Localization of the peptidase activity of human serum butyrylcholinesterase in a approximately 50-kDa fragment obtained by limited alpha-chymotrypsin digestion

A Method for Diagnosing Organophosphate Pesticide Exposure in Humans using Liquid Chromatography Coupled Tandem Mass Spectrometry

Organophosphate (OP) pesticides are commonly utilized worldwide for agricultural purposes and pose a health threat through air, ground, and water contamination. Here, we present a convenient method for diagnosing exposure to OP pesticides in humans. This immunoprecipitation method relies on extraction of butyrylcholinesterase (BChE), a biomarker of OP poisoning that adducts OP compounds, from human serum using agarose beads conjugated to anti-BChE antibodies. Extracted BChE was then digested with pepsin and analyzed for unadducted and OP-adducted peptides by high performance liquid chromatography coupled to tandem mass spectrometry (HPLC-MS/MS). To characterize and validate this method, pooled human plasma was exposed to parathion and dichlorvos to form diethoxyphospho, aged ethoxyphospho and dimethoxyphospho adducts with BChE. Untreated plasma was also analyzed for unadducted peptides. Additionally, samples were analyzed using Ellman's assay to measure BChE functional activity. The percent inhibition of BChE was 53.5±5.76 and 95.2±0.37%, respectively, for plasma treated with parathion for 1 hour and 24 hours. The percent inhibition was 97.2±0.98 for plasma treated with dichlorvos for 1 hour. The percent inhibition was 97.9±0.41% when the plasma treated with parathion for 1 hour, parathion for 24 hour and dichlorvos for 1 hour were mixed. Individual adducts were quantified in a single chromatographic run. Untreated plasma contained 26.4±1.87 ng/mL of unadducted BChE and no adducted peptides. In contrast, the plasma sample treated with both pesticides contained no unadducted BChE, but did contain 9.46±1.10, 10.9±0.98 and 14.1±1.10 ng/mL of diethoxyphospho, aged-ethoxy, and dimethoxyphospho peptides, respectively. The ability to identify and measure BChE and BChE adducts to parathion and dichlorvos is expected to be useful for diagnosing human exposure to multiple OP pesticides.

Improving the ex vivo stability of drug ester compounds in rat and dog serum: inhibition of the specific esterases and implications on their identity

In drug development, it has been noticed that some drug compounds, especially esters, are unstable in serum samples ex vivo. This can lead to a substantial underestimation of the actual drug concentration. The rat and the dog, representing a rodent and non-rodent species, respectively, are widely used in preclinical studies. We studied the degradation of three structurally different drug esters in rat and dog serum. Moreover, the efficiency of selected enzyme inhibitors to prevent these degradations was investigated. Furthermore, we found indications of the identity of the drug-specific esterases by means of their inhibitor sensitivity as well as by protein purification and identification. The studied drugs were sagopilone, drospirenone, and methylprednisolone aceponate (MPA) all of which are used in (pre-)clinical drug development. The sagopilone-cleaving esterases in rat serum were inhibited by serine hydrolase inhibitors. We partly purified these esterases resulting in an activity yield of 5% and a purification factor of 472. Using matrix-assisted laser desorption ionization (MALDI)-time of flight (TOF)-mass spectrometry (MS), the rat carboxylesterase isoenzyme ES-1 was identified in these fractions, thus pointing to its involvement in sagopilone cleavage. Drospirenone cleavage in rat serum was effected by butyrylcholinesterase (BChE) and paraoxonase 1 (PON1) as we deduced from the high efficacy of certain serine hydrolase and metallohydrolase inhibitors, respectively. Likewise, some inhibition characteristics implied that MPA was cleaved in rat serum by BChE and serine proteases. Partial purification of the MPA-specific esterases resulted in activity yields of 1-2%, exhibiting up to 10,000-fold purification. In dog serum, we found that sagopilone was not degraded which was in contrast to MPA and drospirenone. MPA degradation was mainly prevented by serine hydrolase inhibitors. We used a three-step purification to isolate the esterases cleaving MPA. This procedure resulted in an activity yield of 12% and 645-fold purification. By protein identification using liquid chromatography (LC)-electrospray ionization (ESI)-MS, we identified alpha(2)-macroglobulin (alpha(2)M) in the active fractions. We therefore assumed that serine hydrolases, probably butyrylcholinesterase, known to form esteratically active complexes with alpha(2)M, were responsible for MPA cleavage. In contrast, PON1 was assumed to be involved in drospirenone cleavage due to the high efficiency of metallohydrolase inhibitors. This indication was supported by the presence of PON1 in drospirenone-cleaving fractions as we found by affinity chromatography and Western immunoblotting for isolation and detection of PON1, respectively. The identity of the assumed cleaving enzymes remains, however, to be further studied. The inhibitors we found can serve as a tool for stabilizing drug ester compounds in biological samples ex vivo.

Butyrylcholinesterase-Mediated enhancement of the enzymatic activity of trypsin

1. Acetylcholinesterase (AChE, EC 3.1.1.7) and butyrylcholinesterase (BuChE, EC 3.1.1.8) are enzymes that catalyze the hydrolysis of esters of choline. 2. Both AChE and BuChE have been shown to copurify with peptidases. 3. BuChE has also been shown to copurify with other proteins such as transferrin, with which it forms a stable complex. In addition, BuChE is found in association with beta-amyloid protein in Alzheimer brain tissues. 4. Since BuChE copurifies with peptidases, we hypothesized that BuChE interacts with these enzymes and that this association had an influence on their catalytic activities. One of the peptidases that copurifies with cholinesterases has specificity similar to trypsin, hence, this enzyme was used as a model to test this hypothesis. 5. Purified BuChE causes a concentration-dependent enhancement of the catalytic activity of trypsin while trypsin does not influence the catalytic activity of BuChE. 6. We suggest that, in addition to its esterase activity, BuChE may assume a regulatory role by interacting with other proteins.

Selective inactivation of butyrylcholinesterase with metal chelators suggests there is more than one metal binding site

Cholinesterases exhibit functions apart from their esterase activity. We have demonstrated an aryl acylamidase and a zinc stimulated metallocarboxypeptidase activity in human serum butyrylcholinesterase. To establish the presence of zinc binding sites in the enzyme we examined the effect of metal chelators on its catalytic activities. The metal chelators 1,10-phenanthroline and N,N,N',N'-tetrakis (2-pyridyl methyl)ethylene diamine (TPEN) inhibited all the three catalytic activities in the enzyme. However, EDTA inhibited the peptidase activity exclusively without affecting the cholinesterase and aryl acylamidase activities. The catalytic activities were recovered upon removal of the chelator by Sephadex G-25 chromatography. Pre-treatment of the enzyme with any one of the three chelators resulted in the binding of the enzyme to a zinc-Sepharose column or to 65Zn2+. Histidine modification of the enzyme pretreated with chelators resulted in abolition of 65Zn2+ binding and zinc-Sepharose binding. Whereas the binding studies demonstrated removal of a metal from a Zn2+ binding site, attempts to remove the metal responsible for catalytic activity were unsuccessful. Atomic absorption spectroscopy indicated approximately 2.5 mol of zinc per mol of enzyme before treatment with EDTA and 1 mol zinc per mol enzyme after EDTA treatment. The results indicate that there are at least two metal binding sites on butyrycholinesterase. The presence of two HXXE...H sequences in butyrylcholinesterase supports these findings. Our studies implicate a zinc dependent metallocarboxypeptidase activity in the non-cholinergic functions of butyrylcholinesterase.

Butyrylcholinesterase in the life cycle of amyloid plaques

Deposits of diffuse beta-amyloid (Abeta) may exist in the brain for many years before leading to neuritic degeneration and dementia. The factors that contribute to the putative transformation of the Abeta amyloid from a relatively inert to a pathogenic state remain unknown and may involve interactions with additional plaque constituents. Matching brain sections from 2 demented and 4 nondemented subjects were processed for the demonstration of Abeta immunoreactivity, butyrylcholinesterase (BChE) enzyme activity, and thioflavine S binding. Additional sections were processed for the concurrent demonstration of two or three of these markers. A comparative analysis of multiple cytoarchitectonic areas processed with each of these markers indicated that Abeta plaque deposits are likely to undergo three stages of maturation, ie, a "diffuse" thioflavine S-negative stage, a thioflavine S-positive (ie, compact) but nonneuritic stage, and a compact neuritic stage. A multiregional analysis showed that BChE-positive plaques were not found in cytoarchitectonic areas or cortical layers that contained only the thioflavine S-negative, diffuse type of Abeta plaques. The BChE-positive plaques were found only in areas containing thioflavine S-positive compact plaques, both neuritic and nonneuritic. Within such areas, almost all (>98%) BChE-containing plaques bound thioflavine S, and almost all (93%) thioflavine S plaques contained BChE. These results suggest that BChE becomes associated with amyloid plaques at approximately the same time that the Abeta deposit assumes a compact beta-pleated conformation. BChE may therefore participate in the transformation of Abeta from an initially benign form to an eventually malignant form associated with neuritic tissue degeneration and clinical dementia.

Non-classical actions of cholinesterases: role in cellular differentiation, tumorigenesis and Alzheimer's disease

The cholinesterases are members of the serine hydrolase family, which utilize a serine residue at the active site. Acetylcholinesterase (AChE) is distinguished from butyrylcholinesterase (BChE) by its greater specificity for hydrolysing acetylcholine. The function of AChE at cholinergic synapses is to terminate cholinergic neurotransmission. However, AChE is expressed in tissues that are not directly innervated by cholinergic nerves. AChE and BChE are found in several types of haematopoietic cells. Transient expression of AChE in the brain during embryogenesis suggests that AChE may function in the regulation of neurite outgrowth. Overexpression of cholinesterases has also been correlated with tumorigenesis and abnormal megakaryocytopoiesis. Acetylcholine has been shown to influence cell proliferation and neurite outgrowth through nicotinic and muscarinic receptor-mediated mechanisms and thus, that the expression of AChE and BChE at non-synaptic sites may be associated with a cholinergic function. However, structural homologies between cholinesterases and adhesion proteins indicate that cholinesterases could also function as cell-cell or cell-substrate adhesion molecules. Abnormal expression of AChE and BChE has been detected around the amyloid plaques and neurofibrillary tangles in the brains of patients with Alzheimer's disease. The function of the cholinesterases in these regions of the Alzheimer brain is unknown, but this function is probably unrelated to cholinergic neurotransmission. The presence of abnormal cholinesterase expression in the Alzheimer brain has implications for the pathogenesis of Alzheimer's disease and for therapeutic strategies using cholinesterase inhibitors.

Evidence for a Zn(2+)-binding site in human serum butyrylcholinesterase

Purified human serum butyrylcholinesterase after treatment with either of the metal chelators EDTA or NaCN was able to bind to a Zn(2+)-chelate-Sepharose affinity column and was eluted from the column by EDTA or imidazole. Prior EDTA treatment of the enzyme was essential for binding to this affinity column. The enzyme could be labelled with (65)Zn(2+) after EDTA treatment of the enzyme. Diethylpyrocarbonate modification of histidine residues in the EDTA-treated enzyme resulted in the abolition of both binding to the Zn(2+)-chelate-Sepharose column and labelling by (65)Zn(2+). Stoicheiometry of (65)Zn(2+) binding indicated approximately 0.85 mol of Zn(2+)/mol of subunit of the EDTA-treated enzyme. EDTA or NaCN treatment resulted in the loss of thermal stability of the enzyme at 37 degrees C which could not be reversed by Zn(2+). Whereas the cholinesterase activity of butyrlcholinesterase was not affected by EDTA, there was significant loss of its carboxypeptidase activity in the presence of EDTA, and the loss could be reversed by added ZnCl2. These results suggest the presence of a Zn(2+)-binding site on human serum butyrylcholinesterase and the involvement of histidine residues in the metal binding. The presence in human serum butyrylcholinesterase of a sequence HXXE...H found in many known Zn(2+)-containing enzymes supports these findings.

Stimulation by lysosomal enzymes and mannose-6-phosphate of a phosphoprotein phosphatase activity associated with the lysosomal enzyme binding receptor protein from monkey brain

Previously we have isolated a lysosomal enzyme binding receptor protein from monkey brain that exhibits protein kinase activity and undergoes phosphorylation on serine and tyrosine residues. Using the 32P-labelled receptor protein, we have found that the lysosomal enzyme fucosidase and mannose-6-phosphate, which are ligands for the receptor, stimulated a protein phosphatase activity associated with the receptor protein. Stimulation of protein phosphatase activity using the 32P-labelled receptor protein was demonstrated both by the loss in radioactivity of the receptor and by the release of 32P-phosphate. There was no stimulation by a non-lysosomal glycoprotein enzyme, or by the sugars mannose or glucose. Both serine-phosphate and tyrosine-phosphate residues were dephosphorylated. Stimulation of protein phosphatase activity by fucosidase and mannose-6-phosphate was also demonstrated using as substrate histone 32P-labelled, on serine/threonine or tyrosine residues. Insulin-like growth factor II, another known ligand for the lysosomal enzyme binding receptor, did not show any significant effect, either on the phosphorylation or dephosphorylation of the receptor protein. Our previous and present results suggest that a phosphorylation/dephosphorylation mechanism may be operative in the ligand binding and functions of the receptor.

The peptidase activity of human serum butyrylcholinesterase: studies using monoclonal antibodies and characterization of the peptidase

Purified human serum butyrylcholinesterase, which exhibits cholinesterase, aryl acylamidase, and peptidase activities, was cross-reacted with two different monoclonal antibodies raised against human serum butyrylcholinesterase. All three activities were immunoprecipitable at different dilutions of the two monoclonal antibodies. At the highest concentration of the antibodies used, nearly 100% of all three activities were precipitated, and could be recovered to 90-95% in the immunoprecipitate. The peptidase activity exhibited by the purified butyrylcholinesterase was further characterized using both Phe-Leu and Leu-enkephalin as substrates. The pH optimum of the peptidase was in the range of 7.5-9.5 and the divalent cations Co2+, Mn2+, and Zn2+ stimulated its activity. EDTA and other metal complexing agents inhibited its activity. Thiol agents and -SH group modifiers had no effect. The serine protease inhibitors, diisopropylfluorophosphate and phenyl methyl sulfonyl fluoride, did not inhibit. When histidine residues in the enzyme were modified by diethylpyrocarbonate, the peptidase activity was not affected, but the stimulatory effect of Co2+, Mn2+, and Zn2+ disappeared, suggesting the involvement of histidine residues in metal ion binding. These general characteristics of the peptidase activity were also exhibited by a 50 kD fragment obtained by limited alpha-chymotrypsin digestion of purified butyrylcholinesterase. Under all assay conditions, the peptidase released the two amino acids, leucine and phenylalanine, from the carboxy terminus of Leu-enkephalin as verified by paper chromatography and HPLC analysis. The results suggested that the peptidase behaved like a serine, cysteine, thiol-independent metallopeptidase.

Isolation of a tripeptide (Ala-Gly-Ser) exhibiting weak acetylthiocholine hydrolyzing activity from a high-salt soluble form of monkey diaphragm acetylcholinesterase

A high-salt soluble form of acetylcholinesterase (AChE) was purified from monkey (Macaca radiata) whole diaphragm by a two step affinity chromatographic procedure using m-aminophenyl trimethylammonium-chloride hydrochloride-Sepharose and procainamide-Sepharose columns. The purified enzyme showed three major protein bands at 80 kDa, 78 kDa and 60 kDa on SDS-gel electrophoresis. [3H]Diisopropyl fluorophosphate ([3H]DFP) labeled enzyme also gave three radioactive peaks corresponding to these three bands. The purified enzyme pretreated with dithiothreitol and subjected to limited trypsin digestion gave a peptide fragment of molecular weight approximately 300 Da showing weak acetylthiocholine hydrolyzing activity as identified by Sephadex G-25 gel filtration. Sequence analysis showed that the active peptide fragment was a tripeptide with the sequence Ala-Gly-Ser. When the purified AChE was labeled with [3H]DFP, digested with trypsin and subjected to Sephadex G-25 chromatography, a radioactive peak that would correspond to the tripeptide fragment was seen. The kinetics, inhibition characteristics and binding characteristics to lectins of the active peptide fragment was compared with the parent enzyme. A synthetic peptide of sequence Ala-Gly-Ser was also found to exhibit acetylthiocholine hydrolyzing activity. The kinetics and inhibition characteristics of the synthetic peptide was similar to those of the peptide derived from the purified enzyme, except that the synthetic peptide was more specific towards acetylthiocholine than butyrylthiocholine. The specific activity (units/mg) of the synthetic peptide was about 29480 times less than that of the purified AChE.