The platelet-activating factor acetylhydrolase from human erythrocytes. Purification and properties

Behavior of platelet activating factor in membrane-mimicking environment. Langmuir monolayer study complemented with grazing incidence X-ray diffraction and Brewster angle microscopy

1-O-octadecyl-2-acetyl-sn-glycero-3-phosphocholine (PAF) belonging to the class of single-chained ether phospholipids is widely known from its essential biological activities. There is a growing body of evidence that some significant aspects of PAF actions are connected with its capability to direct intercalation into biomembranes' environment. Although this mechanism is of great importance in the perspective of understanding PAF implications in various physiological processes, in the literature, there is a lack of studies devoted to this subject. It is still unknown which is the exact influence of membrane composition, molecular organization, and its other properties on the PAF impact on cells and tissues. Unfortunately, the biological studies carried out on cell cultures do not provide satisfactory results, mainly because of the complexity of natural systems. In order to obtain insight into the behavior of PAF in a lipid environment at the molecular level, the application of appropriate model systems is required. Among them, Langmuir monolayers are very often applied as a simple but very efficient platform for studies of the interactions between membrane lipids. In the present paper, special attention is focused on the issue concerning the interactions between PAF and two representatives of membrane components occurring mainly in the outer leaflet of natural bilayers, namely, cholesterol and DPPC. The application of Langmuir monolayers enabled us to construct the effective model mimicking the exogenous incorporation of PAF into membrane environment. On the basis of the obtained results, a thorough discussion was carried out and the conclusions derived from the traditional thermodynamic analysis were confronted with microscopic analysis of surface domains and the GIXD results. The selection of experimental techniques enables us to obtain information regarding the miscibility and interactions in the binary mixed films as well as the molecular organization of film-forming molecules on water surface. The experiments revealed that the addition of the investigated single-chained ether phospholipid into both cholesterol and DPPC monolayers causes a considerable decrease of monolayer condensation. On the basis of thermodynamic analysis, it was found that PAF mixes and consequently interacts strongly with cholesterol, whereas its interactions with DPPC are thermodynamically unfavorable. Differences between the PAF influence on cholesterol and DPPC monolayer found its corroboration in the results obtained with the GIXD technique. Namely, the monolayer of DPPC can incorporate more PAF than the model membrane containing cholesterol. The obtained results indicate that short chained sn-2 ether phospholipid is able to modify model membrane properties in a concentration-dependent way.

Intracellular erythrocyte platelet-activating factor acetylhydrolase I inactivates aspirin in blood

Aspirin (acetylsalicylic acid) prophylaxis suppresses major adverse cardiovascular events, but its rapid turnover limits inhibition of platelet cyclooxygenase activity and thrombosis. Despite its importance, the identity of the enzyme(s) that hydrolyzes the acetyl residue of circulating aspirin, which must be an existing enzyme, remains unknown. We find that circulating aspirin was extensively hydrolyzed within erythrocytes, and chromatography indicated these cells contained a single hydrolytic activity. Purification by over 1400-fold and sequencing identified the PAFAH1B2 and PAFAH1B3 subunits of type I platelet-activating factor (PAF) acetylhydrolase, a phospholipase A(2) with selectivity for acetyl residues of PAF, as a candidate for aspirin acetylhydrolase. Western blotting showed that catalytic PAFAH1B2 and PAFAH1B3 subunits of the type I enzyme co-migrated with purified erythrocyte aspirin hydrolytic activity. Recombinant PAFAH1B2, but not its family member plasma PAF acetylhydrolase, hydrolyzed aspirin, and PAF competitively inhibited aspirin hydrolysis by purified or recombinant erythrocyte enzymes. Aspirin was hydrolyzed by HEK cells transfected with PAFAH1B2 or PAFAH1B3, and the competitive type I PAF acetylhydrolase inhibitor NaF reduced erythrocyte hydrolysis of aspirin. Exposing aspirin to erythrocytes blocked its ability to inhibit thromboxane A(2) synthesis and platelet aggregation. Not all individuals or populations are equally protected by aspirin prophylaxis, the phenomenon of aspirin resistance, and erythrocyte hydrolysis of aspirin varied 3-fold among individuals, which correlated with PAFAH1B2 and not PAFAH1B3. We conclude that intracellular type I PAF acetylhydrolase is the major aspirin hydrolase of human blood.

Plasma levels of lipoprotein-associated phospholipase A(2) are increased in patients with β-thalassemia

Lipoprotein-associated phospholipase A(2) (Lp-PLA(2)) is an independent cardiovascular risk factor. We investigated the plasma levels of Lp-PLA(2) activity and mass as a function of plasma lipid levels, LDL subclass profile, and oxidative stress in patients with β-thalassemia. Thirty-five patients with β-thalassemia major (β-TM) and 25 patients with β-thalassemia intermedia (β-TI) participated in the study. Lp-PLA(2) activity and mass were measured in total plasma, in apolipoprotein (apo)B-depleted plasma (HDL-Lp-PLA(2)), and in LDL subclasses. Lp-PLA(2) activity produced and secreted from peripheral blood monocytes in culture was also determined. Patients with β-thalassemia are characterized by a predominance of small-dense LDL particles, increased oxidative stress, and very high plasma levels of Lp-PLA(2) mass and activity, despite low LDL-cholesterol levels. A significant positive correlation between plasma Lp-PLA(2) activity or mass and 8-isoprostane (8-epiPGF2a) and ferritin levels as well as intima-media thickness (IMT) values was observed. An increase in secreted and cell-associated Lp-PLA(2) activity from monocytes in culture was observed in both patient groups. The HDL-Lp-PLA(2) activity and mass as well as the ratio of HDL-Lp-PLA(2)/plasma Lp-PLA(2) were significantly higher in both patient groups compared with the control group. In conclusion, patients with β-thalassemia exhibit high plasma Lp-PLA(2) levels, attributed to increased enzyme secretion from monocytes/macrophages and to the predominance of sdLDL particles in plasma. Plasma Lp-PLA(2) is correlated with carotid IMT, suggesting that this enzyme may be implicated in premature carotid atherosclerosis observed in β-thalassemia.

Intracellular PAF catabolism by PAF acetylhydrolase counteracts continual PAF synthesis

Stimulated inflammatory cells synthesize platelet-activating factor (PAF), but lysates of these cells show little enhancement in PAF synthase activity. We show that human neutrophils contain intracellular plasma PAF acetylhydrolase (PLA2G7), an enzyme normally secreted by monocytes. The esterase inhibitors methyl arachidonoylfluorophosphonate (MAFP), its linoleoyl homolog, and Pefabloc inhibit plasma PAF acetylhydrolase. All of these inhibitors induced PAF accumulation by quiescent neutrophils and monocytes that was equivalent to agonist stimulation. Agonist stimulation after esterase inhibition did not further increase PAF accumulation. PAF acetylhydrolase activity in intact neutrophils was reduced, but not abolished, by agonist stimulation. Erythrocytes, which do not participate in the acute inflammatory response, inexplicably express the type I PAF acetylhydrolase, whose only known substrate is PAF. Inhibition of this enzyme by MAFP caused PAF accumulation by erythrocytes, which was hemolytic in the absence of PAF acetylhydrolase activity. We propose that PAF is continuously synthesized by a nonselective acyltransferase activity(ies) found even in noninflammatory cells as a component of membrane remodeling, which is then selectively and continually degraded by intracellular PAF acetylhydrolase activity to modulate PAF production.

Erythrocyte PAF-acetylhydrolase activity in various stages of chronic kidney disease: effect of long-term therapy with erythropoietin

BACKGROUND

Erythrocytes represent an important component of the antioxidant capacity of blood, comprising, in particular, intracellular enzymes, including platelet-activating factor acetylhydrolase (PAF-AH) and glutathione peroxidase (Gpx). We evaluated the erythrocyte PAF-AH and Gpx activities in various stages of chronic kidney disease (CKD), and further investigated whether erythropoietin (EPO) administration in these patients has any influence on the enzyme activities.

METHODS

Thirty-six patients (19 men and 17 women) with CKD (stages 1 to 5) participated in the study. Thirteen of them presented with CKD stage 1 to 2 (group I), whereas 23 patients presented with CKD stage 3 to 5 and randomized into two groups (i.e., groups II and III). Patients of group II (N= 11) were administered EPO subcutaneously, 50 units per kg once per week. In group III (N= 12), EPO was initiated only when the hemoglobin (Hb) levels decreased during follow-up to less than 9 g/dL. All patients were seen on an outpatient basis at 2 and 4 months. Fifteen normolipidemic age- and sex-matched healthy volunteers also participated in the study and were used as controls. The PAF-AH and Gpx activities were determined in isolated washed erythrocytes.

RESULTS

The erythrocyte-associated PAF-AH and Gpx activities were higher in all CKD patient groups at baseline compared to controls, the groups II and III exhibiting significantly higher enzyme activities compared with group I. In all studied populations, both enzyme activities were negatively correlated with the creatinine clearance values. Importantly, the PAF-AH and Gpx activities were progressively decreased during the follow-up in patients not treated with EPO (group III), a phenomenon not observed in patients receiving EPO (group II), or in patients of group I. This reduction in enzyme activities was positively correlated with the decrease in the creatinine clearance values in patients of group III.

CONCLUSION

Significant alterations in the erythrocyte-associated PAF-AH and Gpx activities related to the disease stage are observed in CKD patients. Administration of EPO prevented the reduction in enzyme activities observed during the progression of the renal insufficiency, thus preserving the erythrocyte defense mechanisms against oxidative stress.

Aldose reductase-catalyzed reduction of aldehyde phospholipids

Oxidation of unsaturated phospholipids results in the generation of aldehyde side chains that remain esterified to the phospholipid backbone. Such "core" aldehydes elicit immune responses and promote inflammation. However, the biochemical mechanisms by which phospholipid aldehydes are metabolized or detoxified are not well understood. In the studies reported here, we examined whether aldose reductase (AR), which reduces hydrophobic aldehydes, metabolizes phospholipid aldehydes. Incubation with AR led to the reduction of 5-oxovaleroyl, 7-oxo-5-heptenoyl, 5-hydroxy-6-oxo-caproyl, and 5-hydroxy-8-oxo-6-octenoyl phospholipids generated upon oxidation of 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine (PAPC). The enzyme also catalyzed the reduction of phospholipid aldehydes generated from the oxidation of 1-alkyl, and 1-alkenyl analogs of PAPC, and 1-palmitoyl-2-arachidonoyl phosphatidic acid or phosphoglycerol. Aldose reductase catalyzed the reduction of chemically synthesized 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphatidylcholine (POVPC) with a K(m) of 10 mum. Addition of POVPC to the culture medium led to incorporation and reduction of the aldehyde in COS-7 and THP-1 cells. Reduction of POVPC in these cells was prevented by the AR inhibitors sorbinil and tolrestat and was increased in COS-7 cells overexpressing AR. Together, these observations suggest that AR may be a significant participant in the metabolism of several structurally diverse phospholipid aldehydes. This metabolism may be a critical regulator of the pro-inflammatory and immunogenic effects of oxidized phospholipids.

Detection and characterization of 45 kDa platelet activating factor acetylhydrolase in cerebrospinal fluid of children with meningitis

Platelet activating factor acetylhydrolase (PAF-AH) activity has been identified in cerebrospinal fluid (CSF) samples taken from children with meningitis. We reported that PAF-AH activity is significantly increased, by about 3 fold, in patients with meningitis compared to control subjects. Because of limited knowledge about this enzyme in CSF, we examined the biochemical properties of CSF PAF-AH. PAF-AH of CSF was calcium independent, showed a broad pH spectrum and was relatively heat stable. In addition, this enzyme activity was strongly inhibited by phenylmethanesulfonyl fluoride (PMSF), partially inhibited by p-bromophenacylbromide (p-BPB), uninhibited by iodoacetamide, and moderately stimulated by dithiothreitol (DTT). PAF-AH of CSF did not degrade phospholipid with a long chain fatty acyl group at sn-2 position. This enzyme hydrolyzed PAF and oxidatively modified phosphatidylcholine. Furthermore, we identified a monomeric polypeptide with a molecular weight of approximately 45 kDa by Western blot using human plasma PAF-AH antibody. These results suggested that plasma type PAF-AH activity exist in CSF taken from children with meningitis.

Human group IVC phospholipase A2 (cPLA2gamma). Roles in the membrane remodeling and activation induced by oxidative stress

To create the unique properties of a certain cellular membrane, both the composition and the metabolism of membrane phospholipids are key factors. Phospholipase A(2) (PLA(2)), with hydrolytic enzyme activities at the sn-2 position in glycerophospholipids, plays critical roles in maintaining the phospholipid composition as well as producing bioactive lipid mediators. In this study we examined the contribution of a Ca(2+)-independent group IVC PLA(2) isozyme (cPLA(2)gamma), a paralogue of cytosolic PLA(2)alpha (cPLA(2)alpha), to phospholipid remodeling. The enzyme was localized in the endoplasmic reticulum and Golgi apparatus, as seen using green fluorescence fusion proteins. Electrospray ionization mass spectrometric analysis of membrane extracts revealed that overexpression of cPLA(2)gamma increased the proportion of polyunsaturated fatty acids in phosphatidylethanolamine, suggesting that the enzyme modulates the phospholipid composition. We also found that H(2)O(2) and other hydroperoxides induced arachidonic acid release in cPLA(2)gamma-transfected human embryonic kidney 293 cells, possibly through the tyrosine phosphorylation pathway. Thus, we propose that cPLA(2)gamma is constitutively expressed in the endoplasmic reticulum and plays important roles in remodeling and maintaining membrane phospholipids under various conditions, including oxidative stress.

Platelet-activating factor acetylhydrolase

Platelet-activating factor (PAF) is one of the most potent lipid mediators and is involved in a variety of physiological events. The acetyl group at the sn-2 position of its glycerol backbone is required for its biological activity, and deacetylation of PAF induces loss of activity. The deacetylation reaction is catalyzed by PAF-acetylhydrolase (PAF-AH). A series of biochemical and enzymological studies have revealed that there are at least three types of PAF-AH in mammals, namely the intracellular type I and II and plasma type. Type I PAF-AH is a G-protein-like complex of two catalytic subunits (alpha1 and alpha2) and a regulatory beta subunit. The beta subunit is a product of the LIS1 gene, mutations of which cause type I lissencephaly. Recent studies indicate that LIS1/beta is important in cellular functions such as induction of nuclear movement and control of microtubule organization. Although circumstantial evidence is accumulating supporting the idea that the catalytic subunits are also involved in microtubule function, it is still not known what role PAF plays in the process and whether PAF is a native endogenous substrate of this enzyme. Type II PAF-AH is a single polypeptide and shows significant sequence homology with plasma PAF-AH. Type II PAF-AH is myristoylated at the N-terminus and like other N-myristoylated proteins, is distributed in both the cytosol and membranes. Plasma PAF-AH is also a single polypeptide and exists in association with plasma lipoproteins. Type II PAF-AH as well as plasma PAF-AH may play roles as scavengers of oxidized phospholipids which are thought to be involved in diverse pathological processes, including disorganization of membrane structure and PAF-like proinflammatory actions. In this chapter, author focuses on the structures and possible biological functions of intracellular PAF-AHs.

Transfection of the plasma-type platelet-activating factor acetylhydrolase gene attenuates glutamate-induced apoptosis in cultured rat cortical neurons

Using an adenoviral vector, we induced overexpression of the plasma type of platelet-activating factor acetylhydrolase in cultured rat neurons. Neurons overexpressing this enzyme showed a decrease in glutamate-induced injury, mainly, apparent as decreased apoptosis. Reduction of lipid peroxidation by this enzyme and protection of mitochondrial function were demonstrated, and these may be the basis of the resistance to glutamate-induced neuronal injury that we observed.

Platelet-activating factor acetylhydrolases in health and disease

The platelet-activating factor (PAF) acetylhydrolases catalyze hydrolysis of the sn-2 ester bond of PAF and related pro-inflammatory phospholipids and thus attenuate their bioactivity. One secreted (plasma) and four intracellular isozymes have been described. The intracellular isozymes are distinguished by differences in primary sequence, tissue localization, subunit composition, and substrate preferences. The most thoroughly characterized intracellular isoform, Ib, is a G-protein-like complex with two catalytic subunits (alpha1 and alpha2) and a regulatory beta subunit. The beta subunit is a product of the LIS1 gene, mutations of which cause Miller-Dieker lissencephaly. Isoform II is a single polypeptide that is homologous to the plasma PAF acetylhydrolase and has antioxidant activity in several systems. Plasma PAF acetylhydrolase is also a single polypeptide with a catalytic triad of amino acids that is characteristic of the alpha/beta hydrolases. Deficiency of this enzyme has been associated with a number of pathologies. The most common inactivating mutation, V279F, is found in >30% of randomly surveyed Japanese subjects (4% homozygous, 27% heterozygous). The prevalence of the mutant allele is significantly greater in patients with asthma, stroke, myocardial infarction, brain hemorrhage, and nonfamilial cardiomyopathy. Preclinical studies have demonstrated that recombinant plasma PAF acetylhydrolase can prevent or attenuate pathologic inflammation in a number of animal models. In addition, preliminary clinical results suggest that the recombinant enzyme may have pharmacologic potential in human inflammatory disease as well. These observations underscore the physiological importance of the PAF acetylhydrolases and point toward new approaches for controlling pathologic inflammation.

Phospholipase A(2)s and lipid peroxidation

Lipid peroxidation of membrane phospholipids can proceed both enzymatically via the mammalian 15-lipoxygenase-1 or the NADPH-cytochrome P-450 reductase system and non-enzymatically. In some cells, such as reticulocytes, this process is biologically programmed, whereas in the majority of biological systems lipid peroxidation is a deleterious process that has to be repaired via a deacylation-reacylation cycle of phospholipid metabolism. Several reports in the literature pinpoint a stimulation by lipid peroxidation of the activity of secretory phospholipase A(2)s (mainly pancreatic and snake venom enzymes) which was originally interpreted as a repair function. However, recent experiments from our laboratory have demonstrated that in mixtures of lipoxygenated and native phospholipids the former are not preferably cleaved by either secretory or cytosolic phospholipase A(2)s. We propose that the platelet activating factor (PAF) acetylhydrolases of type II, which cleave preferentially peroxidised or lipoxygenated phospholipids, are competent for the phospholipid repair, irrespective of their role in PAF metabolism. A corresponding role of Ca(2+)-independent phospholipase A(2), which has been proposed to be involved in phospholipid remodelling in biomembranes, has not been addressed so far. Direct and indirect 15-lipoxygenation of phospholipids in biomembranes modulates cell signalling by several ways. The stimulation of phospholipase A(2)-mediated arachidonic acid release may constitute an alternative route of the arachidonic acid cascade. Thus, 15-lipoxygenase-mediated oxygenation of membrane phospholipids and its interaction with phospholipase A(2)s may play a crucial role in the pathogenesis of diseases, such as bronchial asthma and atherosclerosis.

Platelet-activating factor acetylhydrolase gene mutation in Japanese children with Escherichia coli O157-associated hemolytic uremic syndrome

Platelet-activating factor (PAF) may be involved in the pathogenesis of Escherichia coli O157-associated hemolytic uremic syndrome (HUS). PAF is degraded to inactive products by PAF acetylhydrolase. In this study, we investigated whether a PAF acetylhydrolase gene mutation (G-->T transversion at position 994) is involved in HUS in Japanese children. A point mutation in the PAF acetylhydrolase gene (G994T) was identified using polymerase chain reaction in 50 Japanese children with E coli O157-associated HUS and 100 healthy Japanese. We then determined the relationship between the PAF acetylhydrolase G994T gene mutation and clinical features of HUS. There was no difference in genotype and allele frequencies between patients with HUS and healthy controls. The mean duration of oligoanuria was significantly longer in patients with the GT genotype than in those with the GG genotype (P = 0.012). Although 11 of 15 patients (73%) heterozygous for the mutant allele (GT) required dialysis, only 13 of the 35 wild-type homozygotes (GG; 37%) required dialysis (P = 0. 030). Mean plasma PAF acetylhydrolase activity was significantly less in patients with the GT genotype than in those with the GG genotype (P < 0.0001). In conclusion, we have shown an association between the G994T PAF acetylhydrolase gene mutation and the severity of renal damage in E coli O157-associated HUS. Our study suggests that analysis of the PAF acetylhydrolase gene mutation in Japanese children with E coli O157-associated HUS may allow the prediction of the severity of HUS.

PAF-acetylhydrolases

Platelet-activating factor acetylhydrolases (PAF-AHs, EC 3.1.1.47) constitute a unique subfamily of phospholipases A(2), specific for short acyl chains in the sn-2 position of the phospholipid. Their primary substrate is the platelet-activating factor, PAF, from which they cleave an acetyl moiety with concomitant release of lysoPAF. However, some acetylhydrolase will also hydrolyze other polar phospholipids with up to 6-carbons long acyl chains in the sn-2 position. PAF-acetylhydrolases are diverse enzymes, and the well-characterized isoforms are serine-dependent hydrolases, which do not require Ca(2+) for activity. Given the existence of two pools of PAF, intra- and extracellular, the acetylhydrolases can be divided into two subclasses: those found in the cytosol and enzymes secreted to blood plasma or other body fluids. Recent crystallographic studies shed new light on the complex structure-function relationships in PAF-AHs.

Role of platelet-activating factor acetylhydrolase gene mutation in Japanese childhood IgA nephropathy

Platelet-activating factor (PAF) is a potent mediator of inflammatory injury in renal diseases. PAF is degraded to inactive products by PAF acetylhydrolase. Recently, a point mutation (G to T transversion) of the PAF acetylhydrolase gene was observed at position 994, and this mutation was found to contribute to the variability in plasma PAF levels, with undetectable plasma PAF acetylhydrolase activity occurring in homozygous patients (TT genotype) and reduced levels of activity in heterozygous patients (GT genotype). Therefore, we investigated the effect of the PAF acetylhydrolase gene mutation on the pathogenesis and progression of immunoglobulin A (IgA) nephropathy. Genomic DNA was obtained from 89 children with IgA nephropathy and 100 controls. We identified the PAF acetylhydrolase gene mutation (G994T) by polymerase chain reaction. There was no significant difference in genotypic frequency between patients and controls. However, urinary protein excretion at the time of biopsy was significantly greater in patients with the GT/TT genotypes than in those with the GG genotype. The percentage of glomeruli with mesangial cell proliferation was significantly greater in patients with the GT/TT genotypes than in those with the GG genotype. These results indicate the PAF acetylhydrolase gene mutation may influence the degree of proteinuria and the extent of mesangial proliferation in the early stage of childhood IgA nephropathy.

Platelet-activating factor acetylhydrolase gene mutation in Japanese nephrotic children

BACKGROUND

Platelet-activating factor (PAF) may be involved in the pathogenesis of steroid-responsive nephrotic syndrome (SRNS). PAF is degraded to inactive products by PAF acetylhydrolase. We have investigated whether PAF acetylhydrolase gene mutation is involved in SRNS in Japanese children.

METHODS

We identified a point mutation in the PAF acetylhydrolase gene (G994T) using the polymerase chain reaction in 101 Japanese children with SRNS and 100 healthy Japanese.

RESULTS

There was no difference in the genotype and allele frequencies between patients with SRNS and normal controls. The mean number of relapses during the first year after onset was significantly higher in the 26 patients who were heterozygous for the mutant allele (GT) than in 75 wild-type homozygotes (GG) (2.61 +/- 1.98 vs. 1.33 +/- 1.35; P = 0.0019).

CONCLUSIONS

We conclude that analysis of the PAF acetylhydrolase gene mutation at position 994 in Japanese children with SRNS allows the identification of patients who are more likely to have a disease relapse.

Recombinant plasma-type platelet-activating factor acetylhydrolase attenuates NMDA-induced hippocampal neuronal apoptosis

The bioactive lipid platelet-activating factor (PAF) accumulates in brain during injury, seizures and ischemia and may, in addition, be significant in AIDS dementia and in other neurodegenerative diseases. We have used plasma-type recombinant PAF acetylhydrolase (rPAF-AH) to test the hypothesis that PAF accumulation is involved in early events leading to neuronal apoptosis during excitotoxic neuronal injury. Neuronal cultures were labeled with FITC-12-dUTP (TUNEL technique) and propidium iodide, digitized using fluorescence microscopy and a chilled 3CCD color camera, and analyzed with 2D graphics analysis software. N-methyl-D-aspartate (NMDA) (50 microM, 2 hr) induced a 2.5-fold increase in apoptosis of hippocampal neurons compared with controls when analyzed 24 hr after NMDA treatment. Hippocampal neurons receiving rPAF-AH (20 microg/ml) before, during, and after NMDA treatment demonstrated a concentration-dependent neuroprotective effect which resulted in 47% and 30% neuroprotection against 50 and 100 microM NMDA, respectively. The noncompetitive NMDA receptor antagonist MK-801(300 nM) completely inhibited apoptosis caused by NMDA. The neuroprotective effect of rPAF-AH against NMDA-induced apoptosis was confirmed using as additional criteria, histone release, electron microscopy, and DNA laddering. Neuroprotection elicited by rPAF-AH demonstrates that PAF is an injury mediator in NMDA-induced neuronal apoptosis and that the recombinant protein is potentially useful as a therapeutic approach.

Roles of plasma platelet-activating factor acetylhydrolase in allergic, inflammatory, and atherosclerotic diseases

Platelet-activating factor (PAF) mediates a variety of physiologic and pathologic events by activating platelets, neutrophils, monocytes, macrophages, and smooth muscle cells. A strongly oxidizing environment induces fragmentation of the polyunsaturated fatty acids of membrane phospholipids, and the resulting oxidized phospholipids are structurally similar to PAF and mimic its biologic actions. The effects of PAF and oxidized phospholipids are abolished by hydrolysis of the sn-2 residue, a reaction catalyzed by PAF acetylhydrolase. Plasma and intracellular forms of PAF acetylhydrolase have been purified and characterized. The plasma form binds with high affinity to lipoproteins in plasma. Furthermore, changes in the activity of this enzyme are associated with various human diseases and animal models of human pathology, suggesting that it may play important roles in their pathogenesis. Studies that have defined the properties of this enzyme and its roles in physiologic and pathologic processes are reviewed. Such studies have provided insight into the functions of PAF and oxidized phospholipids as well as into the etiology of allergic, inflammatory, and atherosclerotic diseases.

Secretory PAF-acetylhydrolase of the rat hepatobiliary system: characterization and partial purification

Hepatocytes and Kupffer cells in primary culture both secrete plasma-type platelet-activating factor-acetylhydrolase (pPAF-AH) into serum-free culture medium. The rate of secretion of pPAF-AH by Kupffer cells was 20 to 25 times higher than from hepatocytes, and Kupffer cells expressed a higher level of pPAF-AH mRNA than did hepatocytes. Purified liver cell-secreted pPAF-AH exhibited a major protein band of 65-67 kDa on SDS-PAGE; this was the band predominantly labeled when the enzyme catalytic center was reacted with [3H]diisopropylfluorophosphate ([3H]DFP). Rat bile collected from cannulated bile ducts contained significant PAF-AH activity, and bile samples possessed a prominent band at 30-32 kDa, which was the exclusive target for [3H]DFP. Experiments using tunicamycin, an inhibitor of N-linked glycosylation, and endoglycosidase H suggested that pPAF-AH secreted constitutively by cultured hepatocytes and Kupffer cells is glycosylated. The present study supports the notion that hepatic secretion of pPAF-AH into the blood contributes to the regulation of PAF and oxidized phospholipid levels in the circulation, whereas secretion of PAF-AH into the bile may allow hepatic control of these phospholipid signaling molecules in the gastrointestinal tract.

Substrate specificities of neuronal nuclear acetyltransferases involved in the synthesis of platelet-activating factor: differences in the use of 1-alkyl and 1-acyl lysophospholipid acceptors

The selectivity of alkylglycerophosphate (AGP) acetyltransferase and lyso-platelet-activating factor (lyso-PAF) acetyltransferase was studied in neuronal nuclei isolated from cerebral cortices of 15-day-old rabbits. Specifically, 1-alkyl and 1-acyl analogues were compared as acceptors in these acetylation reactions. A number of observations supported one nuclear activity in the acetylation of AGP and lyso-PA. Lyso-PA was a competitive substrate for AGP, Km values for AGP and lyso-PA were similar, as were acetylation rates measured at individual AGP or lyso-PA concentrations, and the acetylation of both substrates was unaffected by preincubations with protein phosphatase 1 (PP-1). In contrast, there were a number of differences seen in the acetylation of lyso-PAF and lyso-PC. The kinetics for lyso-PC acetylation (as a function of lyso-PC concentration) were not hyperbolic, and lyso-PC was not a competitive substrate for the acetylation of lyso-PAF. Unlike acetylation rates with lyso-PAF, lyso-PC acetylation was not reduced by preincubations with PP-1, and was less susceptible to inhibition particularly at high levels of free fatty acid. In addition, rates of acetylation of lyso-PC were selectively increased by the presence of lyso-PA. When neuronal nuclear envelope fractions (NE) were prepared from N1, the specific acetylation activity with lyso-PAF was significantly lower in NE, while the activities for lyso-PC were comparable in NE and the parent N1 fraction. The results with the acetylation of lyso-PC and lyso-PAF suggest that the lyso-PC acetyltransferase may be in a uniquely sequestered state within the neuronal nucleus. This could explain the smaller inhibition of lyso-PC acetylation by free fatty acid, the maintenance of lyso-PC acetylation during PP-1 preincubations, the non-hyperbolic response to lyso-PC concentrations and the selective preservation of lyso-PC acetylation during NE isolation. This protected status could result from a more internal location for this acetyltransferase within the membranes of the nuclear envelope, or possibly an association of the enzyme with the nuclear matrix that is disrupted with the exposure of N1 to lyso-PA.

Cloning and characterization of cDNAs and the gene encoding the mouse platelet-activating factor acetylhydrolase Ib alpha subunit/lissencephaly-1 protein

Platelet-activating factor acetylhydrolases (PAF-AHs) play an important role in the metabolism of PAF, a potent phospholipid mediator affecting various physiological processes. The heterotrimeric form of intracellular PAF-AH consists of two catalytic subunits (PAF-AH Ib beta and PAF-AH Ib gamma) and a potential regulatory subunit (PAF-AH Ib alpha). Hemizygous deletion of the gene encoding the alpha subunit has been implicated in two related neurological disorders: isolated lissencephaly sequence and Miller-Dieker syndrome. Here we report the isolation and characterization of mouse Pafaha/Lis1 cDNAs and the corresponding Pafaha/Lis1 gene. We have cloned five cDNAs representing alternatively polyadenylated messages. Northern blot analysis revealed that the various Pafaha/Lis1 mRNAs are differentially expressed in mouse tissues. The Pafaha/Lis1 gene spans a genomic region of more than 50 kb and consists of 12 exons, the first 2 of which are embedded in CpG islands. We have identified two sites of alternative splicing of Pafaha/Lis1: one affecting the length of the 5' untranslated region, the other potentially resulting in a truncated form of the encoded protein.

Systematic screening of the LDL-PLA2 gene for polymorphic variants and case-control analysis in schizophrenia

Systematic scans of the genome using microsatellite markers have identified chromosome 6p21.1 as a putative locus for schizophrenia in multiply affected families. There is also evidence from a series of studies for a role of abnormal phospholipid metabolism in schizophrenia. In light of these findings, and the role of platelet activating factor in neurotransmission and neurodevelopment, we have examined the LDL-PLA2 (plasma PAF acetylhydrolase, PAF-AH) gene, a serine dependent phospholipase that has been mapped by hybrid mapping to chromosome 6p21.1, as a positional candidate gene for schizophrenia. The gene was systematically screened using SSCP/HD analysis for polymorphisms associated with the disease. Four polymorphic variants were found within the gene and studied in a group of 200 schizophrenic patients and 100 controls. The variant in exon 7 (Iso195Thr) was found to be weakly associated with schizophrenia (p = 0.04) and the variant in exon 11 (Val379Ala) almost reached significance (p = 0.057). After correcting for multiple testing no significant associations were detected. Haplotype analysis combining pairs of polymorphisms also provided no evidence for association of this gene with schizophrenia in our sample of patients.

Protection against oxidative stress-induced cell death by intracellular platelet-activating factor-acetylhydrolase II

Platelet-activating factor-acetylhydrolase (PAF-AH), which removes the acetyl group at the sn-2 position of PAF, is distributed widely in tissues and plasma. Tissue cytosol contains at least two types of PAF-AH, isoforms Ib and II. Isoform Ib is a tertiary G-protein complex-like heterotrimeric enzyme that is involved in brain development such as formation of the brain cortex. Isoform II (PAF-AH(II)), however, is a 40-kDa monomer and has an amino acid sequence that exhibits a 41% identity with that of plasma PAF-AH. Although PAF-AH(II) preferentialy hydrolyzes oxidized phospholipids as well as PAF in vitro, the function of this enzyme has not, as yet, been elucidated. Here, we report that PAF-AH(II) functions as an anti-oxidant phospholipase. PAF-AH(II) was found to be an N-myristoylated enzyme that has never been reported among lipases and phospholipases. In MDBK cells treated with oxidants, PAF-AH(II) translocated from cytosol to membranes within 20 min, whereas in cells treated with anti-oxidants, it translocated, conversely, from membranes to cytosol. Overexpression of PAF-AH(II) in Chinese hamster ovary-K1 cells suppressed oxidative stress-induced cell death, which occurs by apoptosis. These findings suggest that intracellular PAF-AH(II) translocates between cytosol and membranes in response to a redox state of the cell and protects the cell against oxidative stress most probably by hydrolyzing oxidized phospholipids.

Copper-catalyzed oxidation mediates PAF formation in human LDL subspecies. Protective role of PAF:acetylhydrolase in dense LDL

Free radical-mediated oxidation of cholesterol-rich LDL plays a key role in atherogenesis and involves the formation of oxidized phospholipids with proinflammatory biological activity. We evaluated the production of platelet-activating factor (PAF), a potent inflammatory mediator, in human LDL subspecies on copper-initiated oxidation (4 mumol/L CuCl2, 80 micrograms/mL for hours at 37 degrees C). PAF formation was determined by biological assay of HPLC-purified lipid extracts of copper-oxidized lipoproteins; chemical identity was confirmed by gas chromatographic and mass spectrometric analyses. PAF, characterized as the C16:0 molecular species, was preferentially produced in intermediate LDL (d = 1.029 to 1.039 g/mL) (8.6 +/- 5.7 pmol PAF/3 h per mg LDL protein) and light LDL (d = 1.019 to 1.029 g/mL), but was absent from dense LDL particles (d = 1.050 to 1.063 g/mL). As PAF:acetylhydrolase inactivates PAF and oxidized forms of phosphatidylcholine, we evaluated the relationship of lipoprotein-associated PAF:acetylhydrolase to PAF formation. We confirmed that PAF:acetylhydrolase activity was elevated in native, dense LDL (41.5 +/- 9.5 nmol/min per mg protein) but low in LDL subspecies of light and intermediate density (d 1.020 to 1.039 g/mL) (3.5 +/- 1.6 nmol/min per mg protein) [Tselepis et al, Arterioscler Thromb Vasc Biol. 1995;15:1764-1773]. On copper-mediated oxidation for 3 hours at 37 degrees C, dense LDL particles conserved 20 +/- 14% of their initial enzymatic activity; in contrast, PAF:acetylhydrolase activity was abolished in light and intermediate LDL subspecies. Clearly, the elevated PAF:acetylhydrolase activity of dense LDL efficiently diminishes the potential inflammatory role of endogenously formed PAF; nonetheless, formation of proatherogenic lysophospholipids results. In contrast, LDL particles of the light and intermediate subclasses can accumulate PAF on oxidative modification.

Platelet-activating factor (PAF) up-regulates plasma and tissue PAF-acetylhydrolase activity in the rat: effect of cycloheximide

Platelet-activating factor (PAF) is a proinflammatory phospholipid mediator implicated in necrotizing enterocolitis. Regulation of PAF-acetylhydrolase (AH), the enzyme degrading PAF, is poorly understood. In this study we found that administration of a dose of PAF (1.5 microg/kg, i.v.), which does not cause gross intestinal injury, increased plasma and intestinal PAF-AH in the rat. Cycloheximide (CHX, 5 mg/kg, i.v.) reduced the activity of plasma (but not intestinal tissue) AH in control, as well as in PAF-injected rats, and aggravated systemic inflammation and tissue injury in the latter. The intestinal necrosis induced by PAF and CHX was ameliorated by posttreatment with WEB2170 (a PAF antagonist), indicating a role of endogenous PAF in mediating injury. Both WEB2170 and anti-TNF antibody reduced PAF-induced AH activity in intestinal tissue, but not in the plasma. Allopurinol largely prevented the injury induced by PAF and CHX, but had no effect on the up-regulation of AH. We conclude: 1) de novo protein synthesis is required to maintain physiologic AH level in the plasma; 2) PAF up-regulates plasma and intestinal AH activity; 3) CHX enhances the injurious effect of PAF; 4) endogenous PAF and TNF also play a role in the up-regulation of intestinal AH; the former probably mediating the intestinal injury by PAF; and 5) reactive oxygen species may mediate the injurious effect of PAF plus CHX, but do not contribute to the regulation of AH by PAF.

Cell-specific regulation of expression of plasma-type platelet-activating factor acetylhydrolase in the liver

Platelet-activating factor (PAF) is a potent proinflammatory phospholipid mediator that causes hypotension, increases vascular permeability, and has been implicated in anaphylaxis, septic shock and several other inflammatory responses. PAF is hydrolyzed and inactivated by the enzyme PAF-acetylhydrolase. In the intact rat, a mesenteric vein infusion of lipopolysaccharide (LPS) served as an acute, liver-focused model of endotoxemia. Plasma PAF-acetylhydrolase activity increased 2-fold by 24 h following LPS administration. Ribonuclease protection experiments demonstrated very low levels of plasma-type PAF-acetylhydrolase mRNA transcripts in the livers of saline-infused rats; however, 24 h following LPS exposure, a 20-fold induction of PAF-acetylhydrolase mRNA was detected. In cells isolated from endotoxin-exposed rat livers, Northern blot analyses demonstrated that Kupffer cells but not hepatocytes or endothelial cells were responsible for the increased PAF-acetylhydrolase mRNA levels. In Kupffer cells, plasma-type PAF-acetylhydrolase mRNA was induced by 12 h, peaked at 24 h, and remained substantially elevated at 48 h. Induction of neutropenia prior to LPS administration had no effect on the increase in PAF-acetylhydrolase mRNA seen at 24 h. Although freshly isolated Kupffer cells contain barely detectable levels of plasma-type PAF-acetylhydrolase mRNA, when Kupffer cells were established in culture, PAF-acetylhydrolase expression became constitutively activated concomitant with cell adherence to the culture plates. Alterations in plasma-type PAF-acetylhydrolase expression may constitute an important mechanism for elevating plasma PAF-acetylhydrolase levels and an important component in minimizing PAF-mediated pathophysiology in livers exposed to endotoxemia.

A novel cytosolic calcium-independent phospholipase A2 contains eight ankyrin motifs

We report the purification, molecular cloning, and expression of a novel cytosolic calcium-independent phospholipase A2 (iPLA2) from Chinese hamster ovary cells, which lacks extended homology to other phospholipases. iPLA2 is an 85-kDa protein that exists as a multimeric complex of 270-350 kDa with a specific activity of 1 micromol/min/mg. The full-length cDNA clone encodes a 752-amino acid cytoplasmic protein with one lipase motif (GXS465XG) and eight ankyrin repeats. Expression of the cDNA in mammalian cells generates an active 85-kDa protein. Mutagenesis studies show that Ser465 and the ankyrin repeats are required for activity. We demonstrate that iPLA2 selectively hydrolyzes the sn-2 over sn-1 fatty acid by 5-fold for 1,2-dipalmitoyl phosphatidylcholine in a mixed micelle. Moreover, we found the fatty acid preference at the sn-2 position to be highly dependent upon substrate presentation. However, iPLA2 does have a marked preference for 1,2-dipalmitoyl phosphatidic acid presented in a vesicle, generating the lipid second messenger lysophosphatidic acid. Finally the enzyme is able to hydrolyze the acetyl moiety at the sn-2 position of platelet-activating factor.

Deficiency of plasma platelet-activating factor acetylhydrolase: roles of blood cells

Platelet-activating factor (PAF), a potent mediator of inflammation and circulatory shock, is inactivated by the enzyme PAF acetylhydrolase. Plasma PAF acetylhydrolase deficiency occurs even in healthy subjects. We hypothesized that erythrocyte PAF acetylhydrolase could play a supplementary role in this plasma acetylhydrolase deficiency. We examined 1,030 subjects who participated in mass checkups, and assayed plasma and erythrocyte PAF acetylhydrolase. We also investigated the degradation of exogenous PAF by erythrocytes or other blood cells obtained from subjects who exhibited the plasma enzyme deficiency. The incidence of the plasma enzyme deficiency in this general Japanese population was 4.7% (48/1,030). None of the subjects with the deficiency had a history of allergy, circulatory shock, or chronic inflammatory diseases. The mean values for erythrocyte cytosolic PAF acetylhydrolase activity in the normal and deficient subjects were 0.51 +/- 0.15 (SD) and 0.71 +/- 0.28 nkat (nmol/s)/g protein, respectively, and the difference was significant (P < 0.001, Mann-Whitney U-test). The half-life of 10 nmol/l [3H]PAF in plasma from normal subjects was about 5 min, and the half-life in whole blood or erythrocyte suspension in autologous plasma was almost the same as that in plasma. In plasma from deficient subjects, unchanged PAF virtually remained and the degradation in whole blood or erythrocyte suspension was a little faster than in plasma. We conclude that erythrocytes contribute only little to PAF metabolism in normal blood but they account for almost all of the slow PAF degradation in blood from subjects deficient in plasma PAF acetylhydrolase.

Platelet-activating factor acetylhydrolases in Caco-2 cells and epithelium of normal and ulcerative colitis patients

BACKGROUND & AIMS

Platelet-activating factor (PAF) is a potent inflammatory mediator implicated in the pathogenesis of inflammatory bowel disease and necrotizing enterocolitis. Metabolism by platelet-activating factor acetylhydrolase (PAF-AH) is the major pathway for platelet-activating factor degradation. The aim of this study was to investigate the possible role of intestinal epithelium as a source of PAF-AH.

METHODS

Intracellular and secreted PAF-AHs were characterized in human colonic epithelial cells isolated from histologically normal mucosa and inflamed mucosa from patients with ulcerative colitis and in the human intestinal epithelial cell line Caco-2 by measuring the metabolism of [3H]-PAF to [3H]lysoPAF.

RESULTS

Human colonic epithelial cells and Caco-2 cells synthesize and secrete PAF-AH as shown by in vitro hydrolysis of [3H]PAF to [3H]-lysoPAF in cell lysates and conditioned media. Both intracellular and secreted PAF-AHs are calcium-independent and substrate-specific for phospholipids similar to PAF. Epithelial cells from involved areas of resections for ulcerative colitis had increased levels of secreted PAF-AH and decreased levels of intracellular PAF-AH compared with epithelial cells from histologically normal areas.

CONCLUSIONS

Human colonic epithelial cells and Caco-2 cells produce intracellular and secreted PAF-AHs, which are distinct proteins. This is the first demonstration of PAF-AH production by epithelial cells.

PAF-degrading acetylhydrolase is preferentially associated with dense LDL and VHDL-1 in human plasma. Catalytic characteristics and relation to the monocyte-derived enzyme

In human plasma, platelet activating factor (PAF)-degrading acetylhydrolase (acetylhydrolase) is principally transported in association with LDLs and HDLs; this enzyme hydrolyzes PAF and short-chain forms of oxidized phosphatidylcholine, transforming them into lyso-PAF and lysophosphatidylcholine, respectively. We have examined the distribution, catalytic characteristics, and transfer of acetylhydrolase activity among plasma lipoprotein subspecies separated by isopycnic density gradient ultracentrifugation; the possibility that the plasma enzyme may be partially derived from adherent monocytes has also been evaluated. In normolipidemic subjects with Lp(a) levels < 0.1 mg/mL, acetylhydrolase was associated preferentially with small, dense LDL particles (LDL-5; d = 1.050 to 1.063 g/mL) and with the very-high-density lipoprotein-1 subfraction (VHDL-1; d = 1.156 to 1.179 g/mL), representing 23.9 +/- 1.7% and 20.6 +/- 3.2%, respectively, of total plasma activity. The apparent Km values for PAF of the enzyme associated with such lipoproteins were 89.7 +/- 23.4 and 34.8 +/- 4.5 mumol/L for LDL-5 and VHDL-1, respectively: indeed, the Km value for LDL-5 was some 10-fold higher than that of the light LDL-1, LDL-2, and LDL-3 subspecies, whereas the Km of VHDL-1 was some twofold greater than those of the HDL-2 and HDL-3 subspecies. Furthermore, when expressed on the basis of unit plasma volume, the Vmax of the acetylhydrolase associated with LDL-5 was some 150-fold greater than that in LDL-1 (d = 1.019 to 1.023 g/mL). No significant differences in the pH dependence of enzyme activity or in sensitivity to protease inactivation, sulfydryl reagents, the serine protease inhibitor Pefabloc, or the PAF antagonist CV 3988 could be detected between apo B-containing and apo A-I-containing lipoprotein particle subspecies. Incubation of LDL-1 (Km = 8.4 +/- 2.6 mumol/L) and LDL-2 (d = 1.023 to 1.029 g/mL; Km = 8.4 +/- 3.3 mumol/L) subspecies with LDL-5, in which acetylhydrolase had been inactivated by pretreatment with Pefabloc, demonstrated preferential transfer of acetylhydrolase to LDL-5. Acetylhydrolase transferred to LDL-5 from the light LDL subspecies exhibited a Km of 9.4 +/- 2.2 mumol/L, a value characteristic of the particle donors. Finally, acetylhydrolase (Km = 23.4 +/- 7.6 mumol/L) released by adherent human monocytes in culture was found to bind preferentially to small, dense LDL subspecies upon incubation of Pefabloc-inactivated plasma with monocyte supernatant.(ABSTRACT TRUNCATED AT 400 WORDS)

Purification and characterization of platelet-activating factor acetylhydrolase II from bovine liver cytosol

Platelet-activating factor (PAF) acetylhydrolase, which inactivates PAF by removing the acetyl group at the sn-2 position, is distributed widely in plasma and tissues. In a previous study, we demonstrated that the PAF acetylhydrolase activity present in the soluble fraction of bovine brain cortex could be separated chromatographically into three peaks (tentatively designated isoforms Ia, Ib, and II) (Hattori, M., Arai, H., and Inoue, K. (1993) J. Biol. Chem. 268, 18748-18753). In this study, these three isoforms were also detected in kidney and liver cytosols, although their relative activity ratios in these tissues differed. In particular, isoform II was responsible for the majority of the bovine liver PAF acetylhydrolase activity. We purified isoform II from bovine liver cytosol to near homogeneity and demonstrated that it is a single 40-kDa polypeptide. This enzyme was inactivated by diisopropyl fluorophosphate and 5,5'-dithiobis(2-nitrobenzoic acid), suggesting that both serine and cysteine residues are required for the enzyme activity, and [3H]diisopropyl fluorophosphate labeled only the 40-kDa polypeptide, confirming the enzyme's identity. Isoform II showed a comparatively broader substrate specificity than isoform Ib. Isoform II hydrolyzed propionyl and butyroyl moieties at the sn-2 position approximately half as effectively as it did PAF, whereas isoform Ib hardly hydrolyzed these substrates. Taken together with previous data, the current findings indicate that tissue cytosol contains at least two types of PAF acetylhydrolase with respect to polypeptide composition, substrate specificity, and tissue distribution and suggest that these two enzymes may share distinct physiological functions in tissues.

In vivo turnover of phospholipids in rabbit erythrocytes

The rate of phospholipid turnover in erythrocyte membranes in vivo has been studied using a recently developed procedure (Kuypers, F.A., Easton, E.W., van den Hoven, R., Wensing, T., Roelofsen, B., Op den Kamp, J.A.F. and van Deenen, L.L.M. (1985) Biochim. Biophys. Acta 819, 170-178). The technique is based on the application of phospholipid transfer proteins in order to introduce trace amounts of radiolabelled phospholipids in the membrane of isolated erythrocytes, followed by re-injection of the erythrocytes into the bloodstream of the animal. The most abundant species of the phosphatidylcholine (PC) class, 1-palmitoyl,2-linoleoyl PC, has, on the basis of loss of the radioactivity in its fatty acyl part, a relatively high turnover with a half-time value of 1.5 days. Other PC species studied exhibit more moderate turnover rates of about 5 days for 1-palmitoyl,2-oleoyl PC and 1-stearoyl,2-arachidonoyl PC. Dipalmitoyl PC, labelled in the polar headgroup, turns over at a slow rate with a half-time value of 9 days. From these data and the relative abundance of the various species, it can be calculated that, on a daily basis in vivo, about one third of the total PC pool in rabbit erythrocyte membranes is replaced and/or modified by de-/reacylation. The only phosphatidylethanolamine (PE) species studied so far, 1-palmitoyl,2-arachidonoyl PE, appeared to be renewed at a relatively low rate with a half-time value of 12 days. The data demonstrate that the in vivo turnover values of phospholipids in the erythrocyte membrane may depend on their polar head group structure, their localization in the membrane and, to a large extent, on their fatty acid composition.

Density-associated changes in platelet-activating factor acetylhydrolase activity and membrane fluidity of human erythrocytes

Platelet-activating factor acetylhydrolase is known to degrade oxidatively fragmented phospholipids which are similar in structure to platelet-activating factor. We examined changes of acetylhydrolase activity during in vivo aging of human erythrocytes and tried to assess its role in maintaining the membrane properties of erythrocytes. Higher-density erythrocytes are enriched with older cells. Erythrocytes obtained from seven healthy colleagues were separated into four density fractions by centrifugation in discontinuous Percoll density gradients. Both membrane and cytosolic acetylhydrolase decreased with increasing erythrocyte density. Membrane and cytosolic acetylhydrolase activities in the lightest fraction were 2.0 +/- 1.0 (SD) nkat/g protein and 362 +/- 58 pkat/g protein, respectively, and these values were significantly higher than those in the densest fraction: 1.3 +/- 0.7 nkat/g protein and 286 +/- 70 pkat/g protein, respectively. Membrane acyltransferase activity also decreased with red cell density and the average values in the lightest and densest fractions were 51.2 +/- 23.6 and 27.0 +/- 20.2 mukat/g protein, respectively. Generation of thiobarbituric acid-reactive substances induced by t-butyl hydroperoxide treatment decreased with increasing cell density, and the inhibition of acetylhydrolase with diisopropylfluorophosphate resulted in enhanced peroxide-induced lipid oxidation, particularly in lower-density fractions. There was no significant change in basal levels of thiobarbituric acid-reactive substances in red cell membrane. Membrane fluidity was evaluated by fluorescence recovery after photo-bleaching and it decreased as erythrocyte density increased. We conclude that the activity of the deacylation/reacylation cycle maintained by acetylhydrolase and acyltransferase is gradually reduced during in vivo aging of erythrocytes. This may be connected with decreases of polyunsaturated fatty acids and membrane fluidity in old erythrocytes.