Brownian motion on a square Lennard-Jones lattice: Trapping, hopping, and diffusion

Previously we showed that cGMP hydrolysis in rat whole retinal homogenates exhibited a dose-dependent inhibition following developmental lead exposure and a concentration-dependent inhibition with direct Pb2+ exposure. Additionally, developmental lead exposure resulted in a dose-dependent increase in retinal cGMP and rod Ca2+ levels. To determine whether Pb2+ or Ca2+ directly inhibited the rod-specific cGMP phosphodiesterase (PDE) and to examine the kinetic mechanism of this inhibition, purified bovine rod cGMP PDE was assayed in the presence of varying concentrations of cGMP, and Mg2+, Pb2+, and/or Ca2+. Increasing concentrations of the substrate, cGMP, resulted in a shift of the Pb2+ and Ca2+ concentration-response curves to the left, indicating a decrease in the half-maximal inhibitory concentrations of Pb2+ from nanomolar to picomolar levels. Increasing concentrations of the cofactor, Mg2+, resulted in a shift of the Pb2+ and Ca2+ concentration-response curves to the right, indicating a decrease in the inhibition of PDE activity by Pb2+ or Ca2+. A plot of 1/velocity vs 1/Mg2+ as a function of Pb2+ revealed that picomolar concentrations of Pb2+ competitively inhibited PDE relative to millimolar concentrations of Mg2+. Consistent with this finding, Mg2+ reversed the Pb(2+)-induced inhibition of PDE. Our recent kinetic analysis showed that Mg2+ and cGMP bind at interacting sites on the PDE in a random order. The present results reveal that Pb2+ may bind at the same site but with 4-6 log units higher affinity than Mg2+, thus preventing the hydrolysis of cGMP. These findings provide a novel mechanism for understanding the Pb(2+)-induced inhibition of cGMP PDE. These results may have implications for other enzymes using Mg2+ as a cofactor and suggest that Mg2+ may be useful in these situations for reversing the inhibition by Pb2+.     

Formation of unilamellar liposomes from total polar lipid extracts of methanogens

Unilamellar liposomes were formed by controlled detergent dialysis of mixed micelles consisting of acetone-insoluble total polar lipids extracted from various methanogens and the detergent n-octyl-beta-D-glucopyranoside. The final liposome populations were studied by dynamic light scattering and electron microscopy. Unilamellar liposomes with mean diameters smaller than 100 nm were obtained with lipid extracts of Methanococcus voltae, Methanosarcina mazei, Methanosaeta concilii, and Methanococcus jannaschii (grown at 50 degrees C), whereas larger (greater than 100-nm) unilamellar liposomes were obtained with lipid extracts of M. jannaschii grown at 65 degrees C. These liposomes were shown to be closed intact vesicles capable of retaining entrapped [14C]sucrose for extended periods of time. With the exception of Methanospirillum hungatei liposomes, all size distributions of the different liposome populations were fairly homogeneous.     

The effects of age and gender on the single dose pharmacokinetics of avitriptan administered to healthy volunteers

The effects of age and gender on the single dose pharmacokinetics of avitriptan and its three metabolites were assessed in 15 young men, 15 young women, 15 elderly men and 15 elderly women. Avitriptan was administered as a 150-mg capsule after a 10-hour fast and serial plasma and urine samples were collected up to 36 hours after the dose. Plasma samples were analyzed for avitriptan and its metabolites, N-desmethyl avitriptan (ND048), O-desmethyl avitriptan (OD048), and methoxypyrimidinyl piperazine (MPP). Urine samples were analyzed for only avitriptan and MPP. Avitriptan was well tolerated in all four groups. The drug was rapidly absorbed with a median time to maximum plasma concentration (tmax) between 0.5 and 1.5 hours. No significant gender-related differences were found in the maximum plasma concentration (Cmax) and area under the concentration-time curve extrapolated to infinity (AUC0-infinity) of avitriptan. Renal clearance of avitriptan was significantly smaller in young women compared with young men, but this is clinically not relevant because only 2% to 3% of the total dose is excreted unchanged. Compared with the young volunteers, mean Cmax was approximately 50% higher in the elderly but there was no difference in the AUC0-infinity between the 2 age groups. Plasma concentrations of ND048, OD048, and MPP were each 50 to 100 fold lower than those of avitriptan. Hence some age- and gender-related differences found in the pharmacokinetics of avitriptan metabolites are probably not relevant in the assessment of overall safety and efficacy of avitriptan. Based on the pharmacokinetics and tolerability, no age or gender-related dose adjustment is necessary for avitriptan.