Plasma degradation of platelet-activating factor in severely ill patients with clinical sepsis

Platelet-activating factor acetylhydrolase and haemophagocytosis in the sepsis syndrome

Sepsis syndrome (SS) is associated with depressed PAF acetylhydrolase, the enzyme responsible for the degradation of platelet activating factor. PAF acetylhydrolase is in a large part produced by macrophages, whose inadequate activation with haemophagocytosis is frequent in patients with SS. The aim of this study was to test the hypothesis that PAF acetylhydrolase levels could be affected in these critically ill patients, because of the large amounts produced by activated macrophages in vitro and in vivo in animal models. The levels of serum PAF acetylhydrolase were assessed in 90 SS patients, who were divided into three groups: patients with (n = 34) or without haemophagocytosis (n = 31), and patients without thrombocytopenia (n = 25) who were used as a control group. The number of organ dysfunctions was matched between patients with haemophagocytosis and controls. Normal reference values were obtained in 59 randomly selected blood donors. Circulating levels of PAF acetylhydrolase were significantly (p = 0.0001) decreased in patients with SS (57+/-3 nmol/ml/min, n = 90) when compared with healthy subjects (69+/-3 nmol/ml/min, n = 59). PAF acetylhydrolase levels were greater in the presence of a haemophagocytosis but without statistical significance (64.2+/-6.5 vs. 50.1+/-2.8:p = 0.25). Despite the fact that macrophagic activation stimulates the in vitro release of PAF acetylhydrolase, no difference was found between patients with or without haemophagocytosis. The mechanism and the role of the PAF acetylhydrolase reduction in SS patients remain to be determined.

Platelet-activating factor (PAF) receptor and genetically engineered PAF receptor mutant mice

Platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is a biologically active phospholipid mediator. Although PAF was initially recognized for its potential to induce platelet aggregation and secretion, intense investigations have elucidated potent biological actions of PAF in a broad range of cell types and tissues, many of which also produce the molecule. PAF acts by binding to a unique G-protein-coupled seven transmembrane receptor. PAF receptor is linked to intracellular signal transduction pathways, including turnover of phosphatidylinositol, elevation in intracellular calcium concentration, and activation of kinases, resulting in versatile bioactions. On the basis of numerous pharmacological reports, PAF is thought to have many pathophysiological and physiological functions. Recently advanced molecular technics enable us not only to clone PAF receptor cDNAs and genes, but also generate PAF receptor mutant animals, i.e., PAF receptor-overexpressing mouse and PAF receptor-deficient mouse. These mutant mice gave us a novel and specific approach for identifying the pathophysiological and physiological functions of PAF. This review also describes the phenotypes of these mutant mice and discusses them by referring to previously reported pharmacological and genetical data.

Immune dysfunction in trauma

This article documents that all immunomodulation strategies for patients sustaining traumatic injury are still under intense investigation. Although we can speculate that combination strategies may be more beneficial than single-agent immunomodulation approaches, comprehensive clinical studies are required to determine efficacious immune therapy for trauma patients. The only strategy available to clinicians caring for trauma patients is immunonutrition, and this should be strongly considered as a rational approach to improve immune function and reduce septic complications in critically ill or injured patients.

Plasma platelet-activating factor acetylhydrolase activity in human immunodeficiency virus infection and the acquired immunodeficiency syndrome

Platelet-activating factor (PAF) acetylhydrolase (PAF-AH) catalyzes the hydrolysis of PAF, a mediator of inflammation, as well as other biologically active oxidized phospholipids. In humans, plasma PAF-AH activity is bound to low-density lipoprotein (LDL) and high-density lipoprotein (HDL). Higher levels of plasma PAF-AH activity have been found in a variety of diseases, and are thought to be a defense mechanism against the toxic effects of PAF and oxidized phospholipids. We studied plasma PAF-AH activity in patients with human immunodeficiency virus (HIV) infection and acquired immunodeficiency syndrome (AIDS), a disease characterized by chronic HIV infection and a systemic host response. Plasma PAF-AH activity was significantly greater in AIDS patients compared with control subjects (25.2 +/- 2.0 v 17.0 +/- 0.8 nmol/min/mL, P < .001). The higher levels of plasma PAF-AH activity were found in LDL (28.2 +/- 2.2 v 18.3 +/- 1.0 nmol/min/mL for AIDS v controls, respectively, P = .0005), but not in HDL. Plasma PAF-AH activity in AIDS correlated with circulating interferon alfa (r = .575, P = .005) and plasma triglycerides (r = .556, P < .0025). The presence of secondary infection in AIDS did not significantly change plasma PAF-AH activity. The initiation of a new antiretroviral regimen with either a protease inhibitor or the nucleoside analog lamivudine did not significantly decrease plasma PAF-AH activity, despite successful suppression of HIV RNA levels. Plasma PAF-AH activity may be a sensitive marker of the host response to infection, and the higher levels of plasma and LDL-associated PAF-AH activity in patients with HIV infection and AIDS may be a physiological response to protect the host against oxidative injury from PAF and oxidized phospholipids.

Cellular source(s) of platelet-activating-factor acetylhydrolase activity in plasma

Platelet activating factor (PAF) is immediately degraded and inactivated in the bloodstream by plasma PAF acetylhydrolase (PAF-AH). Although plasma PAF-AH-like activity was secreted in vitro from various cell types such as macrophages and hepatocytes, the exact cellular source(s) of this enzyme activity in vivo remains unclear. There is a naturally-occurring missense mutation (V279F) in the plasma PAF-AH gene in the Japanese population which results in complete loss of the enzyme activity. We analyzed 52 Japanese who had received an allogeneic bone marrow transplant and maintained donor-derived hematopoiesis. Ten recipients had chimeric plasma PAF-AH genotypes between the donor-derived peripheral blood leukocytes and the recipient-derived epithelial cells of buccal mucosa. Multiple regression analysis demonstrated that PAF-AH activity in plasma depended on the donor's genotype (standardized regression coefficient = 0.68, P < 0.0001), but not on the recipient's genotype (p = 0.48). One recipient who was a V279F homozygote in leukocytes and wild type homozygote in buccal mucosa had undetectable PAF-AH activity in plasma. We conclude that most of the PAF-AH activity in human plasma originates from hematopoietic lineage cells.

Plasma platelet-activating factor acetylhydrolase deficiency in Japanese patients with asthma

Platelet-activating factor (PAF), a phospholipid with a wide range of proinflammatory actions, is immediately degraded and inactivated in vivo by PAF acetylhydrolase (PAF-AH). Surprisingly, 4% of the Japanese population lacks the extracellular isoform of this enzyme, plasma PAF-AH, due to a genetic missense (V279F) mutation. We studied the association of this mutation with asthma prevalence and phenotypes in the Japanese adult population. The allele frequency of V279F mutation was 18.6% in 279 patients with asthma (28.7% heterozygotes and 4.3% homozygotes) and 21.7% in 217 healthy subjects (32.3% heterozygotes and 5.5% homozygotes). V279F mutant allele prevalence was consistent regardless of asthma type (16.3% in atopic [n = 156] and 21.6% in nonatopic [n = 123]), or the severity of disease (21.7% in patients with mild [n = 97], 17.5% in those with moderate [n = 131], and 15.8% in those with severe [n = 51] asthma). Plasma PAF-AH activity was inversely proportional to the number of mutant alleles, and did not correlate with asthma prevalence, type, or severity. We concluded that plasma PAF-AH deficiency due to V279F mutation is not essential to the pathophysiology of asthma in the Japanese adult population.

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.

Confirmatory platelet-activating factor receptor antagonist trial in patients with severe gram-negative bacterial sepsis: a phase III, randomized, double-blind, placebo-controlled, multicenter trial. BN 52021 Sepsis Investigator Group

OBJECTIVE

To determine the efficacy and safety of using natural platelet-activating factor receptor antagonist (PAFra), BN 52021, to treat patients with severe Gram-negative bacterial sepsis.

DESIGN

A prospective, randomized, double-blind, placebo-controlled, multicenter clinical trial.

SETTING

Fifty-nine academic medical center intensive care units in Europe.

PATIENTS

Six hundred nine patients with severe sepsis, suspected to be related to Gram-negative bacterial infection, who received PAFra or placebo.

INTERVENTIONS

Patients were randomized to receive either a dose of PAFra (120 mg iv) every 12 hrs over a 4-day period or placebo over a 4-day period.

MEASUREMENTS AND MAIN RESULTS

The patients were well matched at study entry for severity of illness and for risk factors known to influence the outcome of sepsis. Among all randomized patients, the 28-day, all-cause mortality rate was 49% (152/308) in the placebo group, and 47% (140/300) in the PAFra group (p=.50). When analyzed on the basis of the previously defined target population, the 28-day, all-cause mortality rate was 50% (115/232) in the placebo group and 44% (94/212) in the PAFra group, yielding a 12% reduction in mortality rate (p=.29). In patients with documented infection involving other organisms, there was no difference between treated and placebo groups. When the outcomes of organ dysfunctions were examined in the overall population and in the documented Gram-negative bacterial infection population, the number of patients who resolved hepatic dysfunction tended to be higher in the treated group than in the placebo group (p=.06). The number of adverse events reported were not different between the two groups.

CONCLUSIONS

A 4-day administration of the studied PAFra (BN 52021) failed to demonstrate a statistically significant reduction in the mortality rate of patients with severe sepsis suspected to be related to Gram-negative bacterial infection. If PAFra treatment has any therapeutic activity in severe Gram-negative bacterial sepsis, the incremental benefits are small and will be difficult to demonstrate in a patient population as defined by this clinical trial.

Impaired anaphylactic responses with intact sensitivity to endotoxin in mice lacking a platelet-activating factor receptor

Platelet-activating factor (PAF) is a potent phospholipid mediator with diverse biological activities in addition to its well-known ability to stimulate platelet aggregation. Pharmacologic studies had suggested a role for PAF in pregnancy, neuronal cell migration, anaphylaxis, and endotoxic shock. Here we show that disruption of the PAF receptor gene in mice caused a marked reduction in systemic anaphylactic symptoms. Unexpectedly, however, the PAF receptor-deficient mice developed normally, were fertile, and remained sensitive to bacterial endotoxin. These mutant mice clearly show that PAF plays a dominant role in eliciting anaphylaxis, but that it is not essential for reproduction, brain development, or endotoxic shock.

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.

Cytokine and lipid mediator blood concentrations after coronary artery surgery

This study investigates whether increased levels of cytokines and lipid mediators may be associated with complications after coronary artery bypass grafting (CABG) with extracorporeal circulation (ECC). Hemodynamic measurements and blood samples were obtained in 32 patients before and after the end of ECC and at the 6th and the 24th postoperative hours. Coagulation and pulmonary and cardiovascular functions were specifically assessed postoperatively at the same time. Patients with cardiovascular dysfunction had higher interleukin 8 (IL-8) levels. Higher platelet-activating factor (PAF) and decreased PAF acetylhydrolase activity (AHA, the enzyme that inactivates PAF) levels were found in patients with moderate cardiovascular dysfunction. Interleukin 6 (IL-6), IL-8, and AHA levels correlated with most hemodynamic parameters and creatine phosphokinase myocardial band levels obtained after surgery. Patients with severe lung injury had lower PAF, 6-keto prostaglandin (Pg)F1alpha, and PgE2 levels and higher thromboxane (Tx) B2 concentrations compared with patients without lung injury. Increased IL6 levels were only associated with moderate lung injury. Impaired hemostasis was associated with higher IL6 levels. AHA, IL-6, and IL-8 seem to be associated with cardiovascular dysfunction. The IL-6 blood levels and the ratio of TxB2/6 keto-PgF1alpha blood levels are increased during post-CABG lung injury. These results identify an association between specific post-CABG complications and the systemic inflammatory response. The clinical significance of this association remains to be evaluated.

IMPLICATIONS

Patients with pulmonary, cardiovascular, or hemostasis dysfunction after cardiopulmonary bypass demonstrate aberrancies in a variety of cytokines and lipid mediators in arterial blood or plasma. The relationship between these findings and inflammatory response-induced complications remains to be determined.

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.

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.

Activated mast cells release extracellular type platelet-activating factor acetylhydrolase that contributes to autocrine inactivation of platelet-activating factor

IgE-dependent and -independent activation of mouse bone marrow-derived mast cells (BMMC) elicited rapid and transient production of platelet-activating factor (PAF), which reached a maximal level by 2-5 min and was then degraded rapidly, returning to base-line levels by 10-20 min. Inactivation of PAF was preceded by the release of PAF acetylhydrolase (PAF-AH) activity, which reached a plateau by 3-5 min and paralleled the release of beta-hexosaminidase, a marker of mast cell exocytosis. Immunochemical and molecular biological studies revealed that the PAF-AH released from activated mast cells was identical to the plasma-type isoform. In support of the autocrine action of exocytosed PAF-AH, adding exogenous recombinant plasma-type PAF-AH markedly reduced PAF accumulation in activated BMMC. Furthermore, culture of BMMC with a combination of c-kit ligand, interleukin-1beta and interleukin-10 for > 24 h led to an increase in plasma-type PAF-AH expression, accompanied by a reduction in stimulus-initiated PAF production. Collectively, these results suggest that plasma-type PAF-AH released from activated mast cells sequesters proinflammatory PAF produced by these cells, thereby revealing an intriguing anti-inflammatory aspect of mast cells.

Loss of activity of plasma platelet-activating factor acetylhydrolase due to a novel Gln281-->Arg mutation

The prevalence of plasma platelet-activating factor (PAF) acetylhydrolase deficiency was investigated in 477 healthy Japanese individuals and 985 patients with various cardiovascular diseases. The genotype for this enzyme with regard to a G994-->T mutation (MM, normal; Mm, heterozygote; mm, mutant homozygote) was determined by an allele-specific polymerase chain reaction in 80 subjects shown to have no or low plasma activity (<10 nmol/min/ml). In 72 subjects, the genotype was consistent with plasma enzyme activity; 44 individuals with no activity were mm, and 28 with low activity were Mm. However, eight subjects with the MM genotype showed plasma enzyme activities of <10 nmol/min/ml. Determination of the DNA sequence of exon 9 of the plasma PAF acetylhydrolase gene in these eight subjects revealed a previously unidentified A1001-->G missense mutation, resulting in a Gln281-->Arg substitution, in a 72-year-old woman with coronary artery disease, essential hypertension, and no plasma enzyme activity. Site-directed mutagenesis in vitro showed that the corresponding recombinant mutant protein lacked PAF acetylhydrolase activity. Thus, the Gln281-->Arg substitution appears responsible for the loss of plasma PAF acetylhydrolase activity.

Platelet-activating factor antagonism improves ventricular contractility in endotoxemia

OBJECTIVES

Endotoxin stimulates platelet-activating factor production and also causes a decrease in myocardial contractility within a few hours in animal models of sepsis. Platelet-activating factor by itself decreases left ventricular contractility. We investigated whether platelet-activating factor contributes substantially to the decrease in left ventricular contractility seen in sepsis.

DESIGN

Prospective, randomized, controlled animal study.

SETTING

University research laboratory.

SUBJECTS

Twenty-two juvenile, cross-bred pigs.

INTERVENTIONS

Anesthetized pigs were pretreated with a platelet-activating factor receptor antagonist (L-659,989) or vehicle (control), and then treated with endotoxin or saline (control). Hemodynamics and left ventricular pressures (Millar catheter) and volumes (conductance catheter) were measured. Left ventricular contractility was assessed using the slope, or maximum elastance (Emax), of the end-systolic pressure-volume relationship.

MEASUREMENTS AND MAIN RESULTS

In the control/endotoxin group, 4 hrs after endotoxin administration, Emax had decreased by 41 +/- 4% (p < .05) and mean arterial pressure had decreased by 32 +/- 3% (p < .05). In the L-659,989/endotoxin group, the decreases in Emax (26 +/- 2%, p < .05) and mean arterial pressure (16 +/- 7%) were significantly attenuated compared with the control/endotoxin group (p < .05).

CONCLUSIONS

We conclude that platelet-activating factor plays a modest but statistically significant role in the early decrease in left ventricular contractility after endotoxin administration. Inhibition of platelet-activating factor during sepsis might be beneficial for left ventricular mechanics and hemodynamics.

Advances in the therapy for sepsis in children

Sepsis remains a major clinical problem in children, with a high mortality rate. Recent discoveries have identified the major cytokines involved in sepsis and several anticytokine strategies have been used in clinical trials. Antiendotoxin, anticytokine, and even steroid therapy have been studied in humans, with limited success. Several of the clinical trials of immunotherapy in sepsis are reviewed, and the inherent pitfalls in the varied clinical approaches investigated are analyzed.

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 acetylhydrolase deficiency. A missense mutation near the active site of an anti-inflammatory phospholipase

Deficiency of plasma platelet-activating factor (PAF) acetylhydrolase is an autosomal recessive syndrome that has been associated with severe asthma in Japanese children. Acquired deficiency has been described in several human diseases usually associated with severe inflammation. PAF acetylhydrolase catalyzes the degradation of PAF and related phospholipids, which have proinflammatory, allergic, and prothrombotic properties. Thus, a deficiency in the degradation of these lipids should increase the susceptibility to inflammatory and allergic disorders. Miwa et al. reported that PAF acetylhydrolase activity is absent in 4% of the Japanese population, which suggests that it could be a common factor in such disorders, but the molecular basis of the defect is unknown. We show that inherited deficiency of PAF acetylhydrolase is the result of a point mutation in exon 9 and that this mutation completely abolishes enzymatic activity. This mutation is the cause of the lack of enzymatic activity as expression in E. coli of a construct harboring the mutation results in an inactive protein. This mutation as a heterozygous trait is present in 27% in the Japanese population. This finding will allow rapid identification of subjects predisposed to severe asthma and other PAF-mediated disorders.

Anti-inflammatory properties of a platelet-activating factor acetylhydrolase

Platelet-activating factor (PAF) is a potent pro-inflammatory phospholipid that activates cells involved in inflammation. The biological activity of PAF depends on its structural features, namely an ether linkage at the sn-1 position and an acetate group at the sn-2 position. The actions of PAF are abolished by hydrolysis of the acetyl residue, a reaction catalysed by PAF acetylhydrolase. There are at least two forms of this enzyme--one intracellular and another that circulates in plasma and is likely to regulate inflammation. Here we report the molecular cloning and characterization of the human plasma PAF acetylhydrolase. The unique sequence contains a Gly-Xaa-Ser-Xaa-Gly motif commonly found in lipases. Recombinant PAF acetylhydrolase has the substrate specificity and lipoprotein association of the native enzyme, and blocks inflammation in vivo: it markedly decreases vascular leakage in pleurisy and paw oedema, suggesting that PAF acetylhydrolase might be a useful therapy for severe acute inflammation.