Determination of prolidase activity using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Prolidase activity assays. A survey of the reported literature methodologies

Prolidase (EC.3.4.13.9) is a dipeptidase known nowadays to play a pivotal role in several physiological and pathological processes. More in particular, this enzyme is involved in the cleavage of proline- and hydroxyproline-containing dipeptides (imidodipeptides), thus finely regulating the homeostasis of free proline and hydroxyproline. Abnormally high or low levels of prolidase have been found in numerous acute and chronic syndromes affecting humans (chronic liver fibrosis, viral and acute hepatitis, cancer, neurological disorders, inflammation, skin diseases, intellectual disability, respiratory infection, and others) for which the content of proline is well recognized as a clinical marker. As a consequence, the accurate analytical determination of prolidase activity is of greatly significant importance in clinical diagnosis and therapy. Apart from the Chinard's assay, some other more sensitive and well validated methodologies have been published. These include colorimetric and spectrophotometric determinations of free proline produced by enzymatic reactions, capillary electrophoresis, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, electrochemoluminescence, thin layer chromatography, and HPLC. The aim of this comprehensive review is to make a detailed survey of the in so far reported analytical techniques, highlighting their general features, as well as their advantages and possible drawbacks, providing in the meantime suggestions to stimulate further research in this intriguing field.

Electrophoretic deposition of Ru(bpy)32+ in vertically-ordered silica nanochannels: A solid-state electrochemiluminescence sensor for prolidase assay

Prolidase (PLD) plays a crucial role as a dipeptidase in various physiological processes, specifically involved in the cleavage of proline-containing dipeptides for efficient recycling of proline. The accurate determination of PLD activity holds significant importance in clinical diagnosis. Herein, a solid-state electrochemiluminescence (ECL) biosensor was developed to address the urgent need for PLD assay. The Ru(bpy)32+ was electrophoretically deposited within the nanochannels of vertically-ordered mesoporous silica film (VMSF) on indium tin oxide (ITO) electrodes. The Ru(bpy)32+-deposited VMSF/ITO (Ru-VMSF/ITO) exhibited a remarkable ECL response towards proline, attributed to the enhanced concentration of the reactants and improved electron transfer resulting from the nanoconfinement effect. As PLD specifically enzymolyzed the Gly-Pro dipeptide to release proline, a proline-mediated biosensor was developed for PLD assay. Increased PLD activity led to enhanced release of proline into the porous solid-state ECL sensors, resulting in a more robust ECL signal. There was a linear relationship between ΔECL intensity and logarithmic concentration of PLD in the range of 10-10000 U/L, with a detection limit of 1.98 U/L. Practical tests demonstrated the reliability and convenience of the proposed bioassay, making it suitable for widespread application in PLD assays.

The use of matrix-assisted laser desorption/ionization mass spectrometry in enzyme activity assays and its position in the context of other available methods

Activity assays are indispensable for studying biochemical properties of enzymes. The purposes of measuring activity are wide ranging from a simple detection of the presence of an enzyme to kinetic experiments evaluating the substrate specificity, reaction mechanisms, and susceptibility to inhibitors. Common activity assay methods include spectroscopy, electrochemical sensors, or liquid chromatography coupled with various detection techniques. This review focuses on the use of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) as a growing and modern alternative, which offers high speed of analysis, sensitivity, versatility, possibility of automation, and cost-effectiveness. It may reveal reaction intermediates, side products or measure more enzymes at once. The addition of an internal standard or calculating the ratios of the substrate and product peak intensities and areas overcome the inherent inhomogeneous distribution of analyte and matrix in the sample spot, which otherwise results in a poor reproducibility. Examples of the application of MALDI-TOF MS for assaying hydrolases (including peptidases and β-lactamases for antibiotic resistance tests) and other enzymes are provided. Concluding remarks summarize advantages and challenges coming from the present experience, and draw future perspectives such as a screening of large libraries of chemical compounds for their substrate or inhibitory properties towards enzymes.

Quantitative analysis of the natural history of prolidase deficiency: description of 17 families and systematic review of published cases

PURPOSE

Prolidase deficiency is a rare inborn error of metabolism causing ulcers and other skin disorders, splenomegaly, developmental delay, and recurrent infections. Most of the literature is constituted of isolated case reports. We aim to provide a quantitative description of the natural history of the condition by describing 19 affected individuals and reviewing the literature.

METHODS

Nineteen patients were phenotyped per local institutional procedures. A systematic review following PRISMA criteria identified 132 articles describing 161 patients. Main outcome analyses were performed for manifestation frequency, diagnostic delay, overall survival, symptom-free survival, and ulcer-free survival.

RESULTS

Our cohort presented a wide variability of severity. Autoimmune disorders were found in 6/19, including Crohn disease, systemic lupus erythematosus, and arthritis. Another immune finding was hemophagocytic lymphohistiocytosis (HLH). Half of published patients were symptomatic by age 4 and had a delayed diagnosis (mean delay 11.6 years). Ulcers were present initially in only 30% of cases, with a median age of onset at 12 years old.

CONCLUSION

Prolidase deficiency has a broad range of manifestations. Symptoms at onset may be nonspecific, likely contributing to the diagnostic delay. Testing for this disorder should be considered in any child with unexplained autoimmunity, lower extremity ulcers, splenomegaly, or HLH.

Prolidase deficiency breaks tolerance to lupus-associated antigens

AIM

Prolidase deficiency is a rare autosomal recessive disease in which one of the last steps of collagen metabolism, cleavage of proline-containing dipeptides, is impaired. Only about 93 patients have been reported with about 10% also having systemic lupus erythematosus (SLE).

METHODS

We studied a large extended Amish pedigree with four prolidase deficiency patients and three heterozygous individuals for lupus-associated autoimmunity. Eight unaffected Amish children served as normal controls. Prolidase genetics and enzyme activity were confirmed. Antinuclear antibodies (ANA) were determined using indirect immunofluorescence and antibodies against extractable nuclear antigens were determined by various methods, including double immunodiffusion, immunoprecipitation and multiplex bead assay. Serum C1q levels were determined by enzyme-linked immunosorbent assay.

RESULTS

Two of the four homozygous prolidase deficiency subjects had a positive ANA. One had anti-double-stranded DNA, while another had precipitating anti-Ro. By the simultaneous microbead assay, three of the four had anti-Sm and anti-chromatin. One of the three heterozygous subjects had a positive ANA and immunoprecipitation of a 75 000 molecular weight protein. The unaffected controls had normal prolidase activity and were negative for autoantibodies.

CONCLUSIONS

Prolidase deficiency may be associated with the loss of immune tolerance to lupus-associated autoantigens even without clinical SLE.

Putative sequences on Ro60 three-dimensional structure accessible for 4-hydroxy-2-nonenal (HNE) modification compared to in vitro HNE modification of Ro60 sequences

We have previously reported accelerated acquisition of new autoreactivity upon immunization with 4-hydroxy-2-nonenal (HNE)-modified Ro60, as well as differential induction of lupus or Sjögren's syndrome by immunization with Ro60 containing varying amounts of HNE. Since the number of HNE molecules on Ro60 appears to be important, we hypothesized that specific sequences on Ro60 are targets for HNE-modification. Using surface plasmon resonance (SPR) we have also shown intramolecular protein-protein interaction between Ro60 and Ro multiple antigenic peptides (MAPs). We also hypothesized that intramolecular protein-protein interaction would be abolished by HNE-modification. To test this hypotheses we investigated (a) the epitopes of Ro60, using 19 Ro MAPs in an in vitro assay (involving HNE-modification of MAPs following immobilization on ELISA plates) to identify targets of HNE modification on Ro60 and (b) the protein-protein interaction between unmodified Ro60 MAPs, immobilized on the sensor surface of BIAcore, and unmodified Ro60 or HNE-modified Ro60 using SPR. New data obtained with SPR strengthens our earlier observation that immunization with HNE-Ro60 induces a stronger response. Unmodified Ro60 bound to several Ro60 MAPs through protein-protein interaction analyzed using SPR. This interaction was totally abrogated using HNE-modified Ro60 suggesting that sequences on Ro had become modified with HNE. When 19 Ro60 MAPs were modified in vitro with HNE, it was found that 10/19 MAPs significantly bound HNE covalently (p<0.001 compared to MAPs binding HNE poorly). The amino acid sequences 126-137, 166-272 and 401-495 on Ro60 were strongly HNE modified. Using computational model system based on the recently published crystal structure for Ro60 enabled us to identify regions on the Ro60 molecule represented by the HNE-modified Ro MAPs, which are part of the exposed tertiary structure of the Ro60 protein.

Strategies for the early detection of drug-induced hepatic steatosis in preclinical drug safety evaluation studies

Hepatic steatosis is characterized by the accumulation of lipid droplets in the liver. Although relatively benign, simple steatosis can eventually lead to the development of steatohepatitis, a more serious condition characterized by fibrosis, cirrhosis, and eventual liver failure if the underlying cause is not eliminated. According to the "two hit" theory of steatohepatitis, the initial hit involves fat accumulation in the liver, and a second hit leads to inflammation and subsequent tissue injury. Because some xenobiotics target liver fatty acid metabolism, especially mitochondrial β-oxidation, it is important to avoid potential drug candidates that can contribute to either the initiation of liver steatosis or progression to the more injurious steatohepatitis. The gold standard for the detection of these types of hepatic effects is histopathological examination of liver tissue. In animal studies, these examinations are slow, restricted to a single sampling time, and limited tissue sections. Recent literature suggests that rapid in vitro screening methods can be used early in the drug R&D process to identify compounds with steatotic potential. Further, progress in the identification of potential serum or plasma protein biomarkers for these liver changes may provide additional in vivo tools to the preclinical study toxicologist. This review summarizes recent developments for in vitro screening and in vivo biomarker detection for steatotic drug candidates.

Improving the solubility and pharmacological efficacy of curcumin by heat treatment

Lipid peroxidation has been implicated in a variety of diseases. 4-Hydroxy-2-nonenal (HNE), a major oxidation by-product, is cytotoxic, mutagenic, and genotoxic, being involved in disease pathogenesis. Naturally occurring pharmacologically active small molecules are very attractive as natural nonsteroidal anti-inflammatory agents. Interest has greatly increased recently in the pharmacotherapeutic potential of curcumin, the yellow pigment found in the rhizomes of the perennial herb Curcuma longa (turmeric). Curcumin is efficacious against colon cancer, cystic fibrosis, and a variety of other disorders. Curcumin's full pharmacological potential is limited owing to its extremely limited water solubility. We report here that the water solubility of curcumin could be increased from 0.6 microg/ml to 7.4 microg/ml (12-fold increase) by the use of heat. Spectrophotometric (400-700 nm) and mass spectrometric profiling of the heat-extracted curcumin displays no significant heat-mediated disintegration of curcumin. Using an enzyme-linked immunosorbent assay that employed HNE modification of solid-phase antigen, we found that the heat-solubilized curcumin inhibited HNE-protein modification by 80%. Thus, inhibition of HNE modification may be a mechanism by which curcumin exerts its effect. We also report a simple assay to detect curcumin spectrophotometrically. Curcumin was solubilized in methanol and serially diluted in methanol to obtain a set of standards that were then read for optical density at 405 nm. Curcumin in the heat-solubilized samples was determined from this standard. Heat-solubilized curcumin should be considered in clinical trials involving curcumin, especially in the face of frustrating results obtained regarding curcumin-mediated correction of cystic fibrosis defects.

Characterization of prolidase activity using capillary electrophoresis with tris(2,2'-bipyridyl)ruthenium(II) electrochemiluminescence detection and application to evaluate collagen degradation in diabetes mellitus

A new method for prolidase (PLD, EC 3.4.13.9) activity assay was developed based on the determination of proline produced from enzymatic reaction through capillary electrophoresis (CE) with tris(2,2'-bipyridyl)ruthenium(II) [Ru(bpy)3(2+)] electrochemiluminescence detection (ECL). A detection limit of 12.2 fmol (S/N = 3) for proline, corresponding to 1.22 x 10(-8) units of prolidase catalyzing for 1 min was achieved. PLD activity determined by CE-ECL method was in agreement with that obtained from the classical Chinard's one. CE-ECL showed its powerful resolving ability and selectivity as no sample pretreatment was needed and no interference existed. The clinical utility of this method was successfully demonstrated by its application to assay PLD activity in the serum of diabetic patients in order to evaluate collagen degradation in diabetes mellitus (DM). The results indicated that enhanced collagen degradation occurred in DM.

Kinetic study of paracetamol on prolidase activity in erythrocytes by capillary electrophoresis with Ru(bpy)(3) (2+) electrochemiluminescence detection

We explored the CE with (bpy) (2+) (3) electrochemiluminescence detection for the kinetic study of drug-enzyme interaction. Effects of four nonsteroidal anti-inflammatory drugs including aspirin, paracetamol, sodium salicylate and phenacetin on prolidase (PLD) activity in erythrocytes were investigated. Aspirin enhanced PLD activity whereas the other three had inhibiting effects. This may reveal their different effects on the collagen biosynthesis and catabolism that influence tumor invasiveness. Kinetic study of paracetamol on PLD showed that the value of Michaelis constant K(m) for PLD was 1.23 mM. The mechanism of PLD inhibition by paracetamol is noncompetitive inhibition, and the inhibitor constant K(i) value obtained in our research was 9.73 x 10(3) microg/L.

A comparative study of proteolysis methods for the measurement of 3-nitrotyrosine residues: enzymatic digestion versus hydrochloric acid-mediated hydrolysis

A common approach for the quantification of 3-nitrotyrosine (NY) in routine analyses relies on the cleavage of peptide bonds in order to release the free amino acids from proteins in tissues or fluids. NY is usually monitored by either GC-MS(/MS) or LC-MS/MS techniques. Various proteolysis methods have been employed to combine digestion efficiency with prevention of artifactual nitration of tyrosine. However, so far, no study was designed to compare the HCl-based hydrolysis method with enzymatic digestion in terms of reliability for the measurement of NY. The present work addresses the digestion efficiency of BSA using either 6M HCl, pronase E or a cocktail of enzymes (pepsin, pronase E, aminopeptidase, prolidase) developed in our laboratory. The HCl-based hydrolysis leads to a digestion yield of 95%, while 25 and 75% are achieved with pronase E and the cocktail of enzymes, respectively. These methods were compared in terms of NY measurement and the results indicate that a prior reduction of the disulfide bonds ensures a reliable quantification of NY. We additionally show that the enzyme efficacy is not altered when the digestion is carried out in the presence of BSA with a high content of NY.

A nonsense mutation of PEPD in four Amish children with prolidase deficiency

Encoded by the peptidase D (PEPD) gene located at 19q12-q13.11, prolidase is a ubiquitous cytosolic enzyme that catalyzes hydrolysis of oligopeptides with a C-terminal proline or hydroxyproline. We describe here four Amish children with a severe phenotype of prolidase deficiency in the Geauga settlements of Ohio as the first report of prolidase deficiency in the Amish population as well as in the United States. The patients presented with infection, hepatosplenomegaly, or thrombocytopenia, in contrast to most cases previously reported in the literature, presenting with skin ulcers. All four patients had typical facial features, classic skin ulcers, and multisystem involvement. Recurrent infections, asthma-like chronic reactive airway disease, hyperimmunoglobulins, hepatosplenomegaly with mildly elevated aspartate transaminase (AST), anemia, and thrombocytopenia were common and massive imidodipeptiduria was universal. Prolidase activity in our patients is nearly undetectable. Direct sequencing of PCR-amplified genomic DNA for all of the exons from the four patients revealed the same homozygous single nucleotide mutation c.793 T > C in exon 11, resulting in a premature stop-codon at amino acid residue 265 (p.R265X). It is speculated that the severe phenotype in these patients might be associated with the type of the PEPD gene mutation.

The role of emerging techniques in the investigation of prolidase deficiency: From diagnosis to the development of a possible therapeutical approach

The aim of the present article is to review the efforts performed in the past two decades by numerous research groups for the development of methods that allow a correct diagnosis of prolidase deficiency (PD), a rare autosomal recessive disorder and for the rationalization of a possible therapeutic intervention on these patients. In particular, the interest of the reader is focused on the application of capillary electrophoresis (i) for the detection of biological markers that reflect the pathological feature of the disease and (ii) for the determination of the efficiency of a carrier system in delivering prolidase inside cells in a possible therapy based on enzyme replacement.