Partitioning and location of Bay K 8644, 1,4-dihydropyridine calcium channel agonist, in model and biological membranes

Abstract

Several lines of evidence suggest that nonspecific drug interaction with the lipid bilayer plays an important role in subsequent recognition and binding to specific receptor sites in the membrane. The interaction of Bay K 8644, a 1,4-dihydropyridine (DHP) calcium channel agonist, with model and biological membranes was examined at the molecular level using small angle x-ray diffraction. Nonspecific drug partitioning into the membrane was examined by radiochemical assay. Nonspecific binding characteristics of [3H] Bay K 8644 were determined in both dipalmitoyl phosphatidylcholine (DPPC) vesicles above and below their thermal phase transition (Tm) and rabbit skeletal muscle light sarcoplasmic reticulum (LSR). In DPPC, the partition coefficient, Kp, was 14,000 above the Tm (55 degrees C) versus 160 in the gel phase (2 degrees C). The Kp determined in LSR membranes was 10,700. These values for both DPPC and LSR membranes can be compared with Kp = 290 in the traditional octanol/buffer system. Using small-angle x-ray diffraction, the equilibrium position of the electron-dense trifluoromethyl group of Bay K 8644 in DPPC (above Tm) and purified cardiac sarcolemmal (CSL) lipid bilayers was determined to be consistently located within the region of the first few methylene segments of the fatty acyl chains of these membranes. This position is similar to that observed for the DHP calcium channel antagonists nimodipine and Bay P 8857. We suggest this particular membrane location defines a region of local drug concentration and plane for lateral diffusion to a common receptor site. Below the DPPC membrane Tm, Bay K 8644 was shown to be excluded from this energetically favored position into the interbilayer water space. Heating the DPPC bilayer above the Tm (55 degrees C) showed that this exclusion was reversible and indicates that drug-membrane interaction is dependent on the bilayer physical state. The absence of any specific protein binding sites in these systems allows us to ascertain the potentially important role that the bulk lipid phase may play in the molecular mechanism of DHP binding to the specific receptor site associated with the calcium channel.

Active Metabolite of Atorvastatin Inhibits Membrane Cholesterol Domain Formation by an Antioxidant Mechanism

“…X-ray diffraction experiments were conducted by aligning the samples at grazing incidence with respect to a collimated x-ray source (CuK␣ radiation; ϭ 1.54 Å). In addition to direct calibration of the detector system, cholesterol monohydrate crystals were used to verify the calibration, sample-to-detector distance, and location of the main beam, as described previously (26). The unit cell periodicity (d-space) of the membrane lipid bilayer was calculated using Bragg's law.…”

“…Corrected diffraction orders obtained from samples in this study were analyzed using Fourier summation to yield one-dimensional electron density profiles (Å versus electrons/Å 3 ) of the membrane lipid bilayer. A detailed explanation of membrane diffraction analysis was described previously (26).…”

Distribution and Fluidizing Action of Soluble and Aggregated Amyloid β-Peptide in Rat Synaptic Plasma Membranes

“…The primary reason for a centrosymmetric structure is that the stacking of the membranes results in a random orientation for the SPM with respect to the multilayer direction. The SPM electron density profiles were normalized to the phospholipid headgroup region, a region of relatively high electron density, after applying identical correction factors to the raw diffraction data (28,29).…”

“…This finding suggests that A␤ modulates membrane function by a nonreceptor-mediated mechanism, potentially as a result of altering the physico-chemical properties of membrane constituents, including lipids and proteins (13)(14)(15)(16). Indeed, previous membrane equilibrium binding experiments have demonstrated that the A␤ [25][26][27][28][29][30][31][32][33][34][35] fragment is highly lipophilic (K P Ͼ 10 2 ); the peptide intercalates deep into the membrane bilayer hydrocarbon core, as determined by small angle x-ray diffraction approaches (15). In addition, aggregated A␤ has strong electrostatic interactions with the surface of model membranes that appear to mediate its neurotoxicity (17).…”

“…A recent study from Cotman and co-workers (12) showed that A␤ neurotoxicity is independent of stereoisomerspecific ligand-receptor interaction because both all-D-and all-L-stereoisomers of A␤ [25][26][27][28][29][30][31][32][33][34][35] and A␤ had similar neurotoxic activity. This finding suggests that A␤ modulates membrane function by a nonreceptor-mediated mechanism, potentially as a result of altering the physico-chemical properties of membrane constituents, including lipids and proteins (13)(14)(15)(16).…”