Identification and characterization of a lysophosphatidic acid receptor

A specific binding site for 1-[H-3]stearoyl-lysophosphatidic acid (stearoyl-LPA) was identified and characterized in membranes prepared from rat brain and Swiss 3T3 fibroblasts. Specific binding of [H-3]LPA to these sites was protein dependent, was saturable, reached equilibrium in 15 min, and was displacable by the addition of excess unlabeled LPA. Scatchard analysis of saturation binding experiments indicated that these sites had affinities of 2.0 +/- 0.5 nhl and 5.4 +/- 2.6 nM and densities of 19 +/- 3 fmol/mu g of protein and 38 +/- 6 fmol/mu g of protein in rat brain and 3T3 cell membranes, respectively. Various LPAs, with different acyl groups in the sn-1-position, competed with [H-3]LPA for these binding sites, with a rank order of potency of 1-oleoyl-LPA < 1-stearoyl-LPA = 1-palmitoyl-LPA < 1-myristoyl-LPA. Phosphatidic acid also bound to these sites, but with lower affinity than any LPA tested. Neither lysophosphatidylcholine, lysophosphatidylethanolamine, nor any free fatty acid competed with [H-3]LPA for these binding sites. Binding of [H-3]LPA to these sites was regulated by nonhydrolyzable guanine nucleotides in both rat brain and 3T3 cell membranes. Furthermore, in 3T3 cells, these sites were regulated by cell density. It was subsequently determined that LPA induced a transient increase in intracellular Ca2+ levels in 3T3 cells. The concentrations required for this response, as well as the rank order of potency of the various LPAs and phosphatidic acid, correlated with the affinity of these compounds for the [H-3]LPA binding site. These results suggest that the specific, high affinity, binding sites for [H-3]LPA are G protein-coupled receptors

Role of Organic Matter in Framboidal Pyrite Oxidation

An experimental system has been set up to investigate the reaction kinetics of framboidal pyrite oxidation in real, reactive acid sulfate soil assemblages. This study was undertaken to determine the degree to which pyrite oxidation rates are reduced by bacteriological reactions and organic matter, which both modify the net reaction mechanisms and compete for available oxygen. The results from these experimental runs not only confirm the role of organic matter in mitigating pyrite oxidation, but indicate that, at least initially, the acidity produced is consumed or otherwise ameliorated by parallel reactions. Tracking pH or [H+] in both a reactor and in soil does not accurately reflect reaction progress, and may not correctly indicate the true level of risk. In comparison, the tracking of pyrite oxidation with the concentration of sulfate in solution is not affected by side reactions or precipitation, and is therefore a better indicator for the rate of pyrite destruction