Turnover of arachidonic acid in the major diacyl and ether phospholipids of human platelets

In this work, the uptake and release of [3H]arachidonic acid by the diacyl and ether species of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) in human platelets were studied. Uptake of [3H]arachidonic acid into 1,2-diacyl-PC and 1,2-diacyl-PE was much greater than into the ether phospholipids of the same class. In [3H]arachidonoyl-labeled platelets stimulated by thrombin, there was a decrease in total [3H] arachidonoyl-PC. This was accounted for mostly by a decrease in 1-acyl-2-[3H]arachidonoyl-PC while the level of 1-O-alkyl-2-[3H]arachidonoyl-PC (a precursor for platelet-activating factor) increased slightly. However, in ionophore A23187-stimulated platelets, the reduction of total [3H]arachidonoyl-PC was due to a decrease in both 1-acyl-2-[3H]arachidonoyl-PC and 1-O-alkyl-2-[3H] arachidonoyl-PC, suggesting that ionophore should yield more platelet-activating factor than thrombin. In both thrombin- and ionophore-stimulated platelets, there was a net increase in total [3H]arachidonoyl-PE. This consisted of a decrease in 1,2-diacyl-PE, which was essentially complete by 1 min, followed by an increase in 1-O-alk-1'-enyl-2-[3H]arachidonoyl-PE, which was slower and not apparent until 3-5 min after thrombin. During reincubation of labeled platelets with saline, the 1-O-alkyl-2-[3H]arachidonoyl-PC increased by a factor of 2, between 0 and 4 h, with no significant change in the radioactivity of any other phospholipid. Thus, upon stimulation of human platelets, arachidonic is released from both 1,2-diacyl-PC and 1,2-diacyl-PE for metabolism by platelet cyclooxygenase and lipoxygenase, while certain ether pools of PC and PE also collect arachidonic acid.     

Ultrastructural localization of Thermoplasma acidophilum surface carbohydrate by using concanavalin A

Surface carbohydrate, presumably the lipopolysaccharide, of Thermoplasma acidophilum was visualized by means of the concanavalin A, horseradish peroxidase, and diaminobenzidine cytochemical staining procedure.     

Hormone-dependent processing of the avian progesterone receptor

Avian progesterone receptor exists as two forms, A and B, with molecular weights of 79,000 and 110,000 daltons, respectively. The origin and significance of these two forms is an area of active investigation and debate. Monoclonal antibodies produced against these two forms were used to examine receptor stability in cytosol and changes in the receptor forms induced by hormone binding. The lability of hormone binding at elevated temperatures is well documented. Analysis by Western blotting showed the receptor was stable in freshly prepared oviduct cytosol for 2 hr at 37°C, while hormone binding was lost within 30 min. However, loss of receptor through degradation was seen when cytosol was prepared from frozen tissue or when homogenization was excessive. Progesterone was injected into diethylstilbestrol-stimulated chicks to examine, in vivo, effects of hormone treatment on receptor forms in the cytosol and nuclear fractions. Progesterone treatment caused a time- and dose-dependent conversion of the A receptor to a form (A′) with a slower electrophoretic mobility. The cytosolic progesterone receptor was divided equally between the B and A forms, while the nuclear receptor was predominantly A′. The amount of nuclear receptor was consistently less than cytosolic receptor. Receptor phosphorylation was analyzed by incubating tissue minces with [³²P]orthophosphate with or without progesterone followed by immune isolation of receptor forms. Progesterone treatment caused a time-dependent increase in cytosol receptor phosphorylation which was evident after 5 min of treatment. This phosphorylation was observed with both the A and B receptor forms. The results indicate that receptor phosphorylation is a very early event during progesterone action.     

Characterization of the plasma membrane bound GTPase from rabbit neutrophils. I. Evidence for an Ni-like protein coupled to the formyl peptide, C5a, and leukotriene B4 chemotaxis receptors

We have characterized the GTPase activity of the Ni-like guanine-nucleotide-binding regulatory protein in rabbit neutrophil plasma membranes. The low Km (3.64 +/- 0.87 X 10(-7) M) GTPase copurified with the formyl peptide receptor in the plasma membrane fraction obtained by discontinuous sucrose density gradient centrifugation. The Vmax (23.9 +/- 2.91 pmol/mg/min) and Km of the unstimulated enzyme were similar to those reported for Ni in other cell types. The activity of the unstimulated enzyme was both magnesium and sodium dependent and linear over the first 4 min of the assay. The chemoattractants, formyl-methionyl-leucyl-phenylalanine (fMLP), C5a, and leukotriene B4 (LTB4) stimulated the GTPase in purified neutrophil plasma membrane preparations, whereas other secretagogues, such as A23187 and PMA, were without effect. Lineweaver-Burk analysis showed an fMLP-induced increase in Vmax (31.94 +/- 4.80 pmol/mg/min) (33.1 +/- 9.5%) but not in Km. The dose-response curve for fMLP stimulation showed an ED50 of 4.1 +/- 1.0 X 10(-8) M and an overall 22.2 +/- 3.1% maximal stimulation. C5a (30 micrograms/ml) increased the activity of the GTPase 21.3 +/- 5.7% and 10(-7) M LTB4 produced a 32.2 +/- 5.4% increase. Activated pertussis toxin treatment of neutrophil plasma membranes inhibited by 72.5 +/- 14.3% the stimulation of GTPase activity induced by fMLP; however, activated cholera toxin had no effect on the inhibition of fMLP stimulation, suggesting a direct role for an Ni-like protein in the coupling process. In contrast to the lack of inhibition of fMLP stimulation by activated cholera toxin treatment of plasma membranes, both pertussis toxin and to a lesser extent cholera toxin treatment reduced fMLP, C5a, and LTB4 stimulation of the GTPase in sonicates prepared from pretreated whole cells. Pertussis toxin inhibited fMLP stimulation of the GTPase by 75 +/- 7%, C5a stimulation was inhibited by 83 +/- 13%, and LTB4 stimulation was inhibited completely. Sonicates prepared from neutrophils treated similarly with cholera toxin showed a smaller inhibition of GTPase activity (50 +/- 4% and 14 +/- 9% for fMLP and LTB4, respectively) with the exception of C5a, where CT inhibition (81 +/- 32%) equaled pertussis toxin inhibition. Similarly, pertussis toxin completely inhibited the release of the granule enzyme N-acetyl-glucosaminidase by all three chemoattractants, whereas cholera toxin, except with C5a stimulation, had little or no effect.(ABSTRACT TRUNCATED AT 400 WORDS)