Modified photometric method for the determination of phospholipase A activities

Bradykinin B2-receptor antagonism attenuates fatal cardiocirculatory breakdown induced by severe experimental pancreatitis

OBJECTIVES

To investigate the impact of the long-acting bradykinin B2 receptor antagonist HOE 140 (Icatibant) on survival time in a model of severe porcine pancreatitis.

DESIGN

Randomized, controlled intervention trial.

SUBJECTS

Thirty domestic pigs of either gender anesthetized by intravenous application of piritramide, midazolam, and pancuronium and mechanically ventilated.

INTERVENTIONS

Pancreatitis was induced by an injection of sodium taurocholate (5%, 1 mL/kg body weight [BW]) and enterokinase (10 U/kg BW). Control animals (group 1, n = 10) underwent the spontaneous course of the disease. In two treatment groups, Icatibant was administered either in a low (100 nmol/kg BW; group 2, n = 10) or in a high dosage (5000 nmol/kg BW; group 3, n = 10).

MEASUREMENTS AND MAIN RESULTS

Mean survival time was significantly prolonged by Icatibant (controls, 6.6 hrs; group 2, 9.8 hrs; p = .022; group 3, 10.9 hrs; p = .007). Six hours postinduction, the decline of total peripheral resistance (52% of baseline) and cardiac index (92% of baseline) in controls was significantly improved by Icatibant, both in the low (16% and 44%; p < .05) and high (6% and 45%; p < .05) dosage. The concentrations of free, nonreceptor-bound kinin in plasma 6 hrs postinduction were significantly lower in controls than in groups 2 and 3 animals (111+/-50 vs. 208+/-40 and 237+/-52 fmol/mL, respectively). Six hours postinduction, the pretreatment with Icatibant was associated with significantly higher plasma concentrations of phospholipase A2 (controls, +1194%; group 2, +2000%; group 3, +2285% of baseline values) and interleukin-1 receptor antagonist (controls, 1900+/-800; group 2, 3100+/-800; group 3, 3600+/-800 pg/mL). In contrast, the increase of urinary trypsinogen activation peptides indicating local pancreatic damage (589+/-114 nmol/L in controls) was substantially attenuated by pretreatment with Icatibant (group 2, 467+/-102, NS; 352+/-91 nmol/L in group 3; p = .022 vs. controls). Systemic inflammatory reactions, however, as quantified by C-reactive protein and the extracellularly discharged neutrophil cytosolic inhibitor leukocyte neutral proteinase inhibitor were not influenced by the bradykinin B2-receptor antagonist.

CONCLUSIONS

Pretreatment with the bradykinin B2 receptor antagonist Icatibant resulted in prolonged survival time and in delayed impairment of major macrocirculatory and pulmonary variables. Icatibant resulted in elevated concentrations of free, circulating kinin. This was associated with increased concentrations of phospholipase A2 and interleukin-1 receptor antagonist, suggesting that circulating kinins strengthen the activation of some mediator cascades, the association of which with the kinin metabolism requires further experimental clarification. Other variables indicating a systemic inflammatory response (C-reactive protein, leukocyte neutral proteinase inhibitor) remained unaffected by Icatibant. Bradykinin antagonism distinctly ameliorated the local pancreatic damage, indicated by increased urinary concentrations of trypsinogen activation peptides. It is concluded that the kinin metabolism plays an important role in the pathophysiology of systemic complications after severe experimental pancreatitis.

Influence of zero-balanced hemofiltration on the course of severe experimental pancreatitis in pigs

OBJECTIVE

To examine the impact of continuous venovenous hemofiltration (CVVH) on the course of experimental pancreatitis in pigs.

SUMMARY BACKGROUND DATA

The activation of different mediator cascades is assumed to trigger multiple organ dysfunction or failure during necrotizing pancreatitis. CVVH has been suggested to be beneficial in those instances by eliminating several inflammatory mediators released in the circulation.

METHODS

Pancreatitis was induced by a combined intraductal injection of sodium taurocholate and enterokinase. Control group animals received no treatment after induction. A second group underwent "therapeutic" CVVH after a 20% decline of mean arterial pressure. In the third group, "prophylactic" CVVH was started simultaneously with the induction of pancreatitis. The concentrations of tumor necrosis factor-alpha, transforming growth factor-beta1, kinin, and phospholipase A2 were measured at different time points in blood (pre- and postfilter) and in the hemofiltrate to calculate the respective sieving coefficients that reflect most accurately the plasma clearance of mediators by CVVH.

RESULTS

Survival time was significantly prolonged both by therapeutic and prophylactic CVVH; it was more pronounced in the latter. CVVH did not influence the increase in transforming growth factor concentrations. However, 6 hours after induction, the increases of plasma concentrations of tumor necrosis factor, phospholipase, and kinin were significantly weakened by CVVH compared with controls. In the treatment groups, the plasma concentrations of tumor necrosis factor and phospholipase showed a significant negative correlation with the respective sieving coefficients, which decreased in the later course of the experiments.

CONCLUSIONS

Experimental necrotizing pancreatitis was associated with a tremendous increase of plasma concentrations of tumor necrosis factor, phospholipase, and kinin. The effective removal of these mediators by CVVH resulted in significantly improved survival time. Animals that received prophylactic CVVH had a longer survival period than those in which CVVH was started after clinical impairment. The decreasing efficiency of CVVH in eliminating inflammatory mediators in the later course of the experiments suggested that the filter membranes were compromised by long-term application. These findings provide further evidence that CVVH offers therapeutic options even in the absence of conventional indications for blood-purifying treatments.

Suppression of cytokine synthesis, integrin expression and chronic inflammation by inhibitors of cytosolic phospholipase A2

To define the isoform of phospholipases A2 active in inflammation we evaluated the effects of low-molecular-weight inhibitors of secretory and cytosolic phospholipases A2. We found that inhibitors of cytosolic phospholipase A2 had therapeutic efficacy in an in vivo model of chronic inflammation (rat adjuvant arthritis), whereas inhibitors of secretory phospholipase A2 had no beneficial effect. In vitro, inhibitors of cytosolic phospholipase A2 diminished surface expression of Mac-1 (CD11b/CD18) beta2-integrin on calcium ionophore-stimulated human blood granulocytes and suppressed synthesis of interleukin-1beta in lipopolysaccharide-stimulated human blood monocytes and U937 cells by reducing mRNA levels. Lipid mediators promote Mac-1 exocytosis and transcription of interleukin-1beta, which further enhances cytosolic phospholipase A2 activity and expression. Thus, superinduction of cytosolic phospholipase A2 may establish a positive feedback loop, converting acute inflammation into chronic inflammation. Consequently, inhibitors of cytosolic phospholipase A2 may prevent inflammation in vivo by interfering with cellular activation and infiltration. We conclude that cytosolic phospholipase A2 but not secretory phospholipase A2 is the predominant enzyme in inflammatory signalling.

Low-dose hydrocortisone infusion attenuates the systemic inflammatory response syndrome. The Phospholipase A2 Study Group

There is increasing evidence that the hypercortisolemia in inflammatory diseases suppresses the elaboration of proinflammatory cytokines, thus protecting the host from its own defence reactions. In severe sepsis and septic shock cortisol levels are usually elevated, but some patients may have relative adrenal insufficiency. This may contribute to the overwhelming systemic inflammatory response syndrome. We evaluated the impact of low-dose hydrocortisone infusion (10 mg/h) on the course of the systemic inflammatory response syndrome. This dose corresponds to a maximum secretory rate of cortisol achieved in corticotropin-stimulated healthy humans. In a prospective observational study 57 surgical patients with severe sepsis or septic shock were studied, of which in addition to the conventional treatment 12 patients were infused with low-dose hydrocortisone, and 45 were treated without any corticosteroid. In the longitudinal analysis the systemic inflammatory response--as judged by body temperature, cardiovascular response, and kinetics of inflammatory mediators such as phospholipase A2, C-reactive protein, and neutrophil elastase--started to differ in favor of the hydrocortisone-treated patients after 2 days of treatment (P < 0.05, Mann-Whitney U test). The difference disappeared after withdrawal of exogenous cortisol. Shock reversal was achieved in all patients treated with low-dose hydrocortisone. The data provide evidence that low-dose hydrocortisone infusion attenuates the systemic inflammatory response in human septic shock. From an immunological point of view a relative cortisol deficiency may contribute to the amplified immune response in systemic inflammatory diseases. A randomized clinical trial must clarify the impact of low-dose hydrocortisone infusion on the clinical course and outcome of septic shock patients.

Inflammatory response in the early prediction of severity in human acute pancreatitis

The role of the inflammatory response in acute pancreatitis and its relation with the clinical course was examined. This study examined if the serial measurement of polymorphonuclear granulocyte (PMN) elastase/A1PI complex, phospholipase A catalytic activity, C reactive protein, and other acute phase proteins, and the protease inhibitor alpha 2-macroglobulin, provides meaningful information for prognosis. Eighty non-consecutive patients with acute pancreatitis, classified according to their clinical outcome into mild (n = 40) and severe pancreatitis (n = 40), were followed up daily. Between 48 hours, median values of PMN-elastase, C reactive protein--and most of the acute phase proteins--and phospholipase A activity, were significantly higher in the severe pancreatitis group. PMN elastase shows a dynamic course and it reaches an early peak value at days 1-2, followed by C reactive protein (days 2-4) phospholipase A (day 3), and a negative peak for alpha 2-macroglobulin (days 4-5). PMN elastase (day 1) and C reactive protein (day 2) were selected by discriminant analysis as the most useful variables studied to allow the early accurate prediction of severity (sensitivity 100%, specificity 95%). Little or no predictive additional value was found for all other variables studied. These results strongly suggest a close relation between inflammatory parameters and clinical course in acute pancreatitis, and discriminant analysis of these variables provides a useful method to classify severity.

Determination of the catalytic activity of phospholipase A2: E. coli-based assay compared to a photometric micelle assay

Phospholipase A2 activity in human sera was determined of the basis of the E. coli assay and compared to a photometric micelle assay. The E. coli assay is based on the hydrolysis of phospholipids from [1-14C]oleic acid-labelled E. coli biomembranes. In the photometric assay the phospholipase A2 acts on mixed phospholipid micelles. The amount of fatty acid produced is quantitated in a subsequent photometric assay by coupling in the reaction to the coenzyme A metabolism. The E. coli membranes are essentially resistant to other lipases in human sera, i.e. lipoprotein lipases, hepatic triacylglycerolipase or pancreatic lipase and thus a very specific substrate for the phospholipase A2 of human serum. The photometric assay, though, is susceptible to other lipases in human serum. The ratio of [1-14C]oleic acid to released total fatty acids served as the basis for the calculation of the true enzymatic activity. The assay closely correlated with the photometric assay based on mixed micelles in the higher ranges of phospholipase A2 activity, but not in the normal range. The sensitivity is higher by at least two powers of 10. The human serum phospholipase A2 strongly preferred E. coli membranes as substrate to the mixed micelles containing phosphatidylcholine/phosphatidylethanolamine. In conclusion, the modified phospholipase A2 assay based on E. coli membranes is a sensitive, specific, reliable, and convenient method for the measurement of phospholipase A2 activity in human sera. The photometric assay suffers from low sensitivity but has the advantage of practicability in a normal routine laboratory, including the amenability to automation.

Differentiation of human phospholipase A2 isoenzymes in serum and other body fluids with use of monoclonal antibodies

Elevated phospholipase A2 activities in serum were measured in patients suffering from acute pancreatitis or various inflammatory diseases. The photometric phospholipase A assay of Hoffmann & Neumann (Klin. Wochenschr. 67 (1989) 106-109) was combined with immunoabsorption by different monoclonal antibodies directed against pancreatic phospholipase A2. Pancreatic phospholipase A2 was purified from human duodenal juice. Monoclonal antibodies were prepared by fusion of spleen cells from immunized mice with P3X63-Ag8-653 myeloma cells. Samples with phospholipase A2 activity were incubated in monoclonal antibody-coated microtitre plates. Phospholipase A2 activities were determined in the monoclonal antibody-treated samples as well as in control samples. The method allows the determination of the fraction of human phospholipase A2 isoenzymes in various biological materials. For pancreatic phospholipase A2 the specific binding capacity was about 60-80%, the unspecific binding was 5-30%. Practically no cross-reactivity was seen with partially purified serum phospholipase A2, with recombinant platelet phospholipase A2, or with the sera of patients with non-pancreatic diseases. In conclusion, the present study confirmed the presence of pancreatic phospholipase A2 in human duodenal juice and in the ascites of necrotizing pancreatitis. However, pancreatic isoenzyme was absent in non-pancreatic inflammatory diseases. Therefore, elevated phospholipase activities in non-pancreatic inflammatory diseases cannot be attributed to the pancreas.

Role of interleukin-6 in acute pancreatitis. Comparison with C-reactive protein and phospholipase A

Plasma values of immunoreactive interleukin-6, C-reactive protein and phospholipase A have been determined in serial samples from 24 patients with acute pancreatitis ('mild' pancreatitis nine, 'severe' pancreatitis 15). Median plasma concentrations of interleukin-6, C-reactive protein, and phospholipase A activity were significantly higher in patients with 'severe' illness (p < 0.001) than those with 'mild' illness. A particularly marked increase in interleukin-6 was found in two patients with necrotising pancreatitis and fatal outcome. Significant correlations between plasma concentrations of interleukin-6 and phospholipase A (p = 0.0218) and C-reactive protein and phospholipase A activity (p < 0.0001) were found in patients with 'severe' disease. These findings in a limited number of patients with acute pancreatitis are promising in that raised interleukin-6 correlated with clinical severity and with two other established markers, C-reactive protein, and phospholipase A activity.

Characterization of cellular and elevated serum phospholipase A2 activities with a comparison of two methods

The phospholipase A assay of Hoffmann et al based on the enzymatic photometric determination of the fatty acids liberated from soy-bean phospholipids was compared with the fluorometric assay of Thurén et al. where a synthetic pyrene-labelled substrate is used. Sera from patients with suspected pancreatitis or sepsis were studied. High values compared well while the Hoffmann method was not sensitive enough to detect slightly elevated values in sera from patients with suspected pancreatitis. The phospholipase A2 activities from enzymes purified from human duodenal juice, human sera from patients with sepsis and rat liver mitochondria were characterized in regard to activity towards several synthetic pyrene-labelled substrates, activation by Ca2+ and inhibition by Sr2+ and Mg2+. The enzyme from serum was distinctly different from both the pancreatic secretory and the mitochondrial ones, both in its substrate specificity pattern and in being most strongly inhibited by Mg2+.

Assay strategies and methods for phospholipases

Of the general considerations discussed, the two issues which are most important in choosing an assay are (1) what sensitivity is required to assay a particular enzyme and (2) whether the assay must be continuous. One can narrow the options further by considering substrate availability, enzyme specificity, assay convenience, or the presence of incompatible side reactions. In addition, the specific preference of a particular phospholipase for polar head group, micellar versus vesicular substrates, and anionic versus nonionic detergents may further restrict the options. Of the many assays described in this chapter, several have limited applicability or serious drawbacks and are not commonly employed. The most commonly used phospholipase assays are the radioactive TLC assay and the pH-stat assay. The TLC assay is probably the most accurate, sensitive assay available. These aspects often outweigh the disadvantages of being discontinuous, tedious, and expensive. The radioactive E. coli assay has become popular recently as an alternative to the TLC assay for the purification of the mammalian nonpancreatic phospholipases. The assay is less time consuming and less expensive than the TLC assay, but it is not appropriate when careful kinetics are required. Where less sensitivity is needed, or when a continuous assay is necessary, the pH-stat assay is often employed. With purified enzymes, when free thiol groups are not present, a spectrophotometric thiol assay can be used. This assay is approximately as sensitive as the pH-stat assay but is more convenient and more reproducible, although the substrate is not available commercially. Despite the many assay choices available, the search continues for a convenient, generally applicable assay that is both sensitive and continuous. The spectrophotometric SIBLINKS assay and some of the fluorescent assays show promise of filling this need.