Studies of the in vitro human plasma degradation of methionine-enkephalin

Plasma proenkephalin A and incident chronic kidney disease and albuminuria in the REasons for Geographic And Racial Differences in Stroke (REGARDS) cohort

BACKGROUND

Plasma proenkephalin A (PENK-A) is a precursor of active enkephalins. Higher blood concentrations have been associated with estimated glomerular filtration rate (eGFR) decline in European populations. Due to the significant disparity in incident chronic kidney disease (CKD) between White and Black people, we evaluated the association of PENK-A with incident CKD and other kidney outcomes among a biracial cohort in the U.S.

METHODS

In a nested cohort of 4,400 participants among the REasons for Geographic And Racial Differences in Stroke, we determined the association between baseline PENK-A concentration and incident CKD using the creatinine-cystatin C CKD-EPI 2021 equation without race coefficient, significant eGFR decline, and incident albuminuria between baseline and a follow-up visit 9.4 years later. We tested for race and sex interactions. We used inverse probability sampling weights to account for the sampling design.

RESULTS

At baseline, mean (SD) age was 64 (8) years, 49% were women, and 52% were Black participants. 8.5% developed CKD, 21% experienced ≥ 30% decline in eGFR and 18% developed albuminuria. There was no association between PENK-A and incident CKD and no difference by race or sex. However, higher PENK-A was associated with increased odds of progressive eGFR decline (OR: 1.12; 95% CI 1.00, 1.25). Higher PENK-A concentration was strongly associated with incident albuminuria among patients without diabetes mellitus (OR: 1.29; 95% CI 1.09, 1.53).

CONCLUSION

While PENK-A was not associated with incident CKD, its associations with progression of CKD and incident albuminuria, among patients without diabetes, suggest that it might be a useful tool in the evaluation of kidney disease among White and Black patients.

Proenkephalin A Adds No Incremental Prognostic Value After Acute Ischemic Stroke

Objective:

The aim of this study was to confirm previous observations that proenkephalin A (PENK-A) may serve as prognostic marker in the setting of acute ischemic stroke in a large stroke cohort.

Methods:

The plasma concentration of PENK-A was measured within 72 hours of symptom onset in 320 consecutively enrolled patients with stroke. The primary outcome measures were unfavorable functional outcome (modified Rankin Scale score 0-2 vs 3-6) and mortality within 90 days. Logistic and cox proportional regression analyses were fitted to estimate odds ratios (ORs), hazard ratios (HRs) and 95% confidence intervals (CIs), respectively, for the association between PENK-A and the primary outcome measures.

Results:

After adjusting for demographic and vascular risk factors, PENK-A was neither independently associated with functional outcome (OR: 1.29, 95% CI: 0.16-10.35) nor mortality (HR: 1.02, 95% CI: 0.14-7.33).

Conclusion:

Among patients with acute stroke, PENK-A does not serve as an independent prognostic marker in this external validation cohort.

Analytical performance of an immunoassay to measure proenkephalin

BACKGROUND

Endogenous opioids, enkephalins, are known to increase with acute kidney injury. Since the mature pentapeptides are unstable, we evaluated the performance of an assay that measures proenkephalin 119-159 (PENK), a stable peptide formed concomitantly with mature enkephalins.

METHODS

PENK assay performance was evaluated on two microtiterplate/chemiluminescence sandwich immunoassay formats that required 18 or 1 h incubation times. PENK concentration was measured in plasma from healthy individuals to establish a reference interval and in patients with varied levels of kidney function and comorbidities to assess the association with measured glomerular filtration rate (mGFR) using iothalamate clearance.

RESULTS

Assay performance characteristics in plasma were similar between the assay formats. Limit of quantitation was 26.0 pmol/L (CV = 20%) for the 1 h assay and 17.3 pmol/L (CV = 3%) for the 18 h assay. Measurable ranges were 26-1540 pmol/L (1 h assay) and 18-2300 pmol/L (18 h assay). PENK concentrations are stable in plasma stored ambient to 10 days, refrigerated to at least 15 days, and frozen to at least 90 days. Results were comparable in paired SST serum and EDTA plasma. Age and sex were not associated with PENK concentrations in healthy individuals (reference interval: 36-97.5 pmol/L). Plasma PENK concentration correlated with mGFR. In a multivariate model PENK concentration, age, sex and transplant status were significant predictors of mGFR, and 49% of predicted GFR values fell within 30% of the mGFR.

CONCLUSIONS

Both assay formats are accurate and precise for measuring clinically relevant PENK concentrations. The association of PENK concentration with mGFR is influenced by gender, age, and history of kidney transplantation. Future studies will determine if blood PENK can be used clinically to estimate GFR and/or detect AKI.

Proenkephalin in Heart Failure

The opioid system is activated in heart failure, which may be cardioprotective but may also be counter-regulatory. Recently, systemic proenkephalin activation has been investigated in various conditions predicting mortality and kidney injury. In acute heart failure, proenkephalin independently predicts mortality and heart failure rehospitalization in addition to traditional risk markers. It also predicts worsening renal function, increasingly recognized as an important risk predictor for poor outcome in heart failure. This article explores the role of enkephalins and delta-opioid receptors in the heart, then reviews studies measuring proenkephalin levels in the circulation and their associations with prognosis.

High Level of Fasting Plasma Proenkephalin-A Predicts Deterioration of Kidney Function and Incidence of CKD

High levels of proenkephalin-A (pro-ENK) have been associated with decreased eGFR in an acute setting. Here, we examined whether pro-ENK levels predict CKD and decline of renal function in a prospective cohort of 2568 participants without CKD (eGFR>60 ml/min per 1.73 m²) at baseline. During a mean follow-up of 16.6 years, 31.7% of participants developed CKD. Participants with baseline pro-ENK levels in the highest tertile had significantly greater yearly mean decline of eGFR (P_trend<0.001) and rise of cystatin C (P_trend=0.01) and creatinine (P_trend<0.001) levels. Furthermore, compared with participants in the lowest tertile, participants in the highest tertile of baseline pro-ENK concentration had increased CKD incidence (odds ratio, 1.51; 95% confidence interval, 1.18 to 1.94) when adjusted for multiple factors. Adding pro-ENK to a model of conventional risk factors in net reclassification improvement analysis resulted in reclassification of 14.14% of participants. Genome-wide association analysis in 4150 participants of the same cohort revealed the strongest association of pro-ENK levels with rs1012178 near the PENK gene, where the minor T-allele associated with a 0.057 pmol/L higher pro-ENK level per allele (P=4.67x10^-21). Furthermore, the T-allele associated with a 19% increased risk of CKD per allele (P=0.03) and a significant decrease in the instrumental variable estimator for eGFR (P<0.01) in a Mendelian randomization analysis. In conclusion, circulating plasma pro-ENK level predicts incident CKD and may aid in identifying subjects in need of primary preventive regimens. Additionally, the Mendelian randomization analysis suggests a causal relationship between pro-ENK level and deterioration of kidney function over time.

Increased myocardial methionine-enkephalin with reduced arterial oxygenation in congenital heart disease

BACKGROUND

  Cardiac opioid peptides have been identified to exert important adaptive metabolic signalling for cardioprotection against ischaemia or hypoxia-related injury.

AIMS

  To determine myocardial methionine-enkephalin content in children with hypoxemic congenital heart defects and to correlate myocardial content of methionine-enkephalin with the extent of arterial oxygen desaturation.

METHODS

  Children (n= 20, median age of 16 months), undergoing cardiac surgical repair (tetralogy of Fallot, 17/20), were included in this study. Arterial oxygen saturation was measured on admission. Myocardial samples obtained during surgery were assayed via radioimmunochemistry for methionine-enkephalin content.

RESULTS

  Greater methionine-enkephalin content was measured in the right ventricles of the patients suffering from recent cyanotic spells compared with those with no recent spells (cyanotic spells: 2418 ± 844 pg/g wet weight tissue, n= 6; no spells: 1175 ± 189 pg/g wet weight tissue, n= 14, P= 0.04). An inverse correlation was evident between the arterial oxygen saturation and myocardial methionine-enkephalin content.

CONCLUSION

  Myocardial methionine-enkephalin levels increase with the severity of hypoxic stress in congenital cardiac disease and may play an important adaptive role in countering adrenergic over-activity and related excess demand on myocardial metabolic capacity.

Neuropeptide proenkephalin A is associated with in-hospital mortality in patients with acute intracerebral hemorrhage

High plasma proenkephalin A (PENK-A) levels are associated with poor clinical outcome after ischemic stroke. However, not much is known regarding the change of its level in acute intracerebral hemorrhage. Thus, we sought to determine PENK-A in plasma of patients with acute spontaneous basal ganglia hemorrhage and evaluate its relation with disease severity and in-hospital mortality. One hundred and two patients and 100 healthy controls were recruited. Plasma samples were obtained on admission for patients and at study entry for controls. Its concentration was measured by chemoluminescence sandwich immunoassay. Plasma PENK-A levels were substantially higher in patients than in healthy controls (235.5±85.4 pmol/L vs. 90.1±31.3 pmol/L; P<0.0001). A forward stepwise logistic regression selected plasma PENK-A as an independent predictor for in-hospital mortality of patients (odds ratio 1.080, 95% confidence interval 1.018-1.147, P<0.001). A multivariate linear regression demonstrated that plasma PENK-A level was positively associated with National Institutes of Health Stroke Scale (NIHSS) score (t=6.189, P<0.001) and hematoma volume (t=5.388, P<0.001). A receiver operating characteristic curve identified a plasma PENK-A level>267.1 pmol/L predicted in-hospital mortality of patients with 80.0% sensitivity and 74.7% specificity (area under curve, 0.836; 95% confidence interval, 0.750-0.902). Its predictive value was similar to NIHSS score's and hematoma volume's (both P>0.05). However, it did not statistically significantly improve the predictive values of NIHSS score and hematoma volume (both P>0.05). Thus, increased plasma PENK-A levels are associated with disease severity and in-hospital mortality after acute intracerebral hemorrhage.

Targeting the opioid growth factor: Opioid growth factor receptor axis for treatment of human ovarian cancer

The opioid growth factor (OGF) – opioid growth factor receptor (OGFr) axis is a biological pathway that is present in human ovarian cancer cells and tissues. OGF, chemically termed [Met5]-enkephalin, is an endogenous opioid peptide that interfaces with OGFr to delay cells moving through the cell cycle by upregulation of cyclin-dependent inhibitory kinase pathways. OGF inhibitory activity is dose dependent, receptor mediated, reversible, protein and RNA dependent, but not related to apoptosis or necrosis. The OGF-OGFr axis can be targeted for treatment of human ovarian cancer by (i) administration of exogenous OGF, (ii) genetic manipulation to over-express OGFr and (iii) use of low dosages of naltrexone, an opioid antagonist, which stimulates production of OGF and OGFr for subsequent interaction following blockade of the receptor. The OGF-OGFr axis may be a feasible target for treatment of cancer of the ovary (i) in a prophylactic fashion, (ii) following cytoreduction or (iii) in conjunction with standard chemotherapy for additive effectiveness. In summary, preclinical data support the transition of these novel therapies for treatment of human ovarian cancer from the bench to bedside to provide additional targets for treatment of this devastating disease.

Biological correlates of migraine and cluster headaches: an overview of their potential use in diagnosis and treatment

Current diagnostic criteria for headaches are based on the International Classification for Headache Disorders, second edition, which is largely built on data obtained from clinical examinations and patients' medical histories. Despite decades of vigorous basic and clinical research, we still lack reliable clinical laboratory diagnostic markers for headaches, which clearly obstructs the physician's ability to optimize and follow the patient's response to treatment protocols as well as holds back the discovery and implementation of new therapeutic modalities. In this paper, we review and discuss current efforts to identify and characterize biochemical and immunological changes in biological fluids and tissue that may be specifically associated with the etiology and/or pathophysiology of migraine and cluster headaches; we also discuss some of the recent genetic findings and ion channel modulation studies that may help to distinguish among various headache populations.

Degradation kinetics of methionine5-enkephalin by cerebrospinal fluid: in vitro studies

Changes in the levels or biochemistry of cerebrospinal fluid (CSF) neuropeptides with opioid-like properties have been suggested to reflect alterations in specific biological processes. We have determined various kinetic parameters for methionine-enkephalin (MET) degradation by CSF samples from nonneurological patients. Study subjects included 9 males (51-67 years of age) and 5 females (47-61 years of age). Aliquots, removed from an incubation vessel containing buffer, CSF, and peptide [tyr-3',5'-H(N)MET], were analyzed for tyrosine and other degradation products. Essentially all of the labeled tyrosine from the added MET was recovered as free amino acid after 60 minutes of incubation (1:2 ratio, vol:vol; optimum pH 7.4; and temperature 37°C); other possible peptide metabolites (>3%) were not detected. Irrespective of age or gender, the peptide's degradation half-life and initial velocity values were in a limited range; t1/2 26.2 ± 5.5 and 20.8-33.8 minutes, and Iv 0.03 ±0.01 and 0.02-0.03 pg of peptide per milligram protein per minute. Km and Vmax values were 0.19 ± 0.02 and 0.17-0.21 mM, and 9.8 ± 2.2 and 7.6-12.0 μmol·L·min, respectively. Neither CSF sample storage time (up to a year) nor repeated freezing and thawing (up to 3 times over a year) altered the kinetics or products of this reaction. These preliminary findings might serve as reference values when conducting similar studies using CSF from patients diagnosed with specific neurological conditions; significant alterations in MET degradation profile in such a population could provide valuable biological markers for diagnostic and treatment purposes.

In vitro Methionine5-Enkephalin Degradation Kinetics by Human Brain Preparations

Incubation of [3H]-tyrosine methionine5-enkephalin (MET) with human brain preparations (100,000g supernatant; sections of the limbic system, thalamus, basal ganglia, cerebellum, and cortex) results in its rapid and complete degradation; over 95% of the initial labeled tyrosine is recovered as the free aminoacid within 10 min. Results show a considerable range in the peptide initial velocity (Iv) and half-life (t1/2) degradation values obtained from different brain sections of individual brains, either from the same or from different main brain areas. This relatively wide range of values was scattered, failing to identify consistent differences between the various brains areas studied. Differences in brain tissue storage time or repeated sample freezing and thawing failed to alter significantly either of these kinetic parameters of MET metabolism. Peptide degradation rate (optimum pH and temperature of 7.4 and 37 degrees C, respectively) was concentration-dependent inhibited by known aminopeptidase inhibitors (puromycin, bacitracin, and bestatin, and to a lesser extent by thioridazine). However, it was not significantly affected by either N-carboxymethyl phenyl leucine, captopril or thiorphan [dipeptidyl peptidase(s) or peptidyl dipeptidase(s) inhibitors, respectively]. A better understanding of the mechanisms regulating brain MET metabolism may contribute to the rational design of pharmacological strategies based in the modulation of its bioavailability.

Proenkephalin A 119-159, a stable proenkephalin A precursor fragment identified in human circulation

In this report, we describe a newly developed sandwich immunoassay using antibodies against the proenkephalin A 119-159 peptide (PENK A 119-159). PENK A 119-159 immunoreactivity was detectable in the circulation of human blood donors and in cerebrospinal fluid (CSF) of patients without a neurologic disorder. The concentration was about 100 times higher in CSF than in serum. Analytical reversed phase HPLC revealed that PENK A 119-159 is the main immunoreactivity in human circulation and CSF. Moreover, PENK A 119-159 is stable in vitro for at least 48 h at room temperature as compared to the low stability of the peptides methionine- and leucine-enkephalin. This suggests the use of PENK A 119-159 measurement as surrogate molecule for the release of the mature peptides derived from proenkephalin A.

Inhibition of human plasma leucine5-enkephalin aminopeptidase hydrolysis by various endogenous peptides and a select number of clinically used drugs

We identified a number of clinically used drugs and biologically active endogenous peptides able to significantly decrease the rate of human plasmatic aminopeptidase (AP) leucine-enkephalin (LEU) degradation. Bacitracin, bestatin, fluvoxamine, and each of 4 peptides tested significantly increased, in a dose-dependent manner (10-10 M), LEU degradation half-life (t1/2) in each of 5 plasma samples studied. Each sample was obtained by pooling equal volume of 6 randomly selected, individual plasmas (4 male and 2 female healthy, drug-free volunteers). Thirty subjects (20 females and 10 males) participated in this study. With the exception of fluvoxamine, this inhibitory effect was lacking in various other commonly used drugs with widely different chemical structures and pharmacological profiles, eg, antidepressants (SSRIs, imipramine-like tricyclics, MAOIs), acute antimigraine agents (triptan class drugs), the nonselective beta-adrenergic antagonist propranolol, and serotonin receptor agonists and antagonists. Agents (concentration 10 M used as illustration), listed in decreasing order of LEU-AP inhibitory activity: substance P > angiotensin III > methionine-enkephalin > angiotensin II > fluvoxamine > bestatin gave t1/2 values (+/- SD) of 39.3 +/- 1.1, 29.4 +/- 0.8, 28.3 +/- 0.8, 27.4 +/- 0.7, 24.5 +/- 1.5, and 23.6 +/- 0.9 minutes, respectively. Control, bacitracin, and fluphenazine (known LEU-AP inhibitors were used for comparison) values of 11.8 +/- 1.0, 31.3 +/- 0.7, and 19.6 +/- 1.0 minutes, respectively. As expected, these drugs significantly decreased the initial velocity of peptide degradation; Iv values (+/- SD) of: 0.17 +/- 0.1 (0.02 +/- 0.01), 0.23 +/- 0.2 (0.02 +/- 0.01), 0.25 +/- 0.2 (0.02 +/- 0.01), 0.26 +/- 0.2 (0.03 +/- 0.01), 0.31 +/- 0.1 (0.03 +/- 0.01), and 0.33 +/- 0.1 (0.03 +/- 0.01), respectively; control, bacitracin, and fluphenazine: 1.10 +/- 0.3 (0.12 +/- 0.03), 0.20 +/- 0.1 (0.02 +/- 0.01), and 0.82 +/- 0.2 (0.08 +/- 0.02) pg LEU/min (pg LEU/mg protein/min), respectively. Results emphasize the selective nature of chemical structures required to significantly inhibit AP activity and provided information that could help the rational design of agents with high specificity in a biologic milieu containing multiple peptidases. In this case, targeted modulation of the bioavailability of LEU and other endogenous AP-degraded hormonal and nonhormonal peptides could be useful in the treatment of the pathophysiology associated with various disease conditions. Whether their development could find useful pharmacological applications remains to be explored.

In vitro human plasma leucine(5)-enkephalin degradation is inhibited by a select number of drugs with the phenothiazine molecule in their chemical structure

A number of drugs with the phenothiazine molecule in their chemical structure inhibit in a dose-dependent manner human plasmatic aminopeptidase leucine(5)-enkephalin (LEU) metabolism. Half-life peptide degradation was significantly increased by thioridazine > fluphenazine > As-1397 [10-(alpha-diethylaminopropionyl)phenothiazine] >/= promethazine >/= chlorpromazine (final drug conc. 10(-4) M); t1/2 (+/- SD) 21.2 +/- 1.1, 19.6 +/- 1.0, 17.2 +/- 0.9, 17.1 +/- 1.0, and 17.1 +/- 1.1 min, respectively. Control and bacitracin (known aminopeptidase inhibitor) values were 11.8 +/- 1.0 and 31.3 +/- 1.7 min, respectively. These drugs significantly decreased (listed in the same order) LEU degradation initial velocity; Iv (+/- SD) 0.77 +/- 0.2, 0.82 +/- 0.2, 0.92 +/- 0.3, 0.93 +/- 0.2, 0.94 +/- 0.3 pg LEU/min, respectively. Control and bacitracin 1.10 +/- 0.3 and 0.20 +/- 0.1 pg LEU/min, respectively. Values represent results from 5 samples, each obtained by pooling 6 individual plasmas (4 male and 2 female; n = 30 healthy, drug-free volunteers). However, neither the phenothiazines ethopropazine, methotrimeprazine, prochlorperazine and trifluoperazine nor the various commonly used heterocyclic antipsychotics tested, e.g., molindone, loxapine, clozapine, haloperidol, sulpiride and thiothixene inhibited plasma LEU degradation kinetics. Our results failed to show correlations between chemical structure, antipsychotic properties and ability to inhibit plasmatic aminopeptidase LEU degradation. Whereas, presence of the phenothiazine molecule appears to be necessary for enzyme inhibition, only five out of nine substituted phenothiazines tested exhibited this effect. Furthermore, there was a lack of correlation between phenothiazines antipsychotic properties and their capacity to inhibit aminopeptidase activity, a property shown by promethazine (antihistaminic) and As-1397 (selective butyrylcholinesterase inhibitor) but lacking in prochlorperazine and trifluoperazine. Our results provide information which could lead to the rational design of agents capable to modulate the bioavailability of enkephalin and other endogenous aminopeptidase-degraded peptides believed to be involved in the etiology and/or pathophysiology associated with various disease conditions. Whether their development could find useful pharmacological applications remains to be explored.

In vitro metabolism of opioid tetrapeptide agonists in various tissues and subcellular fractions from rats

The metabolism of three mu-selective opioid tetrapeptide agonists, Tyr-D-Arg-Phe-Nva-NH(2) (TArPN), Tyr-D-Arg-Phe-Phe-NH(2) (TArPP), and Tyr-D-Ala-Phe-Phe-NH(2) (TAPP), was investigated in different rat tissues. High metabolic activity (<20% peptide remaining after 30 min) was found against the three peptides in the kidney homogenate and against TArPN in spleen homogenate. Low metabolic activity (>80% peptide remaining after 30 min) was found for all peptides in brain homogenate and plasma, and for TArPN and TArPP in blood. The other tissue homogenates, prepared from the small and large intestine, liver and lung, all exhibited intermediate metabolic activity (20-80% peptide remaining after 30 min) against the peptides. In all tissues investigated, the tetrapeptides were metabolized at the C-terminal amide by deamidation.A further in depth metabolic investigation was performed in subcellular fractions isolated from three tissues (small intestine, liver and kidney). In the liver, the deamidation was predominantly localized to the mitochondrial/lysosomal fraction, while hydrolysis at the N-terminal Tyr residue was the major metabolic pathway in the microsomal/brush-border membrane fraction from the kidney and small intestine.