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.

Production of lysophosphatidic acid by lysophospholipase D in incubated plasma of spontaneously hypertensive rats and Wistar Kyoto rats

Lysophosphatidic acid has been identified as a vasopressor principle in incubated mammalian plasma and sera, and shown to be generated extracellulary by lysophospholipase D-like activity. In this study, we monitored the time course of changes in the major phospholipid fractions during incubation of plasma, and found that polyunsaturated lysophosphatidic acids accumulate more rapidly than saturated lysophosphatidic acids at expense of the corresponding lysophosphatidylcholines. We compared the phospholipase activities for producing bioactive LPA in age-matched spontaneously hypertensive rats and Wistar Kyoto rats. The lysophospholipase D activity in rat plasma was found to be independent of strain and age. We suggest that lysophospholipase D functions in rat for persistent production of bioactive LPA in the circulation throughout life. However, our finding that production of LPA in spontaneously hypertensive rats was not greater than that in Wistar Kyoto rats does not seem to support the idea that increased production of LPA is involved in the pathogenesis of hypertension.

Substrate specificity of lysophospholipase D which produces bioactive lysophosphatidic acids in rat plasma

Previously we reported that lysophospholipase D in rat plasma hydrolyzes endogenous unsaturated lysophosphatidylcholines (LPCs) preferentially to saturated LPCs to lysophosphatidic acids with growth factor-like and hormone-like activities. In this study, we examined the possibility that association of LPCs with different proteins in rat plasma has an effect on the preference of lysophospholipase D for unsaturated LPCs. Large portions of various LPCs were found to be recovered in the lipoprotein-poor bottom fraction. Furthermore, the percentages of LPCs associated with albumin isolated from rat plasma were shown not to be consistent with their percentage conversions to lysophosphatidic acids by lysophospholipase D on incubation of rat plasma at 37 degrees C. These results indicate that distinct distributions of LPCs in the plasma protein fractions are not critical factors for the substrate specificity of lysophospholipase D. Experiments with Nagase analbuminemic rats suggested that albumin-LPC complexes are not necessarily required for the hydrolysis by lysophospholipase D; lipoprotein-associate LPCs appeared to be good substrates for the phospholipase. We found that both saturated and unsaturated LPCs are present mainly as 1-acyl isomers in rat plasma. This result indicates that the preference of lysophospholipase D for unsaturated LPCs is not attributable to a difference in position of the acyl group attached to the glycerol backbone of LPC. In addition, lysophospholipase D was also found to attack choline phospholipids with a long chain group and a short chain alkyl group, although their percentage hydrolyses were low. Taken altogether, these results suggest that lysophospholipase D shows higher affinities for free forms of unsaturated acyl type LPCs equilibrated with albumin-bound and lipoprotein-associated forms, than for free forms of saturated acyl type LPCs and analogs of platelet-activating factor.

Magnetic circular dichroism at transition metal L2,3 edges in D03-type (Fe(1-x)Vx)3Al alloys

Magnetic circular dichroism (MCD) spectra have been measured at the Fe and V L2,3 edges of DO3-type (Fe(1-x)Vx)3Al in order to investigate their local magnetic moments and electronic structures. Large MCD is observed at the Fe L2,3 edges, while the V L2,3 MCD shows relatively small intensity with complicated features. Signs of these MCD spectra indicate an antiferromagnetic coupling between the magnetic moments on Fe and V. According to the analysis based on the magneto-optical sum rules, the magnetic moment decreases with x, but remains fairly large for Fe2VAl, which might arise from its marginally magnetic nature.

Magnetic circular dichroism at transition metal L2,3 edges in D03-type (Fe(1-x)Mn(x))3Al alloys

We have measured magnetic circular dichroism (MCD) spectra at the transition-metal L2,3 edges in D03-type (Fe(1-x)Mn(x))3Al in order to investigate their local magnetic moments. The analysis of the spectra shows that Fe has moments much larger than Mn, whose moment is ferromagnetically coupled with the Fe one. This does not lend support to the antiferromagnetic mechanism proposed for the reduction in magnetization as well as a large Mn moment predicted for x = 1/3. The evolution of satellites found in the Mn spectrum with x increased suggests that the change in the electronic state may result in the magnetization reduction.