Plasma platelet-activating factor degradation in patients with severe coronary artery disease

Modulation of oxidative stress, inflammation, and atherosclerosis by lipoprotein-associated phospholipase A2

Lipoprotein-associated phospholipase A(2) (Lp-PLA(2)), also known as platelet-activating factor acetylhydrolase (PAF-AH), is a unique member of the phospholipase A(2) superfamily. This enzyme is characterized by its ability to specifically hydrolyze PAF as well as glycerophospholipids containing short, truncated, and/or oxidized fatty acyl groups at the sn-2 position of the glycerol backbone. In humans, Lp-PLA(2) circulates in active form as a complex with low- and high-density lipoproteins. Clinical studies have reported that plasma Lp-PLA(2) activity and mass are strongly associated with atherogenic lipids and vascular risk. These observations led to the hypothesis that Lp-PLA(2) activity and/or mass levels could be used as biomarkers of cardiovascular disease and that inhibition of the activity could offer an attractive therapeutic strategy. Darapladib, a compound that inhibits Lp-PLA(2) activity, is anti-atherogenic in mice and other animals, and it decreases atherosclerotic plaque expansion in humans. However, disagreement continues to exist regarding the validity of Lp-PLA(2) as an independent marker of atherosclerosis and a scientifically justified target for intervention. Circulating Lp-PLA(2) mass and activity are associated with vascular risk, but the strength of the association is reduced after adjustment for basal concentrations of the lipoprotein carriers with which the enzyme associates. Genetic studies in humans harboring an inactivating mutation at this locus indicate that loss of Lp-PLA(2) function is a risk factor for inflammatory and vascular conditions in Japanese cohorts. Consistently, overexpression of Lp-PLA(2) has anti-inflammatory and anti-atherogenic properties in animal models. This thematic review critically discusses results from laboratory and animal studies, analyzes genetic evidence, reviews clinical work demonstrating associations between Lp-PLA(2) and vascular disease, and summarizes results from animal and human clinical trials in which administration of darapladib was tested as a strategy for the management of atherosclerosis.

Platelet-Activating Factor Acetylhydrolase Is Not Associated with Carotid Intima-Media Thickness in Hypercholesterolemic Sicilian Individuals

Background: Atherosclerosis is a complex, chronic disease that usually arises from the converging action of several pathogenic processes, including hypertension, hyperlipidemia, obesity, and the accumulation of oxidized LDL. Platelet-activating factor acetylhydrolase (PAF-AH) is a LDL- and HDL-bound enzyme that hydrolyzes and inactivates PAF and prevents LDL-cholesterol oxidation, thus delaying the onset of atherosclerotic disease.

Methods: We evaluated the relationship between variants of the PAF-AH gene polymorphisms Arg92His, Ile198Thr, and Ala379Val and the presence of carotid atherosclerosis in 190 hypercholesterolemic Sicilian individuals. Carotid artery intima-media wall thickness (IMT) was measured as an indicator of early atherosclerotic disease. The participants were classified according to having normal (≤1 mm) or abnormal (≥1 mm) IMT and were also investigated for physical characteristics and biochemical indices, including PAF-AH activity.

Results: PAF-AH activity and LDL concentrations were significantly correlated in hypercholesterolemic patients, but plasma PAF-AH activity and HDL were not significantly correlated in either IMT group. No significant differences were detected among the PAF-AH gene polymorphisms in both groups after correction for age, sex, body mass index, plasma glucose and lipid concentrations, PAF-AH activity, blood pressure, and smoking habits. The analysis of PAF-AH genotype distribution showed no significant differences in percentage of 92, 198, and 379 genotypes in both IMT groups.

Conclusion: Our data provided no evidence that PAF-AH polymorphisms influence PAF-AH activity and atherosclerosis in hypercholesterolemic Sicilian patients.

Inflammation, bioactive lipids and atherosclerosis: potential roles of a lipoprotein-associated phospholipase A2, platelet activating factor-acetylhydrolase

It is well established that inflammation is an integral feature of atherosclerosis and of the cardiovascular diseases which it underlies. Oxidative stress is also recognized as a key actor in atherogenesis, in which it is closely associated with the inflammatory response and bioactive lipid formation. Several bioactive lipids have been identified in the atherosclerotic plaque, including the potent inflammatory mediator platelet activating factor (PAF), PAF-like lipids, oxidised phospholipids (oxPL) and lysophosphatidylcholine (lyso-PC). Recent evidence has established a central role of two phospholipases (PL) in atherogenesis, the non-pancreatic Type II secretory phospholipase A(2) (sPLA(2)) and the lipoprotein-associated PLA(2)-alternatively termed as PAF-acetylhydrolase (PAF-AH). sPLA(2) is calcium-dependent and hydrolyses the sn-2 acyl group of glycerophospholipids of lipoproteins and cell membranes to produce lyso-PC and free fatty acids. It is also implicated in isoprostane production from oxPL. sPLA(2) is an acute phase reactant, which is upregulated by inflammatory cytokines and may represent a new independent risk factor for coronary heart disease. In contrast to sPLA(2), PAF-AH is calcium-independent and is specific for short acyl groups at the sn-2 position of the phospholipid substrate and with the exception of PAF, can equally hydrolyze oxPL to generate lyso-PC and oxidized fatty acids. Thus PAF-AH plays a key role in the degradation of proinflammatory oxPL and in the generation of lyso-PC and oxidized fatty acids. PAF-AH equally can also hydrolyze short-chain diacylglycerols, triacylglycerols, and acetylated alkanols, and displays a PLA(1) activity. Whereas sPLA(2) may represent a new independent risk factor for coronary artery disease, the potential relevance of PAF-AH to atherosclerosis remains the subject of debate, and recent results suggest that the potential role of the LDL-associated PAF-AH in atherogenesis may be distinct to that of the HDL-associated enzyme. This review is focused on the main structural and catalytic features of plasma PAF-AH, on the association of the enzyme with distinct lipoprotein particle subspecies, on its cellular sources, and finally on the potential significance of this lipoprotein-associated PLA(2) in cardiovascular disease.

Lp-PLA2: an emerging biomarker of coronary heart disease

Cardiovascular disease is the leading cause of death in most industrialized countries. However, the diagnosis and management of coronary heart disease is far from optimal. Lipoprotein-associated phospholipase A2 (Lp-PLA2), also known as platelet-activating factor acetylhydrolase, is an enzyme that hydrolyses oxidized phospholipids and is primarily associated with low-density lipoprotein. Discussed in this review is the accumulating evidence supporting the view that Lp-PLA2 is a potential biomarker of coronary heart disease and plays and an important proinflammatory role in the progression of atherosclerosis. A new ELISA method for the quantitative measurement of Lp-PLA2 mass in human plasma developed by diaDexus, Inc. is presented. Furthermore, potential clinical applications of Lp-PLA2 mass measurements are proposed.

Inhibition of lipoprotein-associated phospholipase A2 diminishes the death-inducing effects of oxidised LDL on human monocyte-macrophages

The death of macrophages contributes to atheroma formation. Oxidation renders low-density lipoprotein (LDL) cytotoxic to human monocyte-macrophages. Lipoprotein-associated phospholipase A2 (Lp-PLA2), also termed platelet-activating factor acetylhydrolase, hydrolyses oxidised phospholipids. Inhibition of Lp-PLA2 by diisopropyl fluorophosphate or Pefabloc (broad-spectrum serine esterase/protease inhibitors), or SB222657 (a specific inhibitor of Lp-PLA2) did not prevent LDL oxidation, but diminished the ensuing toxicity and apoptosis induction when the LDL was oxidised, and inhibited the rise in lysophosphatidylcholine levels that occurred in the inhibitors' absence. Hydrolysis products of oxidised phospholipids thus account for over a third of the cytotoxic and apoptosis-inducing effects of oxidised LDL on macrophages.

The effect of 3-month ingestion of Ginkgo biloba extract (EGb 761) on pancreatic beta-cell function in response to glucose loading in individuals with non-insulin-dependent diabetes mellitus

In the first report (Journal of Clinical Pharmacology 2000; 40:647-654), it was shown that ingestion of 120 mg of Ginkgo biloba extract (EGb 761) daily for 3 months by normal glucose-tolerant individuals caused a significant increase in pancreatic beta-cell insulin and C-peptide response, measured as the area under the curve (AUC0-->120) during a 2-hour standard (75 g) oral glucose tolerance test (OGTT). This follow-up study was designed to determine the effect of the same Ginkgo biloba treatment on glucose-stimulated pancreatic beta-cell function in non-insulin-dependent diabetes mellitus (NIDDM) subjects. In diet-controlled subjects (fasting plasma glucose [FPG], 117 +/- 16 mg/dl; fasting plasma insulin [FPI], 29 +/- 8 microU/ml; n = 6), ingestion of Ginkgo biloba produced no significant effect on the insulin AUC0-->120 (193 +/- 53 vs. 182 +/- 58 microU/ml/h, before and after ingesting Ginkgo biloba, respectively). In hyperinsulinemic NIDDM subjects taking oral hypoglycemic medications (n = 6) (FPG 143 +/- 48 mg/dl; FPI 46 +/- 13 microU/ml), ingestion of Ginkgo biloba caused blunted plasma insulin levels from 30 to 120 minutes during the OGTT, leading to a reduction of the insulin AUC0-->120 (199 +/- 33 vs. 147 +/- 58 microU/ml/h, before and after Ginkgo biloba, respectively). The C-peptide levels increased, and so the AUC0-->120 did not parallel the insulin AUC0-->120, creating a dissimilar insulin/C-peptide ratio indicative of an enhanced hepatic extraction of insulin relative to C-peptide. Thus, in pancreatic beta-cells that are already maximally stimulated, ingestion of Ginkgo biloba may cause a reduction in plasma insulin levels. Only in NIDDM subjects with pancreatic exhaustion (FPG 152 +/- 46 mg/dl; FPI 16 +/- 8 microU/ml; n = 8), who also took oral hypoglycemic agents, did Ginkgo biloba ingestion significantly increase pancreatic beta-cell function in response to glucose loading (insulin AUC0-->120 increased from 51 +/- 29 to 98 +/- 20 microU/ml/h, p < 0.0001), paralleled by a C-peptide AUC0-->120 increase from 7.2 +/- 2.8 to 13.7 +/- 6.8 (p < 0.0001). Whether this increase is due to "resuscitation" of previously exhausted islets or increased activity of only the remaining functional islets is unclear. However, not even in this group did increased pancreatic beta-cell activity cause a reduction of blood glucose during the OGTT. It is concluded that ingestion of Ginkgo biloba extract by an NIDDM subject may increase the hepatic metabolic clearance rate of not only insulin but also the hypoglycemic agents. The result is reduced insulin-mediated glucose metabolism and elevated blood glucose.

Plasma platelet-activating factor acetylhydrolase deficiency is associated with atherosclerotic occlusive disease in japan

PURPOSE

Plasma platelet-activating factor acetylhydrolase (PAF-AH) is known to catalyze platelet-activating factor, thereby inactivating its inflammatory function. Deficiency of this enzyme is caused by a missense (G(994)-->T) in exon 9 of the plasma PAF-AH gene. In this study, we investigated a possible association of this mutation with the risk of atherosclerotic occlusive disease (AO) in Japanese patients.

METHODS

We studied 104 patients with AO. The control group consisted of 114 subjects matched for age and sex. Plasma PAF-AH activity was measured in the patients with AO.

RESULTS

The prevalence of the mutant genotype (GT + TT ) was significantly more frequent in patients with AO than in control subjects (36.5% vs 23.7%; P <.05). Among the patients with AO, those with the mutant allele had significantly more risk factors of prior stroke or ischemic heart disease than patients with normal genotypes. Plasma PAF-AH activity was higher in patients with AO than in control subjects in normal genotype subgroups.

CONCLUSIONS

The missense (G(994)-->T) in exon 9 of the plasma PAF-AH gene is associated with AO in Japanese people.

Lipoprotein-associated phospholipase A(2), platelet-activating factor acetylhydrolase: a potential new risk factor for coronary artery disease

A specific and robust immunoassay for the lipoprotein-associated phospholipase A(2) (Lp-PLA(2)), platelet-activating factor acetylhydrolase, is described for the first time. The immunoassay was used to evaluate possible links between plasma Lp-PLA(2) levels and atherosclerosis risk amongst susceptible individuals. Such an investigation was important because Lp-PLA(2) participates in the oxidative modification of low density lipoprotein by cleaving oxidised phosphatidylcholines, generating lysophosphatidylcholine and oxidised free fatty acids. The majority of Lp-PLA(2) was found associated with LDL (approximately 80%) and, as expected, enzyme levels were significantly positively correlated to LDL cholesterol. Plasma Lp-PLA(2) levels were significantly elevated in patients with angiographically proven coronary artery disease (CAD) when compared with age-matched controls, even though LDL cholesterol levels did not differ significantly. Indeed, when included in a general linear model with LDL cholesterol and other risk factors, Lp-PLA(2) appeared to be an independent predictor of disease status. We propose, therefore, that plasma Lp-PLA(2) mass should be viewed as a potential novel risk factor for CAD that provides information related to but additional to traditional lipoprotein measurements.

Spectrophotometric assay for serum platelet-activating factor acetylhydrolase activity

We developed a spectrophotometric assay for serum platelet-activating factor acetylhydrolase (PAF-AH, EC 3.1.1.47.) activity using a platelet-activating factor (PAF) analogue with a 4-nitrophenyl group as substrate. PAF-AH hydrolyzes the sn-2 position of the substrate ¿1-myristoyl-2-(p-nitrophenylsuccinyl)phosphatidylcholine, producing p-nitrophenyl succinate. This liberation was spectrophotometrically monitored and the activity determined from the change in absorption. The assay does not require radioisotopes and is applicable to an automatic analyzer. Utilizing this assay with an automatic analyzer, it is possible to measure the activities of thousands of samples in a few hours with excellent precision (CV 0.5%, n=30) and high correlation (r=0.979, n=100) with the results of a conventional radioisotopic assay. The assay should be particularly useful for clinical diagnostics.

Correlations between plasma platelet-activating factor acetylhydrolase (PAF-AH) activity and PAF-AH genotype, age, and atherosclerosis in a Japanese population

Platelet-activating factor acetylhydrolase (PAF-AH), a plasma enzyme that hydrolyzes PAF and oxidized phospholipids, is thought to be involved in protecting cells against oxidative stress. A G(994) (M allele)-->T (m allele) mutation in the plasma PAF-AH gene, which results in a Val(279)-->Phe substitution in the mature protein, leads to a loss of catalytic activity. To elucidate the relationships among PAF-AH enzyme activity, genotype, age, and atherosclerosis, we assayed these parameters in a large Japanese population (n=3932) that consisted of three groups; a control group (healthy individuals; n=1684), a risk-factor group (individuals having at least one conventional risk factor for atherosclerosis; n=1398), and a diseased group (patients who had suffered a myocardial infarction or stroke; n=850). We observed a significantly increased frequency of the m allele in the diseased group as compared with the control or risk-factor groups. Plasma PAF-AH activity increased significantly with age in women in the control group with the MM and Mm genotypes, and in men in the control group with the MM genotype, but not in men with the Mm genotype. In both the risk-factor and diseased groups, however, no correlation was observed between plasma PAF-AH activity and age in subjects with either genotype. These results suggest that in individuals with the MM genotype, plasma PAF-AH activity may be increased in response to stresses induced by PAF and/or oxidized phospholipids that might accumulate with age, but that this response is not evident or reduced in healthy individuals with the m allele, or in subjects with atherosclerotic disease, or having risk factors. Together with our previous findings, the G(994)-->T mutation in the PAF-AH gene may be one of the genetic determinants for atherosclerotic disease in the Japanese population.

Regulation of platelet-activating factor (PAF) activity in human diseases by phospholipase A2 inhibitors, PAF acetylhydrolases, PAF receptor antagonists and free radical scavengers

The aim of this review is to present recent findings indicating the likely involvement of platelet-activating factor (PAF) in human diseases, and possible ways of alleviating its harmful effects. PAF is a potent proinflammatory mediator and promotes adhesive interactions between leukocytes and endothelial cells, leading to transendothelial migration of leukocytes, by a process of juxtacrine intercellular signalling. This process leads to activation of leukocytes and the release of reactive oxygen radicals, lipid mediators, cytokines and enzymes. These reaction products subsequently contribute to the pathological features of various inflammatory diseases. The reactive oxygen radicals cause low density lipoprotein (LDL) oxidation which mediates the development of atherosclerosis. Oxidized LDL may damage cellular and subcellular membranes, leading to tissue injury and cell death. Among the therapeutic approaches considered are agents that inhibit/degrade proinflammatory mediators and thereby have anti-inflammatory and/or anti-atherogenic potential. These include inhibitors of phospholipase A2 activity, PAF-acetylhydrolases, PAF antagonists and free radical scavengers/antioxidants, the latter protecting against oxidized LDL-induced cytotoxicity.

Urinary platelet-activating factor excretion is elevated in non-insulin dependent diabetes mellitus

Proteinuria is currently considered a very sensitive predictor of diabetic nephropathy, but 20-25% of all diabetic patients with negative Albustix reaction excrete higher than normal (< 20 mg/24 h) amounts of albumin in their urine. It is our hypothesis that platelet-activating factor (PAF), a potent glycerophospholipid that acts as a chemical mediator for a wide spectrum of biological activities, including increased vascular permeability, may be produced in significant amounts during periods preceding microalbuminuria. In this study, we compared urinary PAF excretion in Mexican-American subjects who were diagnosed with non-insulin dependent diabetes mellitus (NIDDM) with their healthy control counterparts. The age of the NIDDM subjects (45.9 +/- 2.1 years) was not significantly different from the healthy control group, which was 39.4 +/- 2.7 years (P < 0.0672). The NIDDM subjects (body mass index, 29.9 +/- 1.1 compared to 26.1 +/- 0.9 kg/m2 in healthy controls) were characterized by significantly increased (P < 0.05) fasting plasma glucose (192 +/- 11 vs. 97 +/- 4 mg/dl in healthy controls), fasting insulin (20.9 +/- 2.4 vs. 12.3 +/- 1.6 microU/ml), fasting C-peptide (2.93 +/- 1.26 vs. 1.48 +/- 0.51 ng/ml), and hemoglobin A1c (10.3 +/- 0.7 vs. 5.6 +/- 0.3%), respectively. The urine output for the NIDDM and control subjects were 1942 +/- 191 ml/24 h and 1032 +/- 94 ml/24 h, respectively, and urinary albumin excretion (UAE) rates were estimated to be 38 +/- 7 micrograms/min and 11 +/- 1 micrograms/min, respectively. The NIDDM subjects produced significantly increased levels of urinary PAF (2606.3 +/- 513.1 ng/24 h compared with 77.9 +/- 14.1 ng/24 h in controls (or 1706.3 +/- 420.8 ng/ml compared with 85.4 +/- 17.8 pg/ml of urine, in NIDDM and control subjects, respectively). We found that urinary PAF excretion was significantly correlated with microalbumin excretion (r = 0.7) especially at UAE rates greater than 30 mg/day and more importantly, some NIDDM patients with negative Albustix reaction (i.e. normal UAE) produced significantly more PAF, suggesting that PAF excretion may precede microalbuminuria and that subtle injury to the kidneys are present in NIDDM long before overt albuminuria ensues, urinary PAF measurements could potentially therefore serve as a sensitive indicator of renal injury in diabetes mellitus. These results lend further credence to our hypothesis that PAF may be the biochemical compound linking the various members of the insulin resistance syndrome.

Plasma platelet-activating factor acetylhydrolase activity is not associated with premature coronary atherosclerosis

Platelet-activating factor acetylhydrolase activity in plasma was compared between 72 subjects with angiographically normal coronary arteries and matched controls with clinically significant obstruction. No difference was seen, and we conclude that variation in plasma platelet-activating factor acetylhydrolase activity is not a risk factor for coronary artery disease.

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.

A mutation in plasma platelet-activating factor acetylhydrolase (Val279-->Phe) is a genetic risk factor for stroke

BACKGROUND AND PURPOSE

Platelet-activating factor (PAF) is a phospholipid with multiple actions that include thrombosis and inflammation. It is inactivated by a plasma enzyme, PAF acetylhydrolase. Deficiency of this enzyme in plasma is caused by a missense mutation in the gene (Val279-->Phe). We have studied a possible association of this mutation with the risk of stroke.

SUBJECTS AND METHODS

We studied 120 consecutive patients with cerebral thrombosis. The control group consisted of 134 patients matched for age and sex with minor complaints but without stroke. Genomic DNA was analyzed for the mutant allele by a specific polymerase-chain reaction. Plasma PAF acetylhydrolase activity was determined by the method of Stafforini et al.

RESULTS

The prevalence of the mutant gene was 43.4% in stroke patients (39.2% heterozygotes and 4.2% homozygotes), which was significantly higher than the 25.4% in control subjects (22.4% heterozygotes and 3.0% homozygotes) (chi 2 = 9.22, P < .01). The prevalence was slightly higher in stroke patients without hypertension than those with hypertension, but the difference was not significant. The patients with family histories of stroke had a slightly higher but not a significant prevalence of the mutant gene as compared with those without family histories of stroke. Plasma PAF acetylhydrolase activity was higher in patients than in control subjects, in normal subjects, or patients with a heterozygous genotype.

CONCLUSIONS

These results suggest that plasma PAF acetylhydrolase deficiency may be a risk factor for stroke. This may explain the relatively high prevalence of stroke in Japan, as the mutation is more common among Japanese than Caucasians.

Plasma PAF acetylhydrolase in non-insulin dependent diabetes mellitus and obesity: effect of hyperinsulinemia and lovastatin treatment

Insulin resistance is characterized principally by impaired insulin-mediated glucose uptake which provokes a compensatory increase in pancreatic beta-cell secretory activity. For a time this may produce well-controlled plasma glucose levels but as the insulin resistance worsens the augmented insulin production becomes inadequate to keep plasma glucose at euglycemia leading to the development of non-insulin dependent diabetes mellitus (NIDDM), accompanied by hyperinsulinemia and hyperglycemia. A number of metabolic defects are associated with NIDDM including obesity, hypercoagulability, cardiovascular disease risk factors such as hypertension and dyslipidemia and these constitute the insulin resistance syndrome. The identity of the biochemical factor that might link all these defects is not yet known. We have hypothesized that platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine, PAF) may be such a link. In this study, we measured plasma acetylhydrolase (EC.1.1.48), which degrades PAF to the inactive metabolise lyso-PAF, as a surrogate for PAF activity in three groups of hypercholesterolemic subjects: lean controls (n = 9), non-diabetic obese (n = 6) and NIDDM subjects (n = 6). The ages and body mass indices of the subjects were 46 +/- 3.1 and 24.2 +/- 2.2 for the lean controls, 52 +/- 2.5 and 28.7 +/- 0.9 for the NIDDM subjects and 60 +/- 2 and 27.6 +/- 2.1 for the obese, non-diabetic subjects (mean +/- S.E.M.). The measurements were made before and after therapy with the cholesterol-lowering drug lovastatin, a 3-hydroxy 3 methylglutaryl (HMG) coenzyme. A reductase inhibitor (40 mg/day) for 3 months. Fasting plasma glucose (FPG) levels were 91 +/- 11, 96 +/- 3 and 146 +/- 11 mg/dl, for the lean, obese and NIDDM subjects, respectively, before therapy began. Lovastatin did not affect FPG in any of the three subject groups. Before treatment, the fasting plasma insulin (FPI) levels were 6.1 +/- 0.92, 10.83 +/- 2.03 and 14.68 +/- 3.64 mU/l for the lean, non-diabetic obese and NIDDM subjects, respectively. After lovastatin therapy only the obese group exhibited a significant change in FPI (15.35 +/- 2.47 mU/l) (P < 0.05). Total cholesterol levels were similar in all three groups both before and after lovastatin therapy but within each group lovastatin therapy significantly reduced the total cholesterol by 32, 29 and 34% in the lean, obese and NIDDM subject groups respectively (P < 0.0001). Lovastatin therapy reduced LDL-cholesterol levels by 40, 32 and 46% in the lean, obese and NIDDM subjects, respectively, but produced no significant effect on HDL or triglyceride levels. Before therapy, the plasma acetylyhydrolase activities were 104 +/- 7, 164 +/- 7 and 179 +/- 7 nmol/ml per min in the lean, obese and NIDDM subjects, respectively. Lovastatin therapy reduced plasma acetylhydrolase levels to 70 +/- 7, 87 +/- 6 and 86 +/- 7 nmol/ml per min in the lean, obese and NIDDM subjects, respectively. Plasma acetylhydrolase activity was predominantly (> 80%) associated with LDL cholesterol both before and after lovastatin treatment. Also, plasma acetylhydrolase activity significantly correlated with fasting plasma insulin levels before lovastatin therapy but not after. Taken together, this study clearly implicates PAF metabolism in three defects associated with the insulin resistance syndrome: hypercholesterolemia, obesity and NIDDM. Additionally, we conclude that chronic hyperinsulinemia may play a significant role in the production of plasma acetylhydrolase.

Kinetics of PAF transfer, distribution and metabolism in human blood in vitro

The transfer kinetics of PAF (0.1-100 nM), deposited as a thin film on a plastic surface, to human blood were studied under conditions of physiological significance. Almost all (83-87%) available PAF was solubilized in two waves. Initially, 40-50% of the available PAF was transferred to blood very fast in less than 15 seconds while the rest, 30-45%, obeyed first order kinetics, 0.0226 < k(app) < 0.0319 sec(-1). Blood cells following a parallel to whole blood binding time course bound 20-25% of the solubilized PAF. Dilution of blood up to 1:9 with 0.15M NaCl did not affect PAF transfer parameters favouring an aqueous phase diffusion mechanism. Blood-bound PAF was allocated mainly to plasma, 67 +/- 4%, erythrocytes, 18 +/- 1% and platelets, 5 +/- 1%. These data indicated the role of the high affinity binding sites in human platelets versus the low affinity binding by erythrocytes, although the latter, due to their number, dominated cell bound PAF. Even at 100 nM there were no saturation signs for the transfer of PAF to blood or blood cells. PAF hydrolysis did not affect its binding to the blood elements. Given infinite time only 62 +/- 1% of the blood bound PAF would be metabolised by the rather slow acting, 11.5 > t(1/2) > 6.5 min, PAF-acetylhydrolase.

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.

Purification, properties, sequencing, and cloning of a lipoprotein-associated, serine-dependent phospholipase involved in the oxidative modification of low-density lipoproteins

A novel LDL-associated phospholipase A2 (LDL-PLA2) has been purified to homogeneity from human LDL obtained from plasma apheresis. This enzyme has activity toward both oxidized phosphatidylcholine and platelet activating factor (PAF). A simple purification procedure involving detergent solubilization and affinity and ion exchange chromatography has been devised. Vmax and Km for the purified enzyme are 170 micromol.min-1.mg-1 and 12 micromol/L, respectively. Extensive peptide sequence from LDL-PLA2 facilitated identification of an expressed sequence tag partial cDNA. This has led to cloning and expression of active protein in baculovirus. A lipase motif is also evident from sequence information, indicating that the enzyme is serine dependent. Inhibition by diethyl p-nitrophenyl phosphate and 3,4-dichloroisocoumarin and insensitivity to EDTA, Ca2+, and sulfhydryl reagents confirm that the enzyme is indeed a serine-dependent hydrolase. The protein is extensively glycosylated, and the glycosylation site has been identified. Antibodies to this LDL-PLA2 have been raised and used to show that this enzyme is responsible for >95% of the phospholipase activity associated with LDL. Inhibition of LDL-PLA2 before oxidation of LDL reduces both lysophosphatidylcholine content and monocyte chemoattractant ability of the resulting oxidized LDL. Lysophosphatidylcholine production and monocyte chemoattractant ability can be restored by addition of physiological quantities of pure LDL-PLA2.