An automated chromogenic peptide substrate assay for coagulation factor XII

We have developed an automated chromogenic peptide substrate assay for factor XII (FXIIcs) on a Cobas Mira S Plus clinical chemistry analyser using a new commercially available kit. This was used to determine factor XII (FXII) levels in plasma samples from 320 blood donors, 206 patients with a history of venous thrombosis and 74 lupus anticoagulant positive (LA+) patients. Results were compared with those obtained in a clotting assay for FXII (FXIIct) and an immunochemical assay (FXIIag). A satisfactory correlation coefficient of 0.92 and a regression line equation of y = 7.898 + 0.871x was obtained between FXIIcs and FXIIct in the 320 blood donors. Levels of FXII below the calculated normal range were found in nine blood donors (2.8%) and 16 venous thrombosis patients (7.8%). The blood donors and patients with venous thrombosis with low FXIIcs values had FXII levels below our lower limits of normal for both FXIIct and FXIIag; all were lupus anticoagulant negative. When FXII levels were determined in the 74 LA+ patients, 27 (36.5%) gave markedly lower FXII values in the FXIIct when compared with the FXIIcs. FXIIag levels corresponded with FXIIcs. The automated FXIIcs assay is therefore lupus anticoagulant insensitive and allows us to measure FXII levels accurately and routinely in large numbers of patient samples.

Factor XII determinations in the presence and absence of phospholipid antibodies

Factor XII (FXII) levels were determined in plasma samples from 29 normal donors, 10 patients with inherited FXII deficiency (all lupus anticoagulant [LA] negative) and 67 LA positive patients, using clotting (FXIIct), chromogenic substrate (FXIIcs) and immunochemical (FXIIag) assays. Excellent correlations were obtained in the three FXII assays with the LA negative samples and between the FXIIcs and FXIIag assays in the LA positive samples. Correlations between both the FXIIcs and FXIIag with FXIIct in the LA positive patients were poor. Of 67 LA positive samples studied, 25 (37.3%) showed lower values in the FXIIct assay; 13 (19.4%) of these patients were pseudo FXII deficient with values of FXII below the lower limit of normal. These results indicate that a diagnosis of FXII deficiency can be made inappropriately in the presence of phospholipid antibodies and that such a diagnosis should not be made by FXIIct assay alone.

Influence of heparin, heparin plus aprotinin and hirudin on contact activation in a cardiopulmonary bypass model

In the present study we used an in vitro cardiopulmonary bypass model to compare activation of the FXII-plasma kallikrein systems, coagulation pathway and blood cell changes, with heparin (3 U/ml), heparin plus aprotinin (3 U/ml and 250 KIU/ml) and recombinant hirudin (6 micrograms/ml). After 90 min circulation the results showed that with heparin plus aprotinin and with hirudin the activation of these cascade reactions was markedly lower. In particular kallikrein-like activities and PMN-Elastase-alpha 1-PI levels were significantly lower in the latter two groups. The least activation was detected with hirudin. Our results confirm that the contact systems of blood are activated during CPB with heparin as anticoagulant, that aprotinin reduces this activation, and that recombinant hirudin may be preferred to heparin as an anticoagulant in cardiac surgery.

Heparin-coated oxygenators significantly reduce contact system activation in an in vitro cardiopulmonary bypass model

Over the past decade our group has shown that the contact system of blood is activated in cardiopulmonary bypass (CPB), that heparins enhance this activation and that aprotinin reduces both this activation and blood loss in CPB. We have developed an in vitro CPB model to assess the effects of added components to blood and new components in the artificial devices of CPB. In the present study we have compared membrane oxygenators with or without heparin-coated surfaces under identical conditions in the CPB model. Recalcified ACD blood was circulated in a closed system for 90 min at 28 degrees C. Blood samples were taken at various times during circulation. 4 IU/ml heparin was used with the non-coated oxygenators; no heparin was used in the coated system. Heparin levels were measured in the plasma together with various contact system components. Haemolysis, platelet count, platelet factor 4 and alpha 1-proteinase inhibitor-PMN elastase complexes, were also determined. No heparin was detected during the period of recirculation in samples from the coated oxygenators, showing the excellent adhesive quality of the heparin coating. In keeping with a significant greater fall in the platelet count in non-coated vs coated oxygenators (mean [+/- SD] final counts of 170 +/- 50 x 10(9)/l and 97 +/- 34.2 x 10(9)/l respectively after 90 min circulation), platelet factor 4 levels were significantly higher (682.9 +/- 187.3% and 95.8 +/- 46.5% of the initial value respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

The prolonged activated clotting time (ACT) with aprotinin depends on the type of activator used for measurement

In cardiopulmonary bypass (CPB) surgery high dose heparin is necessary to inhibit the clotting cascade which is activated through damage to the vessels (extrinsic pathway) as well as contact activation of the blood with the various artificial surfaces of the CPB machine (intrinsic pathway). In most European heart surgery centres, the fibrinolytic activation that always occurs in CPB due to contact activation is reduced by the proteinase inhibitor aprotinin. Monitoring of anticoagulation during CPB is performed with the activated whole blood clotting time (ACT). The two machines commonly used for this purpose, Hemotec and Hemochron, use different contact system activators, kaolin and celite. These activators displayed highly significant differences, in both in vitro tests (modified APTT with whole blood in a neutral coagulometer), and ACT in both machines where aprotinin and heparin were used, as well as with parallel measurements with the two machines with blood from patients undergoing CPB with high dose aprotinin therapy (P < 0.001). The Hemotec machine with kaolin as activator was not affected by aprotinin throughout surgery, while the Hemochron clotting times almost doubled as soon as aprotinin and heparin were combined. Our studies show that for the determination of ACT in CPB were high dose aprotinin therapy is used only ACT determinations with machines using kaolin as activator yield accurate results.

Aprotinin in cardiopulmonary bypass--effects on the Hageman factor (FXII)--Kallikrein system and blood loss

Aprotinin has been used in our hospital in open heart surgery for almost 20 years and recently published studies have revealed a reduction in postoperative blood loss under this therapy. In the present study patients undergoing aorto-coronary bypass operations received either 20,000 KIU aprotinin/kg body weight (group 2) or 60,000 KIU aprotinin/kg body weight (group 3). Another group of patients without aprotinin served as a control (group 1) and postoperative bleeding was more pronounced in these patients compared with the other groups. In parallel, slight reductions in kallikrein-like activity were observed in patients treated with aprotinin. Furthermore, we have shown that the main inhibitor of the contact phase, C1-esterase-inhibitor, loses some of its activity against beta-FXIIa in the presence of heparin. Aprotinin was able to partly antagonize this phenomenon. All studies dealing with the effect of aprotinin in extracorporeal circulation demonstrate hyperfibrinolysis in untreated patients. Aprotinin is known to inhibit plasmin at low concentrations and thus reduced the postoperative bleeding tendency (group 2). In addition, plasma kallikrein is inhibited by high aprotinin concentrations and is responsible for a reduced activation of the contact phase system. This effect led to a further reduction in blood loss (group 3).

Chromogenic peptide substrate assays and their clinical applications

Chromogenic peptide substrates were first introduced into research laboratories in the early 1970s and were quickly utilised to develop assays for the determination of enzymes, proenzymes and inhibitors of the coagulation system. These assays were gradually introduced into coagulation and clinical chemistry laboratories as laboratory tools in the diagnosis and treatment of coagulation disorders. From the knowledge of the structures of the natural substrates attacked by enzymes other than those of the coagulation system or by synthesis and random screening, substrates for enzymes of the fibrinolytic, plasma and glandular kallikrein and complement systems were produced. These allowed various research groups to develop assays for components of these systems and subsequently led to the use of these assays in studies on various clinical conditions. Substrates for activated protein C ensured that assays for this enzyme and its inhibitors could be developed and introduced into the haematological routine. With the introduction of substrates for limulus lysate not only were assays for endotoxins in clinical samples produced but the control of all disposable products and injectables for endotoxin contamination can now be effected. Initially high costs and time-consuming manual assays were a hinderence to the general acceptance of the use of chromogenic peptide substrate assays and they were only used routinely in a few specialised laboratories. With the introduction of automated and microtitre plate methods however, these assays are are now available in most hospital laboratories. Since the first chromogenic peptide substrate was described thousands of articles have been published on the use of chromogenic substrate assays to measure proenzymes, enzyme activators, enzyme cofactors and inhibitors in blood and other body fluids in normal subjects and clinical material. We have endeavoured to cover as many of these as possible in this review.

FXII

The plasma protein FXII (Hageman factor) has been shown to be linked with the plasma defence systems of coagulation, fibrinolysis, kallikrein-kinin and complement. It can be activated by surface contact activation and in solution. Surface contact activation is a complex phenomenon involving negatively charged surfaces, FXII, high molecular weight kininogen and plasma kallikrein. Fluid-phase activation can be effected by a variety of serine proteases. In both types of activation the FXII zymogen is converted to active enzymes. FXII levels in plasma are low or undetectable in both inherited deficiencies and in a variety of clinical conditions. FXII levels can also be elevated in some clinical conditions. Although discovered as a clotting protein FXII appears to play an important role in the kallikrein-kinin and fibrinolytic systems and also has effects on cells. Recent studies suggest that therapeutic blockade of activation of FXII can be of benefit in certain clinical conditions.

Modulation of morphological differentiation of human neuroepithelial cells by serine proteases: independence from blood coagulation

We have previously shown that a serum protein, termed differentiation reversal factor (DRF), is responsible for neurite retraction in differentiated cultures of an adenovirus 12 (Ad12) transformed human retinoblast cell line. Data is presented here to show that DRF is identical to the serine protease prothrombin. Both proteins have been immunoprecipitated using an antibody raised against purified prothrombin and have been shown to hydrolyse a specific thrombin substrate only after activation by the snake venom ecarin. Following addition to Ad12 HER 10 cells, which had previously been differentiated by culture in the presence of 2 mM dibutyryl cAMP in serum-free medium, thrombin and prothrombin caused half-maximal retraction of neurites at concentrations of 0.5 ng/ml and 20 ng/ml respectively. Interestingly, activation of prothrombin was shown to be unnecessary for biological activity. Using the inhibitor di-isopropylfluorophosphate (DIP), we have shown that abrogation of the proteolytic activity of thrombin also results in a loss (greater than 2000 fold) of differentiation reversal activity. Thrombin and its zymogen both stimulated the mitosis of differentiated Ad12 HER 10 cells to a similar extent. In addition, differentiation reversal was highly specific since, at physiologically significant concentrations, closely related serine proteases did not cause neurite retraction. Prothrombin and thrombin also reversed morphological differentiation in the SK-N-SH neuroblastoma cell line and in heterogeneous cultures of cells from various regions in the human foetal brain.

Parameters of the kallikrein-kinin, coagulation and fibrinolytic systems as early indicators of kidney transplant rejection

In order to find early indicators of kidney transplant rejection before clinical symptoms were noticed, parameters of the coagulation, fibrinolytic and kallikrein-kinin systems were measured. Nineteen patients were followed before and daily after kidney transplantation during the first week and every second day in the following weeks. All patients received immunosuppressive therapy with cyclosporin and corticoids. Ten patients suffered from transplant rejection. The first rejection occurred on the 7th day after transplantation. Of all the parameters measured, kallikrein inhibition, beta-FXIIa inhibition, plasminogen and antithrombin III were early indicators of kidney transplant rejections. A rise in these parameters could be demonstrated 2-3 days before clinical signs were noticed. In the other 9 patients no significant rises in antithrombin III, plasminogen, kallikrein inhibition and beta-FXIIa inhibition could be found.

Studies on synthetic peptide substrates for F XII enzymes

Screening of chromogenic peptide substrates have shown that FXIIa readily splits substrates of D-Pro-Phe-Arg-pNA (S-2302) and -Gly-Arg-pNA (e.g. S-2222) types. The latter type is preferred in a system where kallikrein is present. By using the substrate S-2222 a method for the determination of beta FXIIa inhibitors has been designed. Chromatography data show that C1-esterase inhibitor is the major inhibitor of beta FXIIa in plasma. Preliminary studies have also been performed on the assay of FXII in human plasma. The procedure to obtain a complete activation of FXII has still to be studied.

Description and evaluation of a new chromogenic substrate assay kit for the determination of prekallikrein in human plasma

Because of an increasing interest in the determination of prekallikrein a kit was made for the determination of this plasma proenzyme. The kit consists of 1) a prekallikrein activator of the cephalin-ellagic acid type containing Factor XII and HMW-kininogen to ensure a total activation of the prekallikrein even in pathological plasmas, 2) a buffer which is optimal for both activation and substrate hydrolysis and 3) the chromogenic substrate S-2302. A control plasma is also included. This kit was evaluated by thirteen research groups as well as by ourselves. Both normal and patient plasmas were analyzed. Good correlations were obtained for prekallikrein levels in plasma samples between the kit method and two other methods (immunochemical and functional). As well as in deficiency states the prekallikrein level was low in pancreatitis (n = 20), cancer (n = 16), early pregnancy with gestosis (n = 15), cirrhosis (n = 9) and cases with thromboembolic disorders (n = 5). The prekallikrein level was high in late pregnancy (n = 4).