Refolded outer membrane protein A of Escherichia coli forms ion channels with two conductance states in planar lipid bilayers.

Outer membrane protein A (OmpA), a major structural protein of the outer membrane of Escherichia coli, consists of an N-terminal 8-stranded beta-barrel transmembrane domain and a C-terminal periplasmic domain. OmpA has served as an excellent model for studying the mechanism of insertion, folding, and assembly of constitutive integral membrane proteins in vivo and in vitro. The function of OmpA is currently not well understood. Particularly, the question whether or not OmpA forms an ion channel and/or nonspecific pore for uncharged larger solutes, as some other porins do, has been controversial. We have incorporated detergent-purified OmpA into planar lipid bilayers and studied its permeability to ions by single channel conductance measurements. In 1 M KCl, OmpA formed small (50-80 pS) and large (260-320 pS) channels. These two conductance states were interconvertible, presumably corresponding to two different conformations of OmpA in the membrane. The smaller channels are associated with the N-terminal transmembrane domain, whereas both domains are required to form the larger channels. The two channel activities provide a new functional assay for the refolding in vitro of the two respective domains of OmpA. Wild-type and five single tryptophan mutants of urea-denatured OmpA are shown to refold into functional channels in lipid bilayers.

Evaluation of a passive dosimeter for collection of 2-bromo-2-chloro-1, 1, 1-trifluoroethane and 2-chloro-1, 1, 2-trifluoroethyl difluoromethyl ether in hospital operating rooms.

Operationally, passive dosimeters are ideally suited for monitoring organic vapors in hospital operating rooms as they are compact, lightweight and do not require tubing or pumps. In this study, a recently developed passive diffusion sampler was used to collect 2-bromo-2-chloro-1, 1, 1-trifluoroethane (Halothane) and 2-chloro-1, 1, 2-trifluoroethyl difluoromethyl ether (Enflurane) in standard air mixtures over the range of 0.2-10 ppm. Additionally, exposures to known concentrations were conducted for various lengths of time. A side-by-side comparison of charcoal tubes (CT) and passive dosimeter collection characteristics were made on known air mixtures and samples collected in operating rooms. The material adsorbed on charcoal from dosimeters and CT was desorbed with carbon disulfide and quantified using gas-liquid chromatography. The overall efficiency of the dosimeters along with quality control data are presented.