INTRODUCTION

Cholinesterases are a family of carboxy esterases, which are present from single-celled organisms to multicellular organisms such as plants, invertebrates and vertebrates in which they appear in embryonic development before synaptogenesis.¹ In 1914, the neurotransmitter acetylcholine (ACh) was discovered which is essential for the central nervous system, the peripheral nodes and the neuromuscular plate. Acetylcholine binds to a specific receptor on the postsynaptic membrane; resulting in the change of membrane potential and may be the synapse.¹ Subsequently, the acetylcholine will be hydrolyzed by acetylcholinesterase (AChE) becoming acetic acid and choline, through the postsynaptic membrane return to the state of rest.² This process prevents overstimulation of muscle fibers due to accumulation of acetylcholine at the neuromuscular junction, which causes a wide range of neurotoxic effects (muscarinic, nicotinic, and neurological).^2,3 See Table 1.Cholinergic syndrome. The accumulation effects of acetylcholine can be evidenced biochemically by suppressed levels of serum pseudocholinesterase (BChE) and erythrocyte cholinesterase (AChE).⁴ These two types of cholinesterase are divided according to their substrate specificity, susceptibility to inhibitors, and energy tissues: Acetylcholinesterase (AChE), or true cholinesterase or E-type specific cholinesterase, is predominant in muscle, erythrocytes, and the nervous system, where BChE levels are lower.²

Butyrylcholinesterase (BChE) or pseudocholinesterase, plasma cholinesterase or cholinesterase type S is present in tissues such as intestine, kidney, heart, lungs and mainly the liver. After being synthesized, it is secreted into plasma.^1,2 The physiological function of BChE is still unknown, it is speculated that it plays an important role in the metabolism of lipids and lipoproteins, regulating the concentration of choline in plasma or preventing the accumulation of butyrylcholine, during the metabolism of fatty acids and lipogenesis. In addition, they have detected functions in the metabolism of drugs such as procaine, succinylcholine and acetylsalicylic acid, as well as the elimination of phosphates and carbamates.⁵

Cholinesterase enzymes are frequently used to biomonitor exposure to organophosphates and carbamates, which are determined to be neurotoxic and cause 80% of acute or chronic pesticide poisonings worldwide.⁶Pesticide absorption pathways are by inhalation, ingestion and through intact skin. Once in the body they suffer metabolic reactions of activation, detoxification and conjugation. Its mechanism of toxic action is the binding and inhibition of stearic enzymes in a stable manner, and they are eliminated relatively quickly by the renal route.³ The inhibition of AChE, caused by carbamates, is of less persistence since it is decarbamilated in a reversible and spontaneous way, with neurotoxic effects similar to organophosphates, but to a lesser extent and with usually rapid recovery.⁶ Approximately 3 million people suffer from poisoning and 200,000 die from pesticide poisoning annually in the world.^2,7 Millions are exposed to hazardous work practices and insecure storage of pesticides; however, it is deliberate self-poisoning that causes most deaths, particularly in Asia.⁸

Chronic intoxications or exposures are associated with immunological effects, carcinogenic effects, reproductive disorders, developmental disorders, neurotoxicity, even exposure to pesticides is associated with the development of Parkinson's, Alzheimer's, anxiety disorders, deficit disorders. attention with hyperactivity, and depression, including areas with an intensive pesticide have a high suicide rate.^9-11.

Cholinesterase levels as a diagnosis

Every good diagnosis should start with a complete medical history, through the suspicion or certainty of exposure to the pesticide, the route of absorption and a compatible clinical picture. Diagnostic confirmation should ideally be done by measuring cholinesterase activity.⁴ However, the results should be interpreted with caution because a wide variability has been observed in the values considered normal because of the characteristics of cholinesterase, individual differences mainly the aggregate clinical conditions, substances that can alter cholinesterase levels, influence of the environment and the possibility of laboratory errors.^7,11,12.

MATERIALS AND METHODS

A systematic review was performed in the PubMed, Crossreff and Google Scholar search engines, using the keywords cholinesterase, acetylcholinesterase, pesticides and poisoning.

• Inclusion criteria: Only original articles in English or Spanish were taken into account, the search did not include a publication date limit, in their methodology they had to include the measurement of plasma cholinesterase, erythrocyte, or both through colorimetric chemical reactions or changes in pH, who would have carried out their measurements in vivo, included experimental and non-experimental investigations.

• Exclusion criteria: Those with a sample of less than 15 people and original articles that did not have the objective of relating cholinesterase measurements with acute or chronic pesticide poisoning.

PROCESS

The review, synthesis and classification of the selected works was carried out by preparing a matrix that includes the name of the first author, date of publication, country of origin, methodology and type of cholinesterase measured, sample size, presence of signs of pesticide poisoning, number of people with inhibited or altered cholinesterase, if there is a statistical association with cholinesterase, exposure time and exclusion criteria. The table is sorted by dates from oldest to most current. See Table 2. Studies with measurements of cholinesterases and pesticide poisoning.

RESULTS

Twenty eight original articles that describe and analyze situations organophosphate or carbamate poisoning and its association with cholinesterase were used to generate this article. Twenty eight point five percent belong to PubMed, 35.7% to Crossreff and the remaining 35.7% is owned by Google Scholar.

Selected articles are conducted primarily in the Americas, predominantly Colombia with 8 items, with the exception of 5 items from which, 3 were conducted in India, 1 in Thailand and 1 in Serbia.

The analytical methods used were the Ellman method, the method of Ellman with DTNA (6-6'-ditiodinicotínico acid), Magnotti, Michel, Lovibond, Wiener, Merckotest, Aldridge and Randox which measure through chemical reactions or changes colored pH. The method most commonly found in the articles as the method of Ellman.

Normal values vary widely considered in each study, according to the methodology, the laboratory, the sample type and population in which was measured for both plasma cholinesterase to erythrocyte cholinesterase. 13 (46.42%) of the items evaluated plasma cholinesterase, 11 (39.28%) evaluated. and 4 erythrocyte cholinesterase (14.28%) of the items evaluated both. Only 10 (35.71%) of the studies collected describe the methodology with which the sample was taken. See Figure 1. Sampling and exclusion criteria for cholinesterase.

The study with the lowest sample had 15 participants and the largest sample had 46.290 participants. 10 (35.71%) of the articles included the exposed group and the control group in 4 (11.2%) of the studies reported taking between 3 and 5 serial samples per person and one article made measurements cholinesterase saliva.

The 28 (100%) articles described their population, as affected by pesticides, with risk factors and little or no protective equipment of these 20 articles (71.4%) reported signs and symptoms of poisoning, of these 20 articles that reported signs and symptoms, 6 (30%) focus on symptoms of chronic poisoning and 14 (70%) articles described signs and symptoms of acute poisoning.

Twenty (71.42%) of the articles reported a decrease in cholinesterase in the study population and 8 articles (28.57%) reported not having detected a decrease in cholinesterase in the study population.

Of the 20 articles that reported a decrease in cholinesterase:

• 9 (45%) articles indicated that less than 15% of their total population studied presented a decrease in measured cholinesterase enzymes.

• 5 (25%) articles reported that between 16% to 50% of their total population had a decrease in measured cholinesterase enzymes.

• 6 (30%) of the articles reported that between 51% to 100% of their study population selected a decrease in measured cholinesterase enzymes.

In 4 (14.28%) articles, they found no statistical association between pesticide exposure and cholinesterase levels. In 9 (32.14%) articles, the exposure time of their population to pesticides was not reported and in the remaining 19 (67.85%) articles, the exposure time was reported between 6 hours to 50 years. Only 14 (50%) of the studies used for this article mentioned or described their exclusion criteria when choosing their sample, ruling out physiological, pathological conditions or the presence of substances that can alter cholinesterase levels. For example, medications used in the treatment of Alzheimer's, such as donepezil or galantamine, have an inhibitory action against BuChE and AchE.¹³

Figure 1
Sampling and exclusion criteria for cholinesterase.

DISCUSSION

Cholinesterase enzymes are widely used as biological markers to detect pesticide poisoning, but the variability of the range considered "normal" prevents comparison between the results in different studies. As already mentioned, the range of normality can vary from individual to individual, from population to population, by pathologies, substances, the environment for example high temperature and humidity less than 40% favor skin absorption by decreasing cholinesterase levels and even by the laboratory and the method used for its analysis. It is true that there is no standardized and universally accepted laboratory technique or method for these types of studies, however, since 1961 international organizations have recommended the Ellman method, which is reflected in its predominance in the analyzed articles.^12,36,37,38 It is desired to highlight some of the main errors that were detected, for which the results of the measurements with acetylcholinesterase are not significant, the lack of standardized analytical and preanalytical parameters can have a great influence on the variability of results since there are modifications to the conventional method there are laboratories that prepare their own reagents, while others use commercial kits.³⁴ It is important to select whether to investigate acute or chronic intoxications to identify the characteristic signs and symptoms, while choosing the most appropriate type of cholinesterase with our study. Butyrylcholinesterase is recommended for acute intoxications and acetylcholinesterase for chronic intoxications.⁶ At the time of selecting the sample, in most of the studies there are no mentioned exclusion criteria aimed at ruling out factors of the individual that could alter the values considered as “normal”, this reason may explain why in some studies they reported cholinesterase inhibition within the control group. The methodology in the sampling is referred to in less than 50% of the studies, but it is of vital importance, considering that it can be contaminated from the venous puncture (by pesticides present in the skin) and by requiring special conditions for its transport (network from cold to approximately 4 degrees Celsius for a maximum of 2 hours) that prevent the inactivation of enzymes during transport. It has been reported that the activity of the AchE in the laboratory at -20 degrees Celsius is stable during the first 7 days, subsequently its activity slowly decreases, it is 91.8% at 34 days, while the activity of BChE in plasma remained unchanged for 34 days. Only one of the studies reported that after the arrival of the laboratory it took 10 hours to perform the analysis and another study reported the analysis immediately.^6,10,12,14.It is believed that better results are obtained in studies where the baseline cholinesterase levels of everyone can be taken prior to exposure to pesticides, but it is known that this is not always possible, it is recommend ed the use of control groups that function as regional baseline samples. Good results are also seen in studies where they take serial samples and compare them to measure enzyme inhibition.

An investigation was added to this study where the measurement of AChE and BChE in saliva is carried out, which is an interesting proposal for the silver problem in some studies that showed the loss of population, who refused to take venous blood because it was an invasive technique; however, it is considered that more research is required for the validation and standardization of this method.. For the analysis of cholinesterase levels, it was found interesting the proposals where the level of intoxication was associated, with cholinesterase levels on a percentage scale. Example. "According to serum PCE levels, the severity of the poisoning was defined according to the scale of Kumary col."⁴

• Latent: BChE level> 50% of normal or> 3,500 IU / L.

• Mild poisoning: BChE level 20-50% of normal or> 1,401- 3,500 IU / L

• Moderate poisoning: BChE level 10-20% of normal or 701- 1,400 IU / L

• Severe poisoning: the BChE level is <10% of normal or <700 IU / L.4

And the Proudfoot classification, where mild organophosphorus toxicity is defined as a BChE reduction of less than 10% (fatigue, headache, nausea and vomiting), moderate toxicity as a 10-50% reduction (myosis, weakness general, dysarthria, fasciculation) and severe toxicity such as> 50% of the reduction (general fasciculation, flaccid paralysis, respiratory distress, loss of consciousness).⁸

CONCLUSION

According to what it was found, it is recommended that researchers who carry out studies with cholinesterase enzymes do not lose sight of the delicacy of these enzymes in order to have the best possible results, not necessarily seek that the results are below a range established by the laboratory and take into account notes that the appearance of symptoms of intoxication depends more on the rate of cholinesterase inhibition than on the absolute level of activity found.³²

CONFLICT OF INTERESTS

The authors declare that they have no conflict of interest.

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Notas de autor

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