotic synergisms, so these techniques have to be standardized; otherwise, contradictory data or even artifacts will be generated.

Assembly of antibodies in lipid membranes for biosensor development

An investigation of the incorporation of antibody in lipid films of a composition that has been used for biosensor preparation is reported. IgG that is incorporated into lipid monolayers prepared from 7:3 mixtures of dipalmitoyl phosphatidylcholine and dipalmitoyl phosphatidic acid is edge-active, and enters and penetrates the fluid region of the mixed-phase system when monolayers are held at low pressure (< 20 mN/m). It was found that there is an "exclusion pressure" observed in pressure-area (pi-A) curves that are collected for monolayers that contain antibody. This term refers to a specific threshold of lateral pressure (which is reached by monolayer compression) that can cause explusion of antibody from the interior of a membrane. Microscopic images of monolayers containing the fluorescent phospholipid nitrobenzoxadiazole dipalmitoyl phosphatidylethanolamine (NBD-PE), or antibody labeled with tetramethylrhodamine isothiocyanate (TRITC), were used to determine the structure of membranes, and the location of effects on structure caused by IgG. Ellipsometric measurements of lipid monolayers that were cast onto silicon wafers by the Langmuir-Blodgett method were used to study the thickness of monolayers and to investigate the structural changes that occurred at the "exclusion pressure." Both the use of fluorescent antigen and ellipsometry indicated that antibody binding activity was present and was dependent on compression pressure. The effects of pH and ionic strength of subphase, antibody concentration, incubation time, and lateral pressure have been examined. The results may indicate the conditions that can be used to improve the incorporation of active IgG for preparation of biosensors that are based on lipid membranes.

The immunochemistry of sandwich ELISAs--V. The capture antibody performance of polyclonal antibody-enriched fractions prepared by various methods

Studies compare the performance of antibody-enriched serum fractions prepared by various methods, when adsorbed on polystyrene microtiter wells as capture antibodies (CAbs) and tested against multivalent antigens. The criteria of performance in the RIA used included antigen capture capacity (AgCC) and the nmol of functional capture sites per microtiter well (CAbt). Affinity purified polyclonal (pAb) and monoclonal antibodies (mAb) were employed as reference CAbs. AgCC was highest for enriched fractions prepared using caprylic acid and a high-pressure SpG affinity column. The performance of capture antibodies is expressed by an equation which was empirically derived and experimentally tested; CAbt x AgCC/ng adsorbed IgG. In terms of this parameter, CAb-enriched fractions prepared with caprylic acid performed best. The data reported also provide insight into solid-phase ligand immunochemistry. Adsorbed polyclonal CAb performed with remarkable homogeneity in percent bound and in Scatchard plots. Values obtained for CAbt from Steward-Petty plots were directly correlated with the length of the LBR of log-log percent bound plots but indicated that less than 10% of the potential capture sites of polyclonal CAbs remained functional after adsorption; mAb showed a more serious loss of activity. The loss of CAbt was a general phenomenon for all preparations tested although relative to their antibody content, certain antibody-enriched fractions retained a higher proportion of CAbt than their affinity-purified counterparts. Comparative studies in which the activity of adsorbed mAb and pAb was compared to the same antibodies immobilized by a non-adsorptive procedure, indicated that adsorbed CAbs also express lower affinity. The studies we report offer a single parameter criterium for comparatively evaluating CAb performance while simultaneously revealing the need to develop immobilization procedures that can preserve CAbt and antibody affinity so that immunoassays with wide dynamic ranges and high AgCC can be developed without waste of antibody.

In vitro and in vivo effect of immunoglobulin G on the integrity of bacterial membranes

The interaction between a modified 7S immunoglobulin (MISG) and bacterial membranes was studied by adopting in vitro as well as in vivo techniques. Preincubation of Escherichia coli and Pseudomonas aeruginosa with MISG resulted in a release of enzymatic markers from the periplasmic space, whereas no cytoplasmic or membrane-bound enzymes were liberated. Due to the interaction of MISG with the outer membrane of gram-negative rods, the bacteria became more susceptible to the antibacterial action of poorly penetrating penicillins because of a significantly increased rate of uptake. These in vitro effects were corroborated under in vivo conditions by adopting the granuloma pouch model. A single intravenous injection of MISG enhanced the therapeutic efficacy of mezlocillin against E. coli; similarly, the antibacterial activity of penicillin G, oxacillin, cephalothin and cefamandole against Staphylococcus aureus was augmented by MISG. These in vivo effects of MISG were not due to an increased rate of phagocytosis or complement activity. Thus, MISG sensitized bacteria to several beta-lactam antibiotics by disorganizing their outer membrane.

In vitro and in vivo effect of immunoglobulin G on the integrity of bacterial membranes

The interaction between a modified 7S immunoglobulin (MISG) and bacterial membranes was studied by adopting in vitro as well as in vivo techniques. Preincubation of Escherichia coli and Pseudomonas aeruginosa with MISG resulted in a release of enzymatic markers from the periplasmic space, whereas no cytoplasmic or membrane-bound enzymes were liberated. Due to the interaction of MISG with the outer membrane of gram-negative rods, the bacteria became more susceptible to the antibacterial action of poorly penetrating penicillins because of a significantly increased rate of uptake. These in vitro effects were corroborated under in vivo conditions by adopting the granuloma pouch model. A single intravenous injection of MISG enhanced the therapeutic efficacy of mezlocillin against E. coli; similarly, the antibacterial activity of penicillin G, oxacillin, cephalothin and cefamandole against Staphylococcus aureus was augmented by MISG. These in vivo effects of MISG were not due to an increased rate of phagocytosis or complement activity. Thus, MISG sensitized bacteria to several beta-lactam antibiotics by disorganizing their outer membrane.