Fluorescence studies of drug and cation interactions with microsomal membranes

Substrate binding site of microsomal cytochrome P-450 directly faces membrane lipids

Cytochrome P-450, purified from liver microsomes of phenobarbital-treated rabbits, was incorporated into dimyristoylphosphatidylcholine liposomes. The binding of benzphetamine to the liposome-bound cytochrome P-450 was examined by measuring the benzphetamine-induced spectral change at various temperatures. The van't Hoff plot of the apparent spectral dissociation constant showed a distinct break at the temperature of phase transition of the synthetic lipid. On the other hand, no such break was observed for benzphetamine binding to microsomal bound cytochrome P-450. These results suggest that the substrate binding site of cytochrome P-450 is embedded in the apolar interior of phospholipid bilayer membranes.

Effect of ions on phospholipid layer structure as indicated by Raman spectroscopy

Various anions and cations are found to induce changes in the layered structure of phosphatidylcholine-water systems as indicated by Raman Spectroscopy. From the ratio of Raman intensities, I1064/I1089, it is inferred that dipositive ions decrease the proportion of gauche character in the hydrocarbon chains, with the relative influence being: Ba2+ less than Mg2+ less than Ca2+ similar to Cd2+. Unipositive ions (Li+, K+ and Na+) produce no observed changes in the Raman spectrum of the lecithin dispersion. The proportion of gauche character of the hydrocarbon chains is found to be nearly independent of the anion for: Br-, Cl-, acetate-, I-, ClO4-, CNS- and SO42-. Dispersions prepared with a solution of KI+I2 produced Raman spectra in which the 1089cm-1 peak, which is characteristic of random lipid chains, was greatly intensified, presumably because of the presence of I3- which is known to penetrate the lipid lamellae. The observed trends are discussed.

The application of fluorescent probes in membrane studies

Fluorescence has been used in biochemical studies for many years but only recently has the information content and the practical applicability of the fluorescence method been fully realized.

Following the early studies of Newton (1954) and Weber (11954) and after the initial utilization of fluorescent probes by Chance and coworkers (Azziet al.1969) and Tasakiet al.(1968), in the study of membranes, the use of fluorescence to provide structural information at microscopic or molecular levels in biological membranes has become widespread. widespread. The application of the fluorescence technique to biological systems has progressed parallel to the development of a theoretical basis for fluorescence data interpretation and the synthesis of a large number of fluorescent probes, organic molecules having fluorescence characteristics that are dependent on their environment.