Metal-ion stimulation and inhibition of lysophospholipase D which generates bioactive lysophosphatidic acid in rat plasma.

We found that lysophospholipase D (LPLD) in rat plasma prefers unsaturated to saturated lysophosphatidylcholines as substrates, generating a biologically active lipid, lysophosphatidic acid, but it does not hydrolyze diacyl-phospholipids. In this study, this LPLD required a metal ion for activity, Co2+ being the most effective, followed in order by Zn2+, Mn2+, and Ni2+. This metal-ion-stimulated LPLD with unique substrate specificity, which has not been described previously, was susceptible to thiol-blocking reagents and serine esterase inhibitors, but not to a histidine-modifying reagent. Consistent with results using thiol-modifying agents, short-chain fatty aldehydes, secondary products of lipid peroxidation, were found to inhibit LPLD. Addition of dibutylhydroxytoluene or butylhydroxyanisole to the plasma increased the activity of this enzyme, probably in a manner independent of its antioxidant activity, since another antioxidant, propyl gallate, was rather inhibitory. These results suggest that rat plasma contains an active LPLD that differs in some properties from other members of the known phospholipase D family detected in animal tissues and body fluids.

Detection of acyl-CoA synthetase, acyl-CoA:lysophospholipid acyltransferase and phospholipase A2 activities in non-pregnant and pregnant guinea-pig uterine tissues.

Acyl-CoA synthetase (ACS), acyl-CoA:lysophospholipid acyltransferase (ACLAT) and phospholipase (PL) A2 activities were detected in guinea-pig endometrium on days 7 and 15 of the cycle, and on days 15, 29 and 36 of pregnancy. Ovariectomy of non-pregnant animals resulted in an increase in the apparent activities of these three enzymes which was reversed by treatment with oestradiol and/or progesterone. ACS, ACLAT and PLA2 activities were detected in day 15 conceptuses, and in the placenta, sub-placenta, chorion and amnion on days 29 and 36 of pregnancy. Apparent activities of the enzymes were generally higher in the fetal membranes than in the placental tissue. This study has established that the enzymes involved in turnover of arachidonic acid in phospholipids are present in tissues in the non-pregnant and pregnant guinea-pig uterus. The higher apparent activities of enzymes (ACS and ACLAT) involved in arachidonic acid uptake compared to the enzyme (PLA2) involved in arachidonic acid release is in agreement with there being very low concentrations of free arachidonic acid in tissues.

Effect of lipoprotein-X on lipid metabolism in rat kidney.

Lipoprotein-X (Lp-X) is found in the plasma of patients with familial lecithin: cholesterol acyltransferase (LCAT) deficiency syndromes. The majority of the patients with this disorder develop progressive glomerulosclerosis. In this study, the effect of Lp-X on lipid metabolism in perfused rat kidney was investigated. Lp-X was isolated from plasma of patients with familial LCAT deficiency by sequential ultracentrifugation and gel filtration column chromatography. Rat kidneys were perfused for 1-2 h with Krebs-Henseleit buffer containing 20 microM [1-(14)C]acetate or 20 microM [Me-3H]choline. In the presence of Lp-X, no significant difference in the incorporation of radioactivity into triglycerides, cholesterol, phosphocholine, CDP-choline and sphingomyelin was observed. However, incorporation of radioactivity into cholesteryl esters and phosphatidylcholine was significantly elevated in Lp-X perfused kidneys. The contents of cholesterol, cholesteryl esters and phosphatidylcholine were also significantly increased in Lp-X perfused kidneys. The increase in lipid content in the Lp-X perfused kidney is attributed to the direct deposition of Lp-X lipids into the organ. The increase in the labelling of cholesteryl esters was attributed to the increase of available substrate (cholesterol) for the acyl-CoA:cholesterol acyltransferase (ACAT) reaction. The increase in phosphatidylcholine labelling was caused by a reduced turnover of the newly synthesized labelled phosphatidylcholine during Lp-X perfusion.

Detergent solubilization of liver microsomal acyl-coenzyme A:1-acyl-glycerophosphocholine acyltransferase in nephrotic rat.

Hyperlipidaemia is a common feature of nephrotic syndrome and this has been thought to involve increased assembly and secretion of very low density lipoprotein (VLDL) in the liver. An important pathway for an indirect modulation of VLDL. Synthesis is the reaction catalyzed by the acyl-coenzyme A:1-acyl-glycero-phosphcholine acyl transferase. We therefore investigated the activity of this enzyme in liver microsomes isolated from puromycin amino nucleoside induced nephrotic rats. When oleoyl-CoA was employed as the acyl-donor, our results indicated that both the total and detergent soluble enzyme activities (112.2 +/- 16.7; 116.1 +/- 17.5 units, respectively) were significantly higher than the corresponding control levels of 91.1 +/- 11.1 and 75.4 +/- 20.9 units respectively. The percentage stimulation by sodium cholate were 176.5 and 192.2 for the control and nephrotic rats, respectively. In absence of sodium cholate, when oleoyl CoA was replaced by arachidonoyl-CoA as acyl-donor, the measured total enzyme activity was only significantly reduced in the control rats (71.1 +/- 8.9 Vs 91.1 +/- 11.1 Units). Oleoyl-CoA as acyl-donor gave higher values for the soluble and residual enzyme activities (90.4 13.3; 99.5 34.5 unit) than the corresponding control levels (75.9 +/- 10.0; 50.5 +/- 34.0 units) as compared to arachidonoyl-CoA. In the control group the difference was only significant in the residual activity (92.9 20.5 Vs 64.7 24.1 units). The addition of monomethylethanomine (200 mM) had little or no effect, while both reduced glutathione (10 mM) and 1,2-diacylglycerol (1 mM) caused significant reduction in measured activity. These results indicated that in nephrotic rats new phospholipid synthesis is enhanced and this could contribute to the increased VLDL assembly and secretion usually associated with nephrotic syndrome.

Changes in microsomal lysophospholipid acyltransferase activity are correlated with rat parotid gland enlargement induced by chronic administration of isoproterenol.

Chronic (5- and 10-day) administration of isoproterenol, an agent that induces the proliferation of salivary gland cells, produced increases in microsomal 1-acyl-sn-glycero-3-phosphate acyltransferase and 1-acyl-sn-glycero-3-phosphocholine acyltransferase activity in rat parotid glands in parallel with gland enlargement. This increased activity was reduced when the treatment was stopped, the reduction corresponding to the reduction in gland weight. There were significant correlations between lysophospholipid acyltransferase activity and gland weight, and between the activities of the two types of lysophospholipid acyltransferase. However, isoproterenol treatment did not affect any of the steps of the subsequent phospholipid N-methylation. These results suggest that the cell proliferation induced by chronic administration of isoproterenol in the parotid gland is accompanied by reversible and selective increases in microsomal lysophospholipid acyltransferases.

Inhibitors of membrane lipid metabolism enhance complement-mediated nucleated cell killing through distinct mechanisms.

The ability of nucleated cells to survive limited complement attack has been attributed to metabolic processes unique to these cells, such as rapid elimination of terminal complement complexes (TCC) from their surfaces. The biochemical processes activated by complement channels responsible for cell defense remain poorly defined. Metabolic inhibitors affecting membrane lipid turnover have been shown to increase the complement-mediated cell death. Whether these metabolic inhibitors increase lytic susceptibility of target cells by reducing the rate of TCC elimination has not been previously evaluated. In the present study, inhibitors of membrane lipid transmethylation and lysolecithin reacylation were evaluated in view of the observations that TCC concurrently increase lipid transmethylation and inhibit lysolecithin reacylation, and the inhibition of lipid transmethylation correlates with increased complement-mediated cell death. We have measured the formation as well as the elimination of C5b-9 on the target membrane that affect the outcome of cell death. Our results in the present communication indicated that inhibitors of transmethylation and lysolecithin reacylation increased TCC-mediated cell death through distinct pathways, the former by allowing more efficient deposition of TCC, and the latter by impairing TCC elimination.