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Biomedical subjects

M M Billah

Publications and source records attributed to M M Billah.

87 records · Page 5Linked to original sources
PubMed1981

The initial action of thrombin on platelets. Conversion of phosphatidylinositol to phosphatidic acid preceding the production of arachidonic acid.

Measurements of phosphatidylinositol (PI) and phosphatidic acid (PA) by phosphorus assays and by radioactivity ([14C]arachidonate) indicate that thrombin induces the degradation of a given fraction of the total PI to PA. The maximal conversion of PI to PA represents approximately one-third of the total PI which can be degraded by thrombin. This same amount of PI is converted to PA even in the presence of 1 mM quinacrine, which completely inhibits the release of arachidonic acid from phospholipids and which reduces by two-thirds the loss of labeled PI. In this case the fall in PI is equal to the amount of PA formed. If thrombin is added to platelets previously maximally stimulated by ionophore A23187, PA is produced from PI in amounts equal to those produced by thrombin in the absence of other stimuli. Furthermore, the resynthesis of PI from PA is also unaffected by quinacrine, and thus the entire thrombin-stimulated PI-cycle is maintained. The data thus indicate the existence of a quinacrine-insensitive phospholipase C which can initially convert a given amount of PI to PA and which is closely associated to the thrombin receptor. The further breakdown of PI and production of arachidonic acid might result from the action of quinacrine-sensitive activities (i.e. phospholipase A2). The simplest scheme is one in which thrombin specifically produces an active fraction of PA which in some way results in the subsequent production of arachidonic acid from various phospholipids (including PI), perhaps by activation of quinacrine-sensitive phospholipase A2.

Arachidonic Acids↗
PubMed1981

The importance of the production of phosphatidic acid for the release of arachidonic acid in stimulated platelets.

Phosphatidic acid is formed by platelets as a result of the combined activities of phospholipase C and diacylglycerol kinase. The initial thrombin-activated phospholipase C-mediated reaction is quinacrine insensitive and is followed by quinacrine-sensitive phospholipase A2 activities. The phosphatidic acid released probably results in cellular calcium gating, which, among other actions, might release arachidonic acid by activating phospholipases A2. Indeed, we have detected a phosphatidic acid-specific phospholipase A2 in platelets, which might have an important role in the liberation of arachidonic acid.

Arachidonic Acid↗
PubMed1980

Rapid acylation and deacylation of arachidonic acid into phosphatidic acid of horse neutrophils.

Horse neutrophils incorporate exogenous [14C]arachidonate into phosphatidic acid very rapidly. This acylation of phosphatidate with arachidonate is followed quickly and spontaneously by its deacylation. This transient formation of arachidonyl-phosphatidate, which reflects a rapidly turning over pool of arachidonate-associated lipid, is not observed with stearic acid or other phospholipids or triglycerides. Phosphatidylcholine, phosphatidylethanolamine, phosphatidylinosinositol, and triglycerides are slowly but increasingly labeled with time. Ionophore A23187 (10 microM) stimulates the extent of labeling of phosphatidate while decreasing the labeling of all the other phospholipids and triglycerides. Phosphatidate is not transiently labeled with [14C]stearate of (32P)orthophosphate, either in the presence or absence of ionophore A23187. When cells are prelabeled for 2 h with very high quantities of (32P)orthophosphate a very substantial fraction (i.e. 20 to 30%) of the phospholipid radioactivity is associated with phosphatidic acid. However, on addition of exogenous arachidonate, there is no increase in [32P]phosphatidate in these prelabeled cells. Thus, the entire phosphatidate molecule does not appear to be turned over during the process described above. Inhibitors of cyclooxygenase and lipoxygenase activities such as BW755C, nordihydroguaiaretic acid, and low concentrations of indomethacin do not affect the labeling of phospholipids. However, eicosatetraynoic acid, an analog of arachidonate, and high concentration (0.1 mM) of indomethacin can block [14C]arachidonate incorporation into lipids. The rapid turnover of the 2-acyl position in phosphatidate might be related to a specific process of fatty acid mobilization within neutrophils.

Acylation↗
PubMed1980

Phospholipase A2 and phospholipase C activities of platelets. Differential substrate specificity, Ca2+ requirement, pH dependence, and cellular localization.

Deoxycholate treatment of horse platelets previously labeled in their phospholipids with [14C]arachidonate produces selective conversion of [14C]phosphatidylinositol (PI) to [14C]1,2-diacylglycerol. This phospholipase C activity, which has a pH optimum of 7.5, is specific for phosphatidylinositol since other phospholipids or neutral lipids are not affected. Although exogenous Ca2+ is not required for activity, ethylene glycol bis(beta-aminoethyl ether)N,N,N',N-tetraacetic acid or EDTA abolishes phosphatidylinositol degradation. However, in the presence of added Ca2+, other phospholipids such as phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylserine (PS) are also degraded but by a phospholipase A2 activity. This activity generates the respective lyso-derivatives as well as various [14C]arachidonate metabolites. The phospholipase A2 activity is further enhanced by increasing the pH (7.5 to 9.5), a condition which severely suppresses the phospholipase C activity. Most of the platelet phospholipase A2 activity is associated with the particulate fractions of the cell, while the phospholipase C activity appears to be almost completely restricted to the soluble fraction. Deoxycholate treatment of the particulate fractions results in cleavage by phospholipase A2 of phosphatidylcholine and phosphatidylethanolamine but not of phosphatidylinositol. The preferred substrates for platelet phospholipase A2 appear to be phosphatidylethanolamine, phosphatidylcholine, and phosphatidylserine, while phosphatidylinositol seems to be degraded nearly exclusively by phospholipase C.

Animals↗
PubMed1979

Phosphatidylinositol metabolism in rat hepatocytes stimulated by glycogenolytic hormones. Effects of angiotensin, vasopressin, adrenaline, ionophore A23187 and calcium-ion deprivation.

1. The effects on phosphatidylinositol metabolism of three Ca(2+)-mobilizing glycogenolytic hormones, namely angiotensin, vasopressin and adrenaline, have been investigated by using rat hepatocytes. 2. All three hormones stimulate both phosphatidylinositol breakdown and the labelling of this lipid with (32)P. 3. The response to angiotensin occurs quickly, requires a high concentration of the hormone and is prevented by [1-sarcosine, 8-isoleucine]angiotensin, a specific angiotensin antagonist that does not prevent the responses to vasopressin and to adrenaline. This response therefore seems to be mediated by angiotensin-specific receptors. 4. [1-Deaminocysteine,2-phenylalanine,7-(3,4-didehydroproline),8-arginine] vasopressin, a vasopressin analogue with enhanced antidiuretic potency, is relatively ineffective at stimulating phosphatidylinositol metabolism. This suggests that the hepatic vasopressin receptors that stimulate phosphatidylinositol breakdown are different in their ligand selectivity from the antidiuretic vasopressin receptors that activate renal adenylate cyclase. 5. Incubation of hepatocytes with ionophore A23187, a bivalent-cation ionophore, neither mimicked nor appreciably changed the effects of vasopressin on phosphatidylinositol metabolism, suggesting that phosphatidylinositol breakdown is not controlled by changes in the cytosol Ca(2+) concentration. This conclusion was supported by the observation that hormonal stimulation of phosphatidylinositol breakdown and resynthesis persists in cells incubated for a substantial period in EGTA, although this treatment somewhat decreased the phosphatidylinositol response of the hepatocyte. The phosphatidylinositol response of the hepatocyte therefore appears not to be controlled by changes in cytosol [Ca(2+)], despite the fact that this ion is thought to be the second messenger by which the same hormones control glycogenolysis. 6. These results may be an indication that phosphatidylinositol breakdown is an integral reaction in the stimulus-response coupling sequence(s) that link(s) activation of alpha-adrenergic, vasopressin and angiotensin receptors to mobilization of Ca(2+) in the rat hepatocyte.

1-Sarcosine-8-Isoleucine Angiotensin II↗
PubMed1977

Permeability characteristics of erythrocyte ghosts prepared under isoionic conditions by a glycol-induced osmotic lysis.

A detailed study has been made of the permeability characteristics of human erythrocyte ghosts prepared under isoionic conditions by a glycol-induced lysis (Billah, M.M., Finean, J.B., Coleman, R. and Michell, R.H. (1976) Biochim. Biophys. Acta 433, 45-54). Impermeability to large molecules such as dextran (average molecular weight 70 000) was restored immediately and spontaneously after each of the 5-7 lyses that were required to remove all of the haemoglobin. Permeabilities to smaller molecules such as MgATP2-, [3H]inositol and [14C]choline were initially high but could be greatly reduced by incubation at 37 degrees C for an hour. The extent of such resealing decreased as the number of lyses to which the ghosts had been subjected increased. Both removal of haemoglobin and permeabilities to small molecules were affected significantly by pH, CA3+ concentrations and divalent cation chelators. Maximum resealing was achieved in ghosts prepared in the basic ionic medium (130 mM KCl, 10 nM NaCl, 2 mM MgCl2, 10 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulphonic acid (HEPES)) at pH 7.0 (0 degrees C) and with a calcium level around 10(-5) M. Acidic pH facilitated the removal of haemoglobin whilst the presence of divalent cation chelators showed down its release. Retention of K+ by ghosts leaded with K+ during the first lysis and subsequently incubated at 37 degrees C was substantial but lation chelators slowed down its released. Retention of K+ by ghosts loaded with K+ during the first lysis and subsequently incubated at 37 degrees C was substantial but little K+ could be retained within the haemoglobin-free ghosts. Permeability of the ghosts to K+ after one lysis was affected by temperature, pH, Ca2+ concentrations and by the presence of divalent cation chelators.

Adenosine Triphosphate↗
PubMed1976

Preparation of erythrocyte ghosts by a glycol-induced osmotic lysis under isoionic conditions.

A procedure has been developed for obtaining haemoglobin-free, erythrocyte ghosts under ionic conditions approximating that of the cell cytoplasm. Haemolysis was effected by incorporating glycol into cells suspended in the isoionic medium and then diluting with a large volume of glycol-free medium. The ghosts were of uniform spherical shape throughout the preparative procedure and were impermeable to macromolecules. Analysis of polypeptides by sodium dodecyl sulphate-gel electrophoresis at each stage of preparation and comparison with ghosts prepared under hypo-ionic conditions served to distinguish membrane components from those of cytoplasm.

Cell Fractionation↗