Platelet-activating factor acetylhydrolase (PAF-AH)

Platelet-activating factor (PAF) is one of the most potent lipid messengers involved in a variety of physiological events. The acetyl group at the sn-2 position of its glycerol backbone is essential for its biological activity, and its deacetylation induces loss of activity. The deacetylation reaction is catalyzed by PAF-acetylhydrolase (PAF-AH). A series of biochemical and enzymological evaluations revealed that at least three types of PAF-AH exist in mammals, namely the intracellular types I and II and a plasma type. Type I PAF-AH is a G-protein-like complex consisting 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 substantial evidence is accumulating supporting the idea that the catalytic subunits are also involved in microtubule function, it is still unknown what role PAF plays in the process and whether PAF is an 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 a role as a scavenger of oxidized phospholipids which are thought to be involved in diverse pathological processes, including disorganization of membrane structure and PAF-like proinflammatory action. In this review, we will focus on the structures and possible biological functions of intracellular PAF-AHs.     

Associations of platelet-activating factor acetylhydrolase (PAF-AH) gene polymorphisms with circulating PAF-AH levels and risk of coronary heart disease or blood stasis syndrome in the Chinese Han population

The circulating level of platelet-activating factor acetylhydrolase (PAF-AH) is a novel biomarker to predict the presence of coronary heart disease. PAF-AH gene polymorphisms may be responsible for the variance of circulating PAF-AH levels in individuals. However, the association of PAF-AH gene polymorphisms with circulating PAF-AH levels and the susceptibility to coronary heart disease (CHD) remains unsolved. Blood stasis syndrome (BSS) of CHD is the most common type of TCM syndromes, and a previous study discovered its relationship with the elevated circulating PAF-AH levels. However, the association of gene polymorphisms and CHD with BSS is unclear at present. In this study, four polymorphisms (R92H, I198T, A379V, V279F) of the PAF-AH gene were genotyped in 570 CHD patients, of which 299 had BSS. In addition, 317 unaffected individuals from the same hospitals served as controls. Plasma PAF-AH levels were measured in 155 controls and 271 CHD patients selected randomly, including 139 CHD patients with BSS. In the Chinese Han population, plasma PAF-AH levels in CHD patients with BSS or without BSS were significantly higher (12.9 ± 6.5 and 11.1 ± 5.0 μM, respectively) than in controls (9.3 ± 5.2 μM); this difference still remained significant after adjustment for traditional risk factors or the inflammatory factors. The R92H polymorphism was highly related to the plasma PAF-AH levels and the risk of CHD, especially among patients with BSS, even with the adjustment for the effects of traditional factors. The I198T polymorphism was highly associated with risk of CHD with BSS, but was associated with neither the risk of CHD with no BSS nor with elevated plasma PAF-AH levels.     

Human macrophages secret platelet-activating factor acetylhydrolase

When monocytes mature to macrophages, their ability to accumulate the pro-inflammatory lipid autacoid, platelet-activating factor (PAF), is markedly decreased (Elstad, M. R. Stafforini, D. M., McIntyre, T. M., Prescott, S. M., and Zimmerman, G. A. (1989) J. Biol. Chem. 264, 8467-8470) in conjunction with a 260-fold increase in the activity of intracellular PAF acetylhydrolase (PAF-AH). We now demonstrate that macrophages also secrete PAF-AH and that the secreted enzyme is biochemically and immunologically identical to the human plasma PAF-AH. It is sensitive to the same active-site-directed inhibitors, has the same electrophoretic mobility, is associated with lipoprotein particles, and transfers between low density lipoprotein and high density lipoprotein in a pH-dependent manner like the plasma PAF-AH. In addition, both activities hydrolyze oxidatively fragmented phospholipids and PAF. These data indicate that macrophages are a cellular source of the plasma PAF-AH. Thus, macrophages secrete an enzyme that inactivates lipid mediators at sites of inflammation and in plasma. These changes during the maturation of monocytes to macrophages may serve to limit the acute inflammatory response.     

Lipoprotein-associated phospholipase A2 (platelet-activating factor acetylhydrolase) and cardiovascular disease

PURPOSE OF REVIEW: Plasma lipoproteins carry a number of highly active enzymes in the circulation. One of these is lipoprotein-associated phospholipase A(2) (Lp-PLA(2)), also known as platelet-activating factor acetylhydrolase. This review addresses the molecular properties of Lp-PLA(2), the controversy surrounding its role in atherosclerosis and the regulation of its plasma levels in humans. RECENT FINDINGS: Recent reports indicate that the enzyme Lp-PLA(2) found in both LDL and HDL may be independently regulated in these lipoprotein subclasses and have distinct roles in atherogenesis. Seminal findings establishing the response-to-retention hypothesis of atherosclerosis support further the potentially damaging role that in-situ release of LDL-associated oxidative products by Lp-PLA(2) may have in the formation of arterial wall lesions. In the mouse, where Lp-PLA(2) circulates mainly bound to HDL, overexpression leads to reduced atherosclerosis, raising the possibility that the enzyme in HDL may have a protective role. Further evidence for a potential protective role is seen in studies of partial or complete deficiency of the enzyme. In the more general setting of population studies, however, it is clear that Lp-PLA(2) is a positive risk factor for coronary disease and measurements of its mass may contribute to the prediction of coronary heart disease risk, especially in individuals with low LDL cholesterol levels. SUMMARY: Lp-PLA(2) is an enzyme with potentially multiple risks in atherosclerosis. In humans the weight of evidence suggests that it is a positive risk factor for coronary heart disease - an observation commensurate with its position in the direct pathological sequence leading from formation of oxidized LDL in the artery wall to cellular dysfunction and formation of lesions.     

Elevated plasma phospholipase A2 and platelet-activating factor acetylhydrolase activity in colorectal cancer

This clinical study reports that blood levels of the pro-inflammatory mediator platelet-activating factor (PAF) did not change in colorectal cancer patients. In contrast, plasma levels of two enzymatic activities, one implicated in PAF production (i.e. phospholipase A2) and one in PAF degradation (i.e. PAF acetylhydrolase activity) were significantly elevated.     

Estrogen effects on platelet-activating factor and platelet-activating factor acetylhydrolase activity in rat uterus during the late stages of pregnancy.

The platelet-activating factor (PAF) concentration of the uterus spontaneously increased during pregnancy. When 17alpha-ethynylestradiol (0.25 mg/kg) was administered subcutaneously to pregnant rats for 3 days starting on Day 17 of pregnancy, some rats delivered prematurely on Day 20. However, none of the vehicle-treated (80% dimethylsulfoxide and 20% ethanol) pregnant rats delivered prematurely. The PAF concentration of the uterus in pregnant rats treated with 17alpha-ethynylestradiol was significantly higher than in those treated with vehicle on Days 19 and 20. On the other hand, the specific activity of uterine PAF-acetylhydrolase (PAF-AH) in pregnant rats treated with 17alpha-ethynylestradiol was significantly lower than in those treated with vehicle on Days 19 and 20, and the plasma PAF-AH activity in pregnant rats treated with estrogen was also significantly lower than in treated with vehicle on Days 18, 19, and 20. These findings indicate that estrogen increases PAF concentrations in the rat uterus, and this was correlated with a decrease in PAF-AH in the uterus and plasma. The increase in PAF concentrations in the uterus may be related to premature delivery and labor caused by PAF's known effect on myometrial contraction.     

Significance of platelet-activating factor acetylhydrolase in patients with non-insulin-dependent (type 2) diabetes mellitus

Background : Non-insulin dependent diabetes mellitus (NIDDM) represents an independent risk factor for cardiovascular diseases (CVD), being characterized by a continnous low-grade inflammation and endothelial activation state. Plasma platelet - activating factor - acetylhydrolases (PAF-AHs) are a subgroup of Ca²⁺ -independent phospholipase A₂ family (also known as lipoprotein-associated phospholipases A₂) that hydrolyze and inactivate the lipid mediator platelet-activating factor (PAF) and/or oxidized phospholipids. This enzyme is considered to play an important role in inflammatory diseases and atherosclerosis. The present study aims to investigate the relations between the levels of PAF-AH activity and LDL-cholesterol/HDL-cholesterol (LDL-ch/HDL-ch) ratio in NIDDM patients as compared to controls. Methods : serum PAF-AH activity was measured in 50 patients with dyslipidemia, in 50 NIDDM patients and in 50 controls (normal lipid and glucose levels). Total cholesterol, LDL-ch, HDL-ch, triglyceride and blood glucose were determined in all subjects. Results : All NIDDM patients display hiperlipidemia, with increased LDL-ch and triglyceride levels. There is a significant correlation between LDL-ch levels (especially LDL-ch / HDL-ch ratio) and PAF-AH activity in dyslipidemic and NIDDM patients. Conclusion : Diabetic and dyslipidemic patients have an increased plasma PAF-AH activity correlated with their LDL-ch levels and mainly with LDL-ch / HDL-ch ratio. Plasma PAF-AH high levels appear to be important as a risk marker for endothelial dysfunction in patients with NIDDM.     

Human plasma platelet-activating factor acetylhydrolase. Purification and properties

Platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is a biologically active phospholipid synthesized by a variety of cell types upon appropriate stimulation. PAF is a potent hypotensive factor and it activates platelets and inflammatory cells at concentrations as low as 10(-10) M. Removal of the acetyl moiety at the sn-2 position abolishes the biological activity and this reaction is catalyzed by a specific acetylhydrolase present in plasma and animal tissues. Ultracentrifugation in density gradients showed that 30% of the activity is associated with high density lipoproteins and 70% with low density lipoproteins. We have purified the plasma low density lipoprotein-associated activity to near homogeneity using a rapid assay based on the separation of [3H]acetate from 1-O-alkyl-2-[3H]acetyl-sn-glycerol-3-phosphocholine on disposable reversed-phase columns. The enzyme was purified by 25,000-fold and approximately 10% of the starting activity was recovered. Plasma PAF-acetylhydrolase has an apparent molecular weight of 43,000, does not require calcium, has preference for micellar versus monomeric substrate, and exhibits surface dilution kinetics. The purified protein has an apparent Km of 13.7 microM and a Vmax of 568 mumol/h/mg with micellar PAF. It can act both on 1-O-alkyl and 1-acyl substrates and on ethanolamine analogs of PAF. However, the enzyme has a marked preference for the sn-2 acetyl residue and therefore can be considered as a specific PAF-acetylhydrolase.     

Trafficking of platelet-activating factor acetylhydrolase type II in response to oxidative stress

Platelet-activating factor acetylhydrolase type II (PAFAH-II) is an intracellular phospholipase A(2) enzyme that hydrolyzes platelet-activating factor and oxidatively fragmented phospholipids. This N-terminally myristoylated protein becomes associated with cytoplasm-facing cell membranes under oxidative stress. The structural requirements for binding of PAFAH-II to membranes in response to oxidative stress are unknown. To begin elucidating the mechanism of trafficking and stress response, we constructed a homology model of PAFAH-II. From the predicted membrane orientation of PAFAH-II, the N-terminal myristoyl group and a hydrophobic patch are hypothesized to be involved in membrane binding. Localization studies of human PAFAH-II in HEK293 cells indicated that an unmyristoylated mutant remained cytoplasmic under stressed and unstressed conditions. The myristoylated wild-type enzyme was partially localized to the cytoplasmic membranes prior to stress and became more localized to these membranes upon stress. A triple mutation of three hydrophobic patch residues of the membrane binding region likewise did not localize to membranes following stress. These results indicate that both the myristoyl group and the hydrophobic patch are essential for proper trafficking of the enzyme to the membranes following oxidative stress. Additionally, colocalization studies using organelle-specific proteins demonstrate that PAFAH-II is transported to the membranes of both the endoplasmic reticulum and Golgi apparatus.     

Effects of platelet-activating factor and platelet-activating factor: acetylhydrolase on in vitro post-thaw boar sperm parameters

Cryopreservation of boar sperm compromises fertility after thawing by reducing sperm longevity and inducing acrosome reaction-like changes. In an attempt to improve the post-thaw motility and acrosome integrity of boar sperm, semen was frozen using a modified Westendorf method in which the medium was supplemented with either platelet-activating factor (PAF) or a recombinant platelet-activating factor:acetylhydrolase (PAF:AH; Pafase) before or after freezing. Platelet-activating factor is a phospholipid that is present in boar semen and PAF:AH is the naturally occurring enzyme that converts PAF to biologically inactive Lyso-PAF. Addition of PAF to the cryopreservation medium improved post-thaw motility immediately after thawing and after 3h incubation at 37 degrees C (60.0+/-0.0% and 25.0+/-2.9%; mean+/-S.E.M.) compared to the control sperm (41.7+/-1.7% and 10.0+/-2.9%; P<0.05). Acrosome integrity was higher immediately after thawing and after 3 and 6h incubation at 37 degrees C when sperm were frozen in the presence of Pafase (55.7+/-3.2%, 45.7+/-3.7% and 23.0+/-3.1%), compared to the control sperm (42.7+/-1.5%, 25.7+/-5.7% and 12.3+/-2.7%) and sperm frozen in the presence of PAF (33.0+/-3.7%, 26.3+/-2.2% and 11.7+/-0.3%; P<0.05). Addition of PAF to sperm after thawing improved motility immediately post-thaw (41.6+/-2.6%), compared with addition of Pafase (23.3+/-2.2%) or the control sperm with no supplementation of the medium (26.7+/-2.2%; P<0.05). However, this beneficial effect was lost by 3h post-thaw. Supplementation of boar semen cryopreservation medium with PAF and Pafase appeared to have beneficial effects on the in vitro quality of the sperm post-thaw.     

Activity of platelet-activating factor (PAF) acetylhydrolase in plasma from patients with ischemic cerebrovascular disease

Platelet-activating factor (PAF) is metabolized by a specific enzyme, PAF acetylhydrolase, which may play an important role in the manifestation of the biological activities of PAF in vivo. The activity of PAF acetylhydrolase in plasma of patients with ischemic stroke was higher than that in healthy controls. The incidence of irreversible platelet aggregation in response to PAF, as well as to ADP, was found to be higher in patients than in controls. The patients whose platelets responded with irreversible aggregation to PAF displayed a higher activity of plasma PAF acetylhydrolase than those with only reversible aggregation. In controls, PAF acetylhydrolase activity correlated positively, although weakly, with LDL-cholesterol, which may reflect the major role of LDL in carrying this enzyme. However, since there was no significant difference in plasma levels of lipids and apoproteins between patients and controls (except for apo B) and there was no significant relationship between the enzyme activity and the levels of other lipids and apoproteins, it is unlikely that increased plasma level of PAF acetylhydrolase activity in stroke patients is accounted for by an abnormality of lipoprotein metabolism. Platelet hyperfunction may be associated with augmented generation of PAF, which, in turn, may bring about the induction of the inactivating enzyme, PAF acetylhydrolase.     

Release of acetylhydrolase from platelets on aggregation with platelet-activating factor

Acetylhydrolase, the enzyme which inactivates platelet-activating factor (PAF, 1-O-alkyl-2-O-acetyl-sn-glycero-3-phosphocholine), was selectively released from bovine platelets by aggregation with physiological concentrations (0.1-10 nM) of PAF with no cell lysis. The release of the acetylhydrolase paralleled that of serotonin. The acetylhydrolase released was active over a broad pH range (pH 5.4-8.6) and was not affected by Ca2+ (1-4 mM) or EDTA (1-8 mM). The Km value of the enzyme was 4.6 microM. Net specific acetylhydrolase activity recovered in the 130,000 x g supernatant after stimulation with PAF could be determined in the presence of EDTA without the activity of Ca2+-dependent phospholipase A2 which was also released from the cells at the same concentration of PAF. The acetylhydrolase was inhibited competitively by specific PAF antagonists, rac-3-(N-n-octadecylcarbamoyloxy)-2-methyoxypropyl-2-thiazolioe thyl phosphate (CV-3988) and (2RS)-1-O-hexadecyl-2-O-ethyl-3-O-(7-thiazolinoheptyl)-glycerol methanesulfonate (ONO-6040). Their Ki values for the enzyme were 1.17 microM and 0.84 microM, respectively. The release of the enzyme could also be detected when the platelets were aggregated with ADP (2.3 microM) or thrombin (0.5 unit). These results suggest that the enzyme released from the aggregated platelets to the blood plasma may also have a physiological function cooperating with the plasma acetylhydrolase.     

Serum platelet-activating factor acetylhydrolase activity: A novel potential inflammatory marker in type 1 diabetes

Plasma activity of the platelet-activating factor acetylhydrolase (PAF-AH) plays an important role in inflammation and atherosclerotic process in chronic diseases. We aimed to evaluate the levels of PAF-AH activity and their association with the metabolic profile and chronic complications in patients with type 1 diabetes. The study included 118 outpatients (54 males) aged 27.1 ± 11.3 years with disease duration of 12.3 ± 8.5 years with (n = 38) or without (n = 80) diabetes complications and 96 control subjects (48 males) matched for age, gender, body mass index and smoking habits. The serum levels of PAF-AH activity were higher in patients either with or without chronic complications (16 ± 5.3 and 14 ± 5.4 nmol/(min mL), respectively) than in controls (13 ± 5.1 nmol/(min mL), P = 0.02). In the total population, PAF-AH activity was correlated with age, HDL-cholesterol, total cholesterol and LDL-cholesterol. In patients, PAF-AH activity was correlated with age, HbA1c, uric acid, HDL-cholesterol, cholesterol, LDL-cholesterol, cholesterol/HDL-cholesterol ratio and the LDL-cholesterol/HDL-cholesterol ratio. It is concluded that PAF-AH plasma activity could be a novel candidate for low-grade inflammatory marker in patients with type 1 diabetes.     

To hydrolyze or not to hydrolyze: the dilemma of platelet-activating factor acetylhydrolase

Mounting ambiguity persists around the functional role of the plasma form of platelet-activating factor acetylhydrolase (PAF-AH). Because PAF-AH hydrolyzes PAF and related oxidized phospholipids, it is widely accepted as an anti-inflammatory enzyme. On the other hand, its actions can also generate lysophosphatidylcholine (lysoPC), a component of bioactive atherogenic oxidized LDL, thus allowing the enzyme to have proinflammatory capabilities. Presence of a canonical lysoPC receptor has been seriously questioned for a multitude of reasons. Animal models of inflammation show that elevating PAF-AH levels is beneficial and not deleterious and overexpression of PAF receptor (PAF-R) also augments inflammatory responses. Further, many Asian populations have a catalytically inert PAF-AH that appears to be a severity factor in a range of inflammatory disorders. Correlation found with elevated levels of PAF-AH and CVDs has led to the design of a specific PAF-AH inhibitor, darapladib. However, in a recently concluded phase III STABILITY clinical trial, use of darapladib did not yield promising results. Presence of structurally related multiple ligands for PAF-R with varied potency, existence of multi-molecular forms of PAF-AH, broad substrate specificity of the enzyme and continuous PAF production by the so called bi-cycle of PAF makes PAF more enigmatic. This review seeks to address the above concerns.     

Serum platelet-activating factor acetylhydrolase activity: a novel potential inflammatory marker in type 1 diabetes

Plasma activity of the platelet-activating factor acetylhydrolase (PAF-AH) plays an important role in inflammation and atherosclerotic process in chronic diseases. We aimed to evaluate the levels of PAF-AH activity and their association with the metabolic profile and chronic complications in patients with type 1 diabetes. The study included 118 outpatients (54 males) aged 27.1+/-11.3 years with disease duration of 12.3+/-8.5 years with (n=38) or without (n=80) diabetes complications and 96 control subjects (48 males) matched for age, gender, body mass index and smoking habits. The serum levels of PAF-AH activity were higher in patients either with or without chronic complications (16+/-5.3 and 14+/-5.4 nmol/(min mL), respectively) than in controls (13+/-5.1 nmol/(min mL), P=0.02). In the total population, PAF-AH activity was correlated with age, HDL-cholesterol, total cholesterol and LDL-cholesterol. In patients, PAF-AH activity was correlated with age, HbA1c, uric acid, HDL-cholesterol, cholesterol, LDL-cholesterol, cholesterol/HDL-cholesterol ratio and the LDL-cholesterol/HDL-cholesterol ratio. It is concluded that PAF-AH plasma activity could be a novel candidate for low-grade inflammatory marker in patients with type 1 diabetes.     

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.     

Study of the paraoxonase and platelet-activating factor acetylhydrolase activities with aging

The purpose of this study was to investigate, with aging, the activity of two enzymes associated to HDL and responsible for its anti-atherogenic activity; paraoxonase (PON1) and platelet-activating factor acetylhydrolase (PAF-AH). Ninety-five subjects aged between 26 and 77 years were recruited for the study. The prevalence of phenotype A, AB, and B in our subjects group was 69.47,21.05 and 9.47% respectively. Plasma as well as HDL paraoxonase activity decreased significantly with aging (r =-0.218, P < 0.039) and (r = -0.280, P < 0.006) respectively. PAF-AH activity was unchanged with aging however, we noted a negative correlation between PAF-AH and PON1 activity in HDL (r = -0.243, P < 0.02) and in LDL vs HDL (r =-0.462, P < 0.001).     

Human plasma platelet-activating factor acetylhydrolase. Association with lipoprotein particles and role in the degradation of platelet-activating factor

Platelet-activating factor (PAF) is a bioactive phospholipid (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) synthesized by a variety of mammalian cell types. PAF induces hypotension, and activates neutrophils and platelets, among other actions. Removal of the acetyl moiety abolishes biological activity, so this reaction may regulate the concentration of PAF and its physiological effects. We have studied the significance of this reaction, which is catalyzed in vitro by an acetylhydrolase present in mammalian plasma, blood cells, and tissues. We have shown that the plasma PAF-acetylhydrolase is responsible for the degradation of PAF in whole human blood and that alternate pathways for PAF degradation in plasma or blood cells are negligible. Human plasma PAF-acetylhydrolase is associated with low and high density lipoproteins (LDL and HDL with apoE). We have confirmed that the activity is a stable component of these particles by density gradient ultracentrifugation, chromatography on heparin-agarose, and immunoprecipitation. The LDL-associated activity accounts for most or all of the PAF degradation that occurs in plasma ex vivo, while the HDL-associated activity contributes little to this process. However, the two activities likely are due to a single protein since the HDL- and LDL-associated PAF-acetylhydrolase activities can transfer from one lipoprotein to the other. These transfer processes are pH-dependent and specific, since they only occur from LDL to a well characterized subclass of HDL (apoE-containing HDL) and vice versa. We discuss the equilibrium between the two particles and the role that this process may have in vivo.     

Liver cells secrete the plasma form of platelet-activating factor acetylhydrolase.

Platelet-activating factor (PAF) is a phospholipid (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) with diverse physiological effects. It has been implicated as a mediator of inflammation, allergy, shock, and thrombosis. Plasma contains an enzyme, PAF acetylhydrolase, that catalyzes the degradation of PAF, and the level of this enzyme may regulate the concentration of PAF in the blood and extracellular spaces under some conditions. Thus, the cellular source(s) of this enzyme and the factors that regulate its synthesis and secretion are issues that may have important physiological and pathological implications. We found that cultures of Hep G2, a human hepatocarcinoma line, secreted PAF acetylhydrolase activity. Optimal secretion occurred in medium that contained serum, and the newly secreted PAF acetylhydrolase was associated with high density and low density lipoproteins (LDL and HDL, respectively), just as the enzyme is in plasma. In the absence of serum. PAF acetylhydrolase was secreted with a particle that had a density similar to HDL. Apolipoproteins B and E were found in the same fractions. We tested the effects of a variety of hormones on the secretion of PAF acetylhydrolase and found that secretion was inhibited by 17 alpha-ethynylestradiol with a maximal effect at 30 microM. This may account for the observation of others that estrogens reduce the activity of PAF acetylhydrolase in the plasma. The PAF acetylhydrolase secreted by Hep G2 cells appeared to be identical to the enzyme in human plasma based on substrate specificity, association with LDL and HDL, response to inhibitors, and reactivity with antibodies against the plasma PAF acetylhydrolase. In conclusion, we have demonstrated that hepatocytes in culture secrete a PAF acetylhydrolase that is apparently identical to the plasma form. The secretion is constitutive but may also be regulated in response to hormonal stimulation.