Release of fatty acid-binding protein from isolated rat heart subjected to ischemia and reperfusion or to the calcium paradox

Elevated levels of both cardiomyocyte membrane and myofibril damage markers predict adverse outcomes in patients with chronic heart failure

BACKGROUND

Recent studies have shown the presence of ongoing myocardial damage in patients with chronic heart failure (CHF) detected by myofibril and membrane damage markers, cardiac troponin T (TnT) and heart-type fatty acid-binding protein (H-FABP), which identifies patients at increased risk of a future cardiac event (CE: death or rehospitalization because of worsening CHF). There is a difference between TnT and H-FABP in their release kinetics following myocardial damage.

METHODS AND RESULTS

TnT and H-FABP were measured in 103 patients with CHF and in 31 controls. Patients were classified into 4 groups based on detectable (>or=0.01 ng/ml) or undetectable TnT (TnT+ or TnT-) and H-FABP >or= or <4.5 ng/ml (mean + 2 standard deviations in controls) (high-H-FABP or low-H-FABP). Kaplan-Meier analysis showed that the CE-free rate (n=43) was significantly lower in patients with TnT+ and high-H-FABP than in patients in the other 3 groups (patients with TnT+ and low-H-FABP, TnT- and high-H-FABP, and TnT- and low-H-FABP; p=0.02, p=0.001 and p=0.0002, respectively). In stepwise multivariate Cox proportional hazard analysis, TnT+ (p=0.01) and high-H-FABP (p=0.04) were independent predictors of future CE.

CONCLUSIONS

Elevated levels of both TnT and H-FABP predict adverse outcomes in CHF patients.

Analytical and clinical performance of a fully automated cardiac multi-markers strategy based on protein biochip microarray technology

OBJECTIVES

The analytical and clinical performance of the Evidence Cardiac Panel were evaluated.

DESIGN AND METHODS

The Evidence Cardiac Panel, an automated protein biochip microarray system, allows the simultaneous determination of creatine kinase MB (CK-MB), myoglobin (MYO), glycogen phosphorylase BB (GPBB), heart-type fatty acid-binding protein (H-FABP), carbonic anhydrase III (CA III), cardiac troponin I (cTnI). Precision: 3 levels of quality control (QC) and 2 in house pools (P) were assayed. Method comparison: MYO and cTnI concentrations measured on Evidence (E) and on Dimension RxL (D) analyzers were compared. Clinical study: 132 non-consecutive patients admitted to the Emergency Department for chest pain were enrolled.

RESULTS AND CONCLUSIONS

The between-day imprecision was CK-MB=6.80-10.08%; MYO=5.36-16.50%; GPBB=6.51-12.12%; H-FABP=6.26-12.63%; CA III=6.98-13.61%; cTnI=6.02-9.80%. Method comparison: E-MYO vs. D-MYO, Bias=-29.22, 95% CI from -40.25 to -18.18; E-cTnI vs. D-cTnI, Bias=-2.75, 95% CI from -4.04 to -1.46. In patients studied (at discharge: AMI, acute myocardial infarction n=42; non-AMI, n=90) H-FABP showed the highest accuracy (ROC analysis, AUC=0.92) and "cTnI+H-FABP" the greatest diagnostic efficacy (89.4%) in AMI diagnosis.

Heart fatty acid binding protein in the diagnosis of myocardial infarction: where do we stand today?

Heart fatty acid binding protein (hFABP) is a novel small cytosolic protein that is abundant in the heart. It is highly cardiac-specific (i.e. expressed primarily in cardiac tissue), but is also expressed at low concentrations in tissues outside the heart. After myocardial ischemic damage, hFABP can be detected in the blood as early as 1-3 h after onset of chest pain, with peak values reached at 6-8 h and plasma levels returning to normal within 24-30 h. hFABP's clinical diagnostic value is very limited in the presence of renal failure and skeletal muscle diseases as it is completely renally eliminated. In these conditions, the diagnosis of acute myocardial infarction (AMI) may be overestimated. The combination of initial hFABP release after symptom onset, rapid kidney clearance from the circulation and high cardiac specificity suggests great potential for clinical use. Serial measurements of hFABP in the first 24 h after onset of symptoms in AMI patients can: (a) identify patients who are susceptible to reperfusion strategies, (b) detect perioperative AMIs, (c) distinguish patients who reperfuse their infarct-related artery from those who do not, as early as 30 min after starting thrombolytic treatment, (d) detect re-infarction if it occurs within 10 h after symptom onset, and (e) permit an accurate estimation of myocardial infarct size providing important prognosis information.

Urinary fatty acid binding protein in renal disease

The number of patients with end stage renal failure has been increasing throughout the world. The importance of measuring clinical parameters in renal injury has been emphasized for administering appropriate treatment and preventing a worsening of the disease. However, there are no clinically useful markers in predicting and monitoring the progression of renal disease. Liver type fatty acid binding protein (L-FABP) of 14.4 kDa is expressed in human proximal tubules. In order to evaluate the clinical significance of urinary L-FABP as a biomarker in renal disease, a monoclonal antibody against human L-FABP was developed and a two step sandwich enzyme linked immunosorbent assay (ELISA) method was established for determining human L-FABP in urine. In some clinical studies, urinary excretion of L-FABP was shown to be an excellent clinical marker that can help predict and monitor the progression of renal disease. The dynamics of renal L-FABP in pathophysiological settings has been revealed in experimental studies using transgenic mice with the human L-FABP gene. This review presents recent findings on the function and pathophysiological role of L-FABP, and summarizes the clinical importance of measuring urinary L-FABP in renal disease.

Unbound Free Fatty Acids and Heart-Type Fatty Acid–Binding Protein: Diagnostic Assays and Clinical Applications

Background: A biomarker that reliably detects myocardial ischemia in the absence of necrosis would be useful for initial identification of unstable angina patients and for differentiating patients with chest pain of an etiology other than coronary ischemia, and could provide clinical utility complementary to that of cardiac troponins, the established markers of necrosis. Unbound free fatty acids (FFAu) and their intracellular binding protein, heart-type fatty acid–binding protein (H-FABP), have been suggested to have clinical utility as indicators of cardiac ischemia and necrosis, respectively.

Methods: We examined results of clinical assessments of FFAu and H-FABP as biomarkers of cardiac ischemia and necrosis. Data published on FFAu and H-FABP over the past 30 years were used as the basis for this review.

Results: Although little clinical work has been done on FFAu since the initial reports, recent studies documented an association between increased serum FFAs and ventricular dysrhythmias and death in patients with acute myocardial infarction (AMI). Recent data suggest that serum FFAu concentrations increase well before markers of cardiac necrosis and are sensitive indicators of ischemia in AMI. H-FABP is abundant in cardiac muscle and is presumed to be involved in myocardial lipid homeostasis. Similar to myoglobin, plasma H-FABP increases within 3 h after AMI and returns to reference values within 12–24 h.

Conclusions: FFAu may have a potential role in identifying patients with cardiac ischemia. H-FABP is useful for detecting cardiac injury in acute coronary syndromes and predicting recurrent cardiac events in acute coronary syndromes and in congestive heart failure patients. Assays are available for both markers that could facilitate further clinical investigations to assess their possible roles as markers of cardiac ischemia and/or necrosis.

Evaluation of H-FABP as a marker of ongoing myocardial damage using hGH transgenic mice

BACKGROUND

There are few heart-specific and highly sensitive biomarkers of cardiac disorders in experimental animals. To evaluate ongoing myocardial damage in experimental mice, available and reliable biomarkers are needed. We investigated whether or not heart-type fatty acid-binding protein (H-FABP) is useful as a biomarker for predicting ongoing myocardial disorders, by using CAG/EGFP-WAP/hGH transgenic male mice with heart disease induced by overexpression of human growth hormone (hGH).

METHODS

Blood samples were collected from transgenic and control male mice at 8, 12, 16, and 36 weeks of age and were measured for aspartate aminotransferase (AST), creatine kinase (CK), lactate dehydrogenase (LDH), and H-FABP. The hearts of the transgenic mice were examined histopathologicaly and the results were compared with those of control mice.

RESULTS

At 36 weeks of age, significant increases in AST, CK, and LDH values were observed in the transgenic mice compared to the control mice. Minute histological changes along with focal and slight degeneration of cardiomyocytes were observed in the transgenic hearts at 12 weeks of age, but the only chemical value to change was that of H-FABP, which increased significantly.

CONCLUSIONS

H-FABP is available as a biomarker for predicting ongoing cardiomyocyte damage in the mouse model.

Heart fatty acid binding protein in the rapid evaluation of myocardial damage following valve replacement surgery

BACKGROUND

Myocardial damage occurs following valve replacement surgery. We estimated the value of heart fatty acid binding protein (H-FABP) in these patients.

METHODS

Sixty elected patients were enrolled and distributed into single (group A) and double (group B) valve replacement groups. The clinical data were outlined and blood samples were collected perioperatively for determination of plasma levels of H-FABP, cardiac troponin-I (cTn-I), and CK-MB in both groups.

RESULTS

56 patients completed the study and no significant difference of clinical data was observed except CPB time and ACC time between groups. H-FABP level elevated quickly after reperfusion and peaked significantly earlier than cTn-I and CK-MB, it also declined rapidly but did not return to baseline at 24 h after reperfusion. Three markers' levels were all higher in group B than in group A after reperfusion with significant differences at their peaks and thereafter. Patients with postoperative complications had significantly higher H-FABP levels than usual. H-FABP peak level associated well with the length of CPB and ACC as well as with other 2 markers' peak levels in both groups.

CONCLUSION

Compared with cTn-I and CK-MB, H-FABP is an earlier and potentially useful marker in the rapid evaluation of myocardial damage following valve replacement surgery with CPB.

Novel markers for heart failure diagnosis and prognosis

PURPOSE OF REVIEW

This paper reviews recent advances in heart failure biomarkers for identification of disease precursors, subclinical disease, and onset or progression of overt disease.

RECENT FINDINGS

Heart failure biomarkers can be categorized empirically as neurohormonal mediators, markers of myocyte injury and remodeling, and indicators of systemic inflammation. Brain natriuretic peptide is the most widely studied, with a potentially important but evolving role for determining prognosis and as a surrogate endpoint in clinical trials. Strong evidence exists for use of brain natriuretic peptide in the diagnosis of acute heart failure and for improved clinical outcomes with a brain natriuretic peptide-guided approach to heart failure care. The use of brain natriuretic peptide as a screening tool for asymptomatic left ventricular systolic dysfunction, or to distinguish systolic from diastolic heart failure, is not supported by current data. Markers of myocyte injury, including troponins, heart-type fatty acid binding protein, and myosin light chain-1, may further improve heart failure prognostication in conjunction with plasma brain natriuretic peptide. Biomarkers of matrix remodeling and inflammation have emerged as potential preclinical indicators to identify individuals at risk of developing clinical heart failure. A role for cellular adhesion molecules may also emerge in identifying those at risk for cardiovascular thrombotic complications, such as stroke.

SUMMARY

The spectrum of heart failure biomarkers and their potential clinical applications continues to grow. Ongoing research on multimarker strategies will likely identify biomarker combinations that are optimal at various stages during the evolution of heart failure, ranging from their use for screening, diagnosis, determining prognosis, and guiding management.

Fatty acid-binding proteins as plasma markers of tissue injury

BACKGROUND

One of the novel and promising plasma markers for detection of tissue injury is the family of 15 kDa cytoplasmic fatty acid-binding proteins of which various tissue-specific types occur.

AIMS AND OBJECTIVES

The present status of heart-type fatty acid-binding protein (H-FABP) as a diagnostic and prognostic marker for acute and chronic cardiac injury, as well as the preliminary diagnostic use of other types of FABP for detecting injury in other organs, is reviewed.

METHODS

This review is based on an overview of the literature on clinical diagnostics of various forms of organ injury, and uses additional literature on physiological aspects relevant for the interpretation of plasma marker concentrations.

RESULTS

H-FABP not only proves to be an excellent early marker for cardiac injury in acute coronary syndromes, but also allows detection of minor myocardial injury in heart failure and unstable angina. Preliminary results indicate that sensitivity, rule-out power and prognostic value of H-FABP in cardiac injury surpass the performance of the standard early marker myoglobin. The liver only contains liver-type FABP (L-FABP), but co-expression of H-FABP and L-FABP occurs in the kidney. Similarly, intestinal-type FABP (I-FABP) and L-FABP are found in intestines, and brain-type FABP (B-FABP) and H-FABP occur in the brain. Preliminary but promising applications of these proteins have been demonstrated for liver rejection, viability selection of kidneys from non-heart-beating donors (NHBD), inflammatory and ischemic bowel disease, traumatic brain injury and in the prevention of muscle injury in trained athletes.

CONCLUSIONS

Further study of the diagnostic and prognostic use of various FABP types is warranted, but their clinical application will require further commercialization of automated and rapid assays.

Heart-Fatty Acid-Binding Protein as a Marker for Early Detection of Acute Myocardial Infarction and Stroke

Heart-fatty acid-binding protein (H-FABP) is a small cytosolic protein involved in intracellular fatty acid transport. This protein, highly concentrated in the heart, is quickly released into plasma after myocardial injury. Results from numerous studies suggest that H-FABP is an excellent marker for the early detection of myocardial damage. H-FABP is also expressed in the brain, although in lower concentrations than in the heart. Recent preliminary studies also investigated the usefulness of H-FABP for the diagnosis of acute and chronic neurological disorders. These potential applications of H-FABP in clinical practice are reviewed in this article, with a strong focus on the early diagnosis of acute myocardial infarction and stroke.

A continuous displacement immunoassay for human heart-type fatty acid-binding protein in plasma

Human heart-type fatty acid-binding protein (FABP) is suggested as an early plasma marker of acute myocardial infarction (AMI), and several studies have proved that, for early diagnosis of AMI, FABP performs better than myoglobin, which is a more often used early marker protein. Because serial measurement of biochemical markers in plasma is now universally accepted as an important determinant in AMI diagnosis, a rapid and continuous measuring method for FABP would be desirable. The aim of the present study was to develop an immunoassay based on the principle of displacement and using a column for rapid and continuous measurement of FABP in plasma. Glass columns filled with Sepharose-bound FABP were loaded with a horseradish peroxidase (HRP)-labeled antibody (Ab) and equilibrated with human plasma. After reaching a stable baseline, human plasma spiked with FABP or plasma from AMI patients was added. The Ab-HRP complex dissociated due to the presence of FABP in the plasma and was subsequently quantified. For plasma from AMI patients (n=5), the Ab-HRP level thus measured correlated with the corresponding plasma FABP concentration (R=0.96). The results of this study show the feasibility of a sensor for continuous monitoring of FABP in plasma.

Biochemical markers of myocyte injury in heart failure

This review discusses the role of biochemical markers of myocyte injury in patients with chronic congestive heart failure. Heart specific assays have been developed for the measurement of cardiac troponin T (cTnT), cardiac troponin I (cTnI), heart type fatty acid binding protein (H-FABP), and myosin light chain 1 (MLC-1). Concentrations of these biochemical markers increase in the absence of ischaemic events in the subset of patients with heart failure whose long term outcomes are most adverse. The markers are easy to measure serially and it is therefore easy to follow patients without inter-observer variability. The serial clinical use of these markers, separately or in combination, will sharpen our understanding of the state of heart failure.

Do we need additional markers of myocyte necrosis: the potential value of heart fatty-acid-binding protein

Heart fatty-acid-binding protein (FABP) is a small cytosolic protein that is abundant in the heart and has low concentrations in the blood and in tissues outside the heart. It appears in the blood as early as 1.5 h after onset of symptoms of infarction, peaks around 6 h and returns to baseline values in 24 h. These features of H-FABP make it an excellent potential candidate for the detection of acute myocardial infarction (AMI). We review the strengths and weaknesses of H-FABP as a clinically applicable marker of myocyte necrosis in the context of acute coronary syndromes.

Development of a quantitative lateral-flow assay for rapid detection of fatty acid-binding protein

Using human heart-type fatty acid-binding protein (H-FABP) as an early cardiac marker to confirm or exclude a diagnosis of acute myocardial infarction (AMI) soon after admission requires a rapid assay system. Due to the requirement of skillful technicians and complicated assay procedures, most immunochemical assays for H-FABP are of limited use for routine clinical practice. In the present study, we describe a rapid lateral-flow assay for detection of H-FABP. Fifty-one human samples were evaluated using a conventional ELISA and a newly developed lateral-flow assay. A good agreement between the two methods was found according to Bland and Altman plot. The correlation found was y=0.9685 x -0.6270 (r(2)=0.9585). The detector antibody labeled with colloidal gold was mixed with those without label to extend the linear range of the calibration curve up to 125 microg/l H-FABP with r(2)=0.9832. The detection limit of the assay was 2.8 microg/l. The test-strips can be stored either at 4 degrees C and room temperature for up to 1 year without significant loss of activity. Finally, a one-step FABP test so-called CardioDetect(R), which was derived from the serum lateral-flow assay has been designed for qualitative determination of H-FABP in whole blood samples. It requires no sample pretreatment and gives results within 15 min. Thirty-eight patients presenting with chest pain and suspected AMI were studied. Using an upper reference level of 7 microg/l, the specificity of the rapid test was 94%. Both sensitivity and negative predictive value (NPV) were 100%, implying that 100% of non-AMI patients could be excluded with no false-negative results. With this rapid and sensitive immunotest, H-FABP could soon be introduced into clinical practice.

Use of cytosolic and myofibril markers in the detection of ongoing myocardial damage in patients with chronic heart failure

PURPOSE

Measurement of serum levels of cytosolic and myofibril components of cardiac tissue could indicate ongoing myocardial damage in patients with chronic heart failure.

METHODS

We correlated serum levels of a cytosolic marker (heart-type fatty acid-binding protein) and a myofibril marker (troponin T) with the severity of symptoms (based on the New York Heart Association [NYHA] class), neurohumoral derangement, and subsequent cardiac events in 56 patients with chronic heart failure.

RESULTS

Mean (+/- SD) levels of heart-type fatty acid-binding protein were greater in patients with NYHA class III or IV heart failure (9.9 +/- 5.2 ng/mL) than in those with NYHA class II (4.9 +/- 1.9 ng/mL, P <0.0001). Detection of troponin T (> or =0.02 ng/mL) was also more common in patients with worse heart failure (81% [13/16] in class III or IV vs. 43% [17/40] in class II, P = 0.02). Significant correlations were found between heart-type fatty acid-binding protein levels and plasma levels of A-type natriuretic peptide (r = 0.45, P = 0.0004), B-type natriuretic peptide (r = 0.66, P <0.0001), and norepinephrine (r = 0.36, P = 0.006). Male sex (hazard ratio [HR] = 5.0; 95% confidence interval [CI]: 1.3 to 19), detectable troponin T levels (HR = 7.0; 95% CI: 1.1 to 44), heart-type fatty acid-binding protein (HR = 2.6 per 3.9-ng/mL increase; 95% CI: 1.1 to 6.5), and left ventricular ejection fraction (HR = 3.6 per 15% decrease; 95% CI: 1.2 to 11) were independently associated with subsequent cardiac events (8 deaths or 10 readmissions because of worsening heart failure).

CONCLUSION

Heart-type fatty acid-binding protein and troponin T are markers of ongoing myocardial damage, and are associated with subsequent cardiac events in patients with chronic heart failure.

Demonstration of ischemia-reperfusion injury separate from postoperative infarction in coronary artery bypass graft patients

BACKGROUND

In patients undergoing coronary artery bypass grafting there are two possible causes of myocardial injury: (1) global ischemic myocardial injury during aortic cross-clamping and subsequent reperfusion, and (2) postoperative myocardial infarction. We studied the use of cardiac marker proteins to specifically and separately detect such injury.

METHODS

Serum levels of enzymes (creatine kinase and creatine kinase-MB) and nonenzymatic proteins (fatty acid-binding protein and myoglobin) were measured in 8 low-risk patients undergoing coronary artery bypass grafting with cardiopulmonary bypass, 8 low-risk patients undergoing coronary artery bypass grafting without cardiopulmonary bypass, and 39 high-risk patients undergoing coronary artery bypass grafting with cardiopulmonary bypass, of whom 7 experienced a postoperative myocardial infarction.

RESULTS

At 0.5 hours after reperfusion significantly increased plasma levels of all markers were noted in patients having the operation with cardiopulmonary bypass, but not in patients having the operation without cardiopulmonary bypass. In patients who had a postoperative myocardial infarction, a second significant increase of each marker was found, but that of fatty acid-binding protein was recorded 4 hours earlier than that of creatine kinase, creatine kinase-MB, or myoglobin.

CONCLUSIONS

Perioperative myocardial injury can be diagnosed from the release of cardiac marker proteins into plasma already at 0.5 hours after the start of reperfusion. For early assessment of postoperative myocardial infarction, fatty acid-binding protein is a more suitable plasma marker than are creatine kinase, creatine kinase-MB, or myoglobin.

Early detection of cardiac damage with heart fatty acid-binding protein after cardiac operations

BACKGROUND

It is still difficult to evaluate myocardial damage in the acute phase of reperfusion in cardiac operations. We investigated the clinical significance of human heart fatty acid-binding protein (HH-FABP) for detecting myocardial damage after cardiac operations earlier than creatine kinase MB isoform or troponin-T.

METHODS

Blood samples from 20 patients who underwent coronary artery bypass grafting were collected serially after reperfusion to measure serum levels of creatine kinase-MB, troponin-T, and HH-FABP.

RESULTS

Serum HH-FABP levels peaked earliest after reperfusion. In addition, the maximum serum HH-FABP level was predictable immediately after reperfusion. The maximum serum HH-FABP level correlated with the maximum serum creatine kinase-MB or troponin-T level, as well as with the aortic cross-clamp time or the maximum dose of catecholamines administered after reperfusion.

CONCLUSIONS

Measurements of HH-FABP allow for earlier evaluation of myocardial damage in the acute phase of reperfusion. Human heart fatty acid-binding protein may be a useful indicator of myocardial damage after cardiac operations.

One-step enzyme-linked immunosorbent assay (ELISA) for plasma fatty acid-binding protein

To allow a more rapid determination of heart-type fatty acid-binding protein (FABP) concentration in plasma a direct non-competitive (sandwich-type) ELISA was developed which uses high-affinity monoclonal antibodies to FABP. Total performance time of the one-step immunoassay is 45 min. The standard curve was linear between 0.2-6 micrograms/L, and the within-run and between-run coefficients of variations were below 6 and 11%, respectively. The serum FABP concentration measured in 79 healthy individuals was 1.6 (0.8) [mean (SD), range 0.3-5.0] micrograms/L. The assay can be used for rapid plasma or serum FABP measurement in the early diagnosis of acute myocardial infarction.

Human heart-type cytoplasmic fatty acid-binding protein as an indicator of acute myocardial infarction

Human heart-type cytoplasmic fatty acid-binding protein (HH-FABPc) has been proposed as an early biochemical indicator of acute myocardial infarction (AMI). However, skeletal muscles also contain HH-FABPc identical to that found in the heart. Before HH-FABPc can be clinically employed as an indicator of AMI, its content in various tissues other than the heart must be known. Accordingly, we measured the HH-FABPc content of various human muscles and organs, using a sandwich enzyme-linked immunosorbent assay (ELISA) for HH-FABPc. HH-FABPc was abundant in the ventricles (0.46 mg/g wet weight and 1.5% of the cytoplasmic protein in the left ventricle), while the atria contained slightly less HH-FABPc (0.25 mg/g wet weight and 0.7% of the cytoplasmic protein in the left atrium). Of the skeletal muscles tested, the diaphragm contained about one-quarter of the HH-FABPc content of the heart, but other skeletal muscles contained very low levels of this protein. Other than the muscles, the kidneys contained less than one-tenth of the HH-FABPc in the heart, and negligible amounts were found in the liver and small intestine. The distribution of HH-FABPc in the heart and skeletal muscles was comparable to that of cardiac-specific creatine kinase (CK-MB) activity, and was inverse to the distribution of myoglobin. The plasma myoglobin/HH-FABPc ratio, determined in patients with AMI and those without AMI, closely reflected that in the heart and skeletal muscles. These findings indicate that HH-FABPc may be useful as a specific indicator of AMI, and the plasma myoglobin/HH-FABPc ratio could provide valuable information for the diagnosis of AMI.

Development of a sandwich enzyme-linked immunosorbent assay for the determination of human heart type fatty acid-binding protein in plasma and urine by using two different monoclonal antibodies specific for human heart fatty acid-binding protein

We have developed a sandwich enzyme-linked immunosorbent assay (ELISA) for the determination of human heart type fatty acid-binding protein (H-FABP) in human plasma and urine using the combination of two distinct monoclonal antibodies (MAbs) directed against human H-FABP purified from human heart muscle. The total assay time of the ELISA is practically much shorter than that of the competitive enzyme immunoassay (EIA) we previously reported. The immunoreactive mass of human H-FABP was specifically measured using a horseradish peroxidase (HRPO)-labeled anti-human H-FABP MAb as an enzyme-linked MAb, and anti-human H-FABP MAb immobilized on the polystyrene microtiter plate as a solid-phase MAb, and purified human H-FABP as standard materials. The assay range of the ELISA was 0-250 ng/ml of plasma and urine. The ELISA yielded a coefficient of variation of less than 10% in inter- and intra-assays, and the good linearity was obtained in dilution test using clinical samples. Anticoagulants, except sodium fluoride and a high concentration of hemoglobin and bilirubin, did not interfere with the assay of plasma samples. A high concentration of hemoglobin, bilirubin and immunoglobulin, and contamination with seminal plasma did not interfere with the assay of urine samples. The average recovery of purified human H-FABP added to human plasma and urine samples was 98.5% and 97.0%, respectively. Myoglobin and myosin did not crossreact in the ELISA. The minimum detection limit of the ELISA was 1.25 ng/ml. The immunoreactive masses of human H-FABP in plasma and urine samples, obtained from one hundred normal healthy subjects were quantified by the sandwich ELISA. The normal mean (+/- SD) level of human H-FABP mass in plasma was 3.65 +/- 1.81 ng/ml, and that in urine was 3.20 +/- 2.70 ng/ml. In conclusion, this sandwich ELISA is a useful tool for the sensitive and precise determination of human H-FABP in human plasma and urine, and it may be used specifically for clinical investigation and diagnosis of myocardial injury.

Fatty-acid-binding protein as a plasma marker for the estimation of myocardial infarct size in humans

BACKGROUND

There are substantial amounts of cytoplasmic heart-type fatty-acid-binding protein (FABP) (15 kDa) in myocardial tissue. The rapid release of FABP into plasma during ischaemia indicates the possibility of using this protein as a biochemical marker for ischaemic myocardial injury.

OBJECTIVE

To study the completeness of the release of FABP from damaged tissue in patients with acute myocardial infarction (AMI) and the suitability of serial plasma FABP concentrations for estimation of myocardial infarct size.

METHODS

Immunochemically assayed FABP and enzymatically assayed creatine kinase isoenzyme MB (CK-MB) and alpha-hydroxybutyrate dehydrogenase (HBDH) were determined serially in plasma samples from 49 patients with AMI who had been treated with thrombolytic agents within six hours after the onset of AMI. Previously validated circulatory models and a value of 2.6 h-1 for the fractional clearance rate of FABP from plasma were used to calculate cumulative protein release into plasma.

RESULTS

Release of FABP was completed earlier (24-36 h) after AMI than that of CK-MB (50-70 h) and that of HBDH (> 70 h). However, infarct size estimated from the cumulative release of the proteins and expressed as gram equivalents of healthy myocardium per litre of plasma yielded a comparable value of 4-6 for both FABP and the two enzymes.

CONCLUSION

The data indicate that FABP released from the heart after AMI is quantitatively recovered in plasma and that FABP is a useful biochemical plasma marker for the estimation of myocardial infarct size in humans.

Human heart-type cytoplasmic fatty acid-binding protein in serum and urine during hyperacute myocardial infarction

We have previously reported that serum and/or urinary human heart-type cytoplasmic fatty acid-binding protein (HH-FABPc) can be used as an early indicator of myocardial injury (Clin Biochem 1991; 24: 195-201). To confirm the usefulness of HH-FABPc as an early diagnostic indicator of acute myocardial infarction (AMI), its serum and urinary levels were measured in samples obtained within 6 h after the onset of acute coronary syndrome related symptoms. Samples were collected from 97 patients, who were composed of 63 with AMI, 24 with unstable angina and 10 with chest pain syndrome. The positivity of serum and urinary HH-FABPc and cardiac creatine kinase isozyme MB (CK-MB) was analyzed in these samples. Serum HH-FABPc levels in AMI were above normal in 91.4% (64/70) of the samples tested within 3 h of the onset of symptoms and in 100% (111/111) of those tested at 3-6 h. Elevated urinary HH-FABPc levels in AMI were obtained in 88.9% (8/9) of samples at 0-3 h and in 75% (6/8) at 3-6 h. CK-MB activity in AMI was positive in 20% (8/40) and 66.3% (53/80) of serum samples at 0-3 h and 3-6 h, respectively. HH-FABPc was always positive when a serum sample was positive for CK-MB. Serum HH-FABPc at 0-6 h in chest pain syndrome and in unstable angina were positive in 17.8% (5/28) and 56.7% (34/60), respectively. The elevated HH-FABPc in serum and urine was noted much earlier than that of CK-MB during the hyperacute phase of AMI. HH-FABPc showed high positive value in unstable angina, but it was low in normal coronary patients having chest pain. However, HH-FABPc level in unstable angina and chest pain syndrome was lower than that of AMI. Thus, HH-FABPc may be a valuable indicator for the diagnosis of hyperacute myocardial infarction.

Release of fatty acid-binding protein and long chain fatty acids from isolated rat heart after ischemia and subsequent calcium paradox

To obtain insight into the relation between the release of heart-type fatty acid-binding protein (H-FABPc) and of long-chain fatty acids (FA) from injured cardiac tissue, rat hearts were Langendorff perfused according to the following scheme: 30 min normoxia, 60 min ischemia, 30 min reperfusion, 10 min Ca2+ free perfusion and finally 10 min Ca2+ repletion. During this protocol right ventricular (Qrv) and interstitial effluent samples (Qi) were collected at regular intervals. During reperfusion a total of 0.8 +/- 0.1 nmol H-FABPc but no FA were detected in the effluents. However, during Ca2+ readmission, 45 +/- 4 nmol H-FABPc (80-90% of total tissue content) was released with an initial (first 3 min) simultaneous release of FA (FA/H-FABPc ratio 0.90 +/- 0.07 mol/mol). Thereafter, FA release continued at 10-15 nmol per min mainly in Qrv while the rate of H-FABPc release decreased. During Ca2+ repletion, tissue FA content raised rapidly from 168 +/- 20 to 1918 +/- 107 nmol/g dry weight. These findings suggest that after severe cardiac damage initially FA is released bound to H-FABPc, whereas further FA release occurs in a non-protein bound manner.

Fatty acid-binding proteinuria diagnoses myocardial infarction in the rat

Cytoplasmic heart-type fatty acid-binding protein (H-FABPc) is a low molecular weight protein with abundant presence in the myocardium. Upon ischemia it is released from the heart and can subsequently be detected in plasma and urine. In this study, the value of measurement of H-FABPc excreted into urine for the diagnosis of myocardial infarction (MI) is investigated in the rat. To this end, firstly the kinetic behaviour of H-FABPc in plasma was examined and its release into urine quantified. After injection of purified H-FABPc in normal animals, a net recovery in urine of 14-29% was found. The kinetic behaviour of H-FABPc in plasma was characterized by a total clearance of 0.33 ml/min and a half-life value of total elimination of about 270 min. Knowing these plasma characteristics of H-FABPc, a comparison was made between the cumulative amounts of H-FABPc released in the 24-hour urine of MI rats and of sham-operated animals. In MI rats, with a mean morphometric MI size of 43%, the mean total amount of H-FABPc excreted into urine was 79 micrograms, while in sham-operated rats this was 23 micrograms. This difference between both groups is significant (p < 0.001). It is concluded that urinary H-FABPc can be used as a noninvasive marker for MI in the rat.

Significance of cytoplasmic fatty acid-binding protein for the ischemic heart

Ischemia of the heart is accompanied by the tissue accumulation of long-chain fatty acids and their metabolic derivatives such as beta-hydroxy fatty acids and fatty acyl-CoA and acyl-L-carnitine esters. These substances might be detrimental for proper myocardial function. Previously, it has been suggested that intracellular lipid binding proteins like cytoplasmic fatty acid-binding protein (FABP) and acyl-CoA binding protein (ACBP) may bind these accumulating fatty acyl moieties to prevent their elevated levels from potentially harmful actions. In addition, the suggestion has been made that the abundantly present FABP may scavenge free radicals which are generated during reperfusion of the ischemic heart. However, these protective actions are challenged by the continuous physico-chemical partition of fatty acyl moieties between FABP and membrane structures and by the rapid release of FABP from ischemic and reperfused cardiac muscle. Careful evaluation of the available literature data reveals that at present no definite conclusion can be drawn about the potential protective effect of FABP on the ischemic and reperfused heart.

Immunohistochemical distribution of heart-type fatty acid-binding protein immunoreactivity in normal human tissues and in acute myocardial infarct

The cellular distribution of heart-type fatty acid-binding protein (H-FABP) immunoreactivity was examined in normal human tissues using a polyclonal antibody against human H-FABP. Immunoreactivity was detected in cardiomyocytes of both ventricles and atria as well as in all striated muscles investigated. In addition, staining was frequently observed in parietal cells of the stomach, renal epithelial cells, acinar and ductal cells of the breast, ductal cells of the salivary gland, corpus luteum, and Leydig cells of the testis. Adipocytes and vascular endothelial cells were positive but other tissues and cells examined were negative. Old infarcts of the heart replaced by fibrous connective tissues were not labelled. Necrotic cardiomyocytes and morphologically normal cardiomyocytes in acute ischaemic lesions 1 h after onset showed reduced or no H-FABP immunoreactivity. Thus, decreased immunoreactivity for H-FABP may be a good histological marker of damaged cardiomyocytes.

Release of heart fatty acid-binding protein into plasma after acute myocardial infarction in man

The release of cytoplasmic heart fatty acid-binding protein (H-FABP) into the plasma of cardiac patients up to 38 hr after the onset of the first clinical symptoms of acute myocardial infarction (AMI) was studied, using a sensitive direct and noncompetitive Enzyme Linked Immunosorbent Assay of the antigen capture type (sandwich ELISA), newly developed for the measurement of small amounts of human H-FABP in plasma samples. Plasma levels of H-FABP were compared with plasma activity levels of the myocardial cytoplasmic enzymes creatine kinase MB (CK-MB) and alpha-hydroxybutyrate dehydrogenase (alpha-HBDH). Upper normal levels of H-FABP (19 micrograms/l), CK-MB (10 U/l) and alpha-HBDH (160 U/l) as determined in plasma from 72 blood donors served as threshold levels. H-FABP levels were significantly elevated above their threshold level within 3 hr after AMI. Peak levels of H-FABP, CK-MB and alpha-HBDH were reached 4.1 +/- 0.9 hr, 8.4 +/- 1.4 hr and 25.0 +/- 9.5 hr (means +/- S.D., n = 10) after acute myocardial infarction, respectively. Serial time curves of the plasma contents of H-FABP reveal that after myocardial infarction H-FABP is released in substantial amounts from human hearts. In 18 out of 22 patients with established AMI the plasma FABP level was at or above the threshold level in blood-samples taken within 3.5 hr after the first onset of symptoms of AMI, while for CK-MB this applied to 9 patients and for alpha-HBDH to 6 patients. These findings suggest that for an early indication of acute myocardial infarction in man cytoplasmic heart fatty acid-binding protein is more suitable than heart type creatine kinase MB and/or alpha-hydroxybutyrate dehydrogenase.

A sandwich enzyme linked immuno-sorbent assay for the determination of rat heart fatty acid-binding protein using the streptavidin-biotin system. Application to tissue and effluent samples from normoxic rat heart perfusion

An enzyme linked immuno-sorbent assay (ELISA) of the sandwich type for the determination of heart-type fatty acid-binding protein (H-FABPc) was developed, making use of the streptavidin-biotin system. The assay turned out to be virtually disturbance insensitive and showed a detection limit for H-FABPc of 0.2 micrograms/l with an intra- and inter-assay variation of 5% and 14%, respectively. The H-FABPc content of adult rat heart muscle was found to be 0.740 +/- 0.120 mg/g wet weight. The H-FABPc content of a number of skeletal muscles varied from 0.013 to 0.303 mg/g wet weight and was related to the content of type I muscle fibers of these tissues, suggesting a role for H-FABPc in intracellular fatty acid metabolism. The assay was further applied to study the release of H-FABPc from isolated rat heart during normoxic Langendorff perfusion, as compared to that of lactate dehydrogenase (LDH), into fluid derived from the right ventricular cavity (Qrv) and that from the interstitial space (Qi). Total release of H-FABPc per 15 min amounted to 0.015 +/- 0.010% but that of LDH to 0.080 +/- 0.040% of their total tissue content. Furthermore, for both H-FABPc and LDH 80% was released into Qi, which only accounted for 1-2% of total flow. These findings suggest that during normoxic perfusion of rat heart H-FABPc, and LDH are released from different cellular compartments and that the bulk amount of released intracellular proteins is transported via the lymph instead of being directly released into the bloodstream.

Serum and urinary human heart fatty acid-binding protein in acute myocardial infarction

A competitive enzyme immunoassay (C-EIA) was developed for the measurement of serum and urinary levels of human heart fatty acid-binding protein (hh-FABP), and the appearance and time-course changes of hh-FABP levels were evaluated in patients with acute myocardial infarction (AMI). Control serum and urinary hh-FABP levels, which were determined in 86 serum and 42 urine samples from 86 patients without AMI, were found to range between 0 and 2.8 ng/mL. Serial determinations performed on 11 patients with AMI demonstrated that hh-FABP levels were significantly elevated in the first serum and urine samples obtained within 14 h of the onset of clinical symptoms. Two serum and 2 urine samples obtained only 1.5 h after the onset of symptoms already showed elevated hh-FABP levels, while in the same serum samples the activity of the myocardial-specific isoenzyme of creatine kinase (CK-MB) was still normal. Maximal serum and urinary hh-FABP levels appeared between 5 and 10 h after symptoms developed, and fell sharply towards normal thereafter. The hh-FABP levels in serum and urine both peaked earlier than the elevation of CK-MB activity in serum. The presence of hh-FABP in serum and/or urine seems to be a marker for myocardial damage and could be used as a useful tool for the early diagnosis of AMI.

Release of fatty acid-binding protein from ischemic-reperfused rat heart and its prevention by mepacrine

In an attempt to resolve the issue of whether there is a loss of fatty acid binding protein (H-FABP) from heart during ischemia and reperfusion, and to further examine the role of this protein in ischemic-reperfusion injury, the amount of H-FABP of heart was monitored during ischemia and reperfusion. Excellent correlation was obtained between the loss of H-FABP from heart and its appearance in the perfusate buffer when examined by Western blot using the specific antibody to H-FABP. Further quantitation was achieved by densitometric scanning of the Western blot and rocket electrophoresis. Maximum release of H-FABP was observed within 20 min of reperfusion, the total release being 10% of the H-FABP content of the heart. Mepacrine, a membrane stabilizer and a phospholipase inhibitor, reduced the release of H-FABP from the heart and prevented the accumulation of nonesterified fatty acids in the tissue during ischemia and reperfusion. In view of the established role of H-FABP in the preservation of membrane phospholipids by either scavenging free radicals during ischemia and reperfusion or by modulating the enzymes of phospholipid synthesis, it seems likely that the loss of H-FABP may have some contribution towards the ischemic-reperfusion injury.

Cellular fatty acid-binding proteins: current concepts and future directions

At least three different proteins are implicated in the cellular transport of fatty acid moieties: a plasmalemmal membrane and a cytoplasmic fatty acid-binding protein (FABPPM and FABPC, respectively) and cytoplasmic acyl-CoA binding protein (ACBP). Their putative main physiological significance is the assurance that long-chain fatty acids and derivatives, either in transit through membranes or present in intracellular compartments, are largely complexed to proteins. FABPC distinguishes from the other proteins in that distinct types of FABPC are found in remarkable abundance in the cytoplasmic compartment of a variety of tissues. Although their mechanism of action is not yet fully elucidated, current knowledge suggests that the function of this set of proteins reaches beyond simply aiding cytoplasmic solubilization of hydrophobic ligands, but that they can be assigned several regulatory roles in cellular lipid homeostasis.

Functions of fatty acid binding proteins

Cytosolic fatty acid binding proteins (FABP) belong to a gene family of which eight members have been conclusively identified. These 14-15 kDa proteins are abundantly expressed in a highly tissue-specific manner. Although the functions of the cytosolic FABP are not clearly established, they appear to enhance the transfer of long-chain fatty acids between artificial and native lipid membranes, and also to have a stimulatory effect on a number of enzymes of fatty acid metabolism in vitro. These findings, as well as the tissue expression, ligand binding properties, ontogeny and regulation of these proteins provide a considerable body of indirect evidence supporting a broad role for the FABP in the intracellular transport and metabolism of long-chain fatty acids. The available data also support the existence of structure- and tissue-specific specialization of function among different members of the FABP gene family. Moreover, FABP may also have a possible role in the modulation of cell growth and proliferation, possibly by virtue of their affinity for ligands such as prostaglandins, leukotrienes and fatty acids, which are known to influence cell growth activity. FABP structurally unrelated to the cytosolic gene family have also been identified in the plasma membranes of several tissues (FABPpm). These proteins have not been fully characterized to date, but strong evidence suggest that they function in the transport of long-chain fatty acids across the plasma membrane.

The fatty acid-binding protein from human skeletal muscle

Fatty acid-binding protein (FABP) was isolated from human skeletal muscle by gel filtration and anion- and cation-exchange chromatography. The isolation procedure, however, with rat and pig skeletal muscle gave mostly inactive preparations. Rat muscle FABP preparations contained parvalbumin as a contaminant. FABP from human muscle had a Mr of about 15 kDa, a pI value of 5.2, and a Kd value with oleic acid of 0.50 microM. Skeletal muscle and heart FABPs and their antisera showed a strong cross-reactivity on Western blots and in enzyme-linked immunosorbent assays (ELISA). No cross-reactivity was observed with liver FABP and its antiserum. On the basis of amino acid composition, electrophoretic behavior, fatty acid binding, and immunochemical properties, human skeletal muscle FABP must be similar or closely related to human heart FABP. The FABP content determined by ELISA was comparable in various human muscles and cultured muscle cells, but lower than that in rat muscles.

Free radical scavenging by myocardial fatty acid binding protein

Recent investigations have indicated the presence of a fatty acid binding protein (FABP) in mammalian heart. This protein binds free fatty acids and their esters with high affinity, however, its physiological role remains unknown. Since FABP constitutes a significant amount of cystolic protein, it is likely that it would be a target for free radical attack. To test this hypothesis, FABP was examined for scavenging against free radicals such as the superoxide anion (O2-), hydroxyl radical (OH.) and hypochlorite radical (OCl.) which may be present in an ischemic reperfused heart. Our results suggest that FABP scavenges O2-, OH. and OCl. as indicated by the FABP inhibition of O2- -dependent reduction of cytochrome c, OH.-dependent hydroxybenzoic acid formation and OCl.-mediated chemiluminescence response. FABP was found to be a more potent scavenger of these free radicals compared to bovine serum albumin. Furthermore, FABP was more effective in scavenging OH. than O2-, and inhibited OH. mediated lipid peroxidation process. These results indicate that FABP can scavenge free radicals which may be present in an ischemic/reperfused heart and, thus, may play a significant physiological role in the heart during ischemia and reperfusion.