Lipid measurement devices

For the biochemist, the advance of new, sensitive temperature-measuring devices comes just at a time when large amounts of very pure proteins, nucleotides, DNA, RNA, lipids, etc., are becoming available. Thus, the ubiquitous nature of heat can be utilized, providing both the thermodynamic and kinetic data needed for any theory of biomolecular interaction. 

Progress was recently made in the construction of an AC admittance modulation system for surface-stabilized lipid membrane biosensors that operated on the basis of the control of the ion permeation by artificial ion channels [40]. A portable admittance modulation measurement device was designed to measure both the in-phase and out-of-phase signal components for determination of the effective ion current and membrane capacitance, respectively [40]. The sensitivity and detection limit of this AC system were tested by studying the interaction of valinomycin with planar BLMs. The electrochemical phenomena were monitored through the in-phase component and measured as conductance changes of the membrane, providing a detection limit of 1 nM for valinomycin. 

The key to resolving the above-mentioned apparent paradox is to recognize the heterogeneous nature of photochemical reactions in biomembranes. Intuitively, the electric currents generated by photoreactions must interact with the inert supporting structure — the lipid bilayer — as well as the measuring device. Equivalent circuit analysis thus provides a shortcut to a better understanding. 

We believe that the type of investigations that are outlined briefly above provide interesting new information on the properties of lipids from both technical and biophysical points of view. Measurements on electrolyte-lipid semiconductor systems should provide useful information complementary to that obtained from BLM investigations. Furthermore, the gas sensitivity of the electrical properties of lipids could be utilized in practical devices. 

It is recognized that filtration is operational, that colloidal-bound PCB congeners are not retained by the filter, and that operational dissolved measurements may be biased positively by colloidal material. Techniques to measure truly dissolved PCBs include gas sparging, differential diffusion into membrane-bound lipids (e.g., semipermeable membrane devices, [230]), and selective adsorption (e.g. non-equilibrium solid phase microextraction [231, 232]). Unfortunately, none of these techniques has sufficient sensitivity to reliably and unambiguously measure truly dissolved PCB congeners at the levels present in the Great Lakes. 

The aggregate states of lipids are discernable from measurements provided by a host of analytical devices, which yield information on physical properties. These techniques may also be used to give clues as to the interaction of the lipid carrier and the drug. 

For measurement of phosphatidylcholine in serum the samples were preincubated with 0.2-0.5 U phospholipase D (EC 3.1.4.4) in the presence of Triton X-100 and calcium chloride (Mascini et al. 1986 Campa-nella et al., 1988). The liberated choline was assayed by means of the choline oxidase sensor. To assay lecithin in foodstuff the samples were extracted with ethanol and diluted with buffer. In the LCA 400 Lipid Analyzer of Toyo Jozo (Japan) the same principle is used for serum samples. However, this device appears not to be suitable for routine application and has been withdrawn from the market. 

Polymer films have been very widely applied to modify the surfaces of chemical sensors both for solution and gas phase measurements. Solution coating has been used to coat quartz crystal oscillators [91-93, 112] and SAW devices [113, 114] with various polymeric adsorbates and polymer/lipid mixtures [90, 94] to prepare arrays of sensors for use in gas sensing and odor evaluation and discrimination. 

An extension of artificial membranes for ion selective electrochemical work was the construction of biomimetic ion channel sensors [65]. These devices were based on Langmuir-Blodgett deposition of charged lipid membranes onto a glassy carbon electrode. This work indicated that a conductive zone can be opened reversibly by a stimulant-membrane interaction by surface charge alterations. This work has demonstrated how the concept of the conductivity measurement could be extended to the more common and useful technique of cyclic voltammetry. 

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