Organofosfatlı pestisit zehirlenmeleri ve serum paraoksonaz 1 PON1 enziminin organofosfat metabolizmasındaki rolü

Öz

Dünya Sağlık Örgütü’nün verilerine göre tüm dünyada yılda 3 milyona yakın pestisit zehirlenmesi meydana gelmekte, bunların 220.000’i ölümle sonuçlanmaktadır. Son yıllarda sıklıkla kullanılan pestisitlerden olan organofosfatlar, sinir sistemi üzerinde etki gösteren kimyasallardır. Organofosfatlar asetilkolinesteraz enzimini baskılayarak nörotoksisiteye yol açarlar. Paraoksonaz 1 PON 1; EC 3.1.8.1 serumda yüksek dansiteli lipoprotein HDL üzerinde yer alan, kalsiyuma bağımlı glikoprotein yapısında bir enzimdir. Memelilerin birçok organında PON1 aktivitesi tespit edilmesine karşın, kuşlar, balıklar ve böceklerde paraoksonaz aktivitesi sıfıra yakındır. PON1’in hidrolize ettiği substratlar arasında paration, diazinon ve klorprifos gibi organofosfatlı insektisitlerin aktif formu olan toksik okson metabolitleri; sarin, ve soman gibi sinir gazları; fenil asetat gibi aromatik esterler; homogentisik asit lakton, dihidrokumarin ve homosistein tiolakton gibi birçok aromatik ve alifatik lakton ile siklik karbonatlar yer almaktadır. Organofosfatlı pestisitlerin toksikolojisi ile ilgili yapılan ilk çalışmalar düşük serum PON1 aktivitesine sahip olmanın organofosfatlı bileşiklerin akut etkilerine karşı duyarlılığı arttırdığını ortaya koymuştur. PON1’i kodlayan genin sekansının belirlenmesinin ardından, kişiler arasında enzimin aktivitesinde ve ifade edilme seviyesinde farklılıklara yol açan polimorfizmler tespit edilmesi, PON1 aktiviteleri düşük olan insanların organofosfat zehirlenmelerine karşı daha hassas olabileceğini düşündürmüştür. Son yıllarda yapılan çalışmalarda saflaştırılmış PON1’in organofosfatlı pestisitlere maruz kalan hayvanlara enjekte edilmesi, dolayısı ile serum PON1 seviyesinin yapay olarak arttırılması ile klorprifos ve diazinon gibi bazı organofosfatların toksik etkilerini azaltmanın mümkün olduğu gösterilmiş, ancak bu uygulamanın paration maruziyetine karşı etkili olmadığı görülmüştür. Her ne kadar paraoksonu hidrolizleyen enzim olarak tanınsa da, PON1’in paraoksonaz aktivitesi nispeten zayıftır. Organofosfat maruziyetine karşı koruyucu olarak kullanılabilmesi için PON1’in katalitik verimi arttırılmalı ve yeterli miktarda elde edilebilmelidir. Son yıllarda protein mühendisliği yöntemleri kullanılarak PON1’in bazı amino asitlerinde değişiklikler yapılmış, bu PON1 varyantları bakteriyel sistemlerde yeterli seviyelerde ifade edilmiş ve bazı organofosfatlara karşı enzim aktivitesinde artışlar sağlanmıştır. Bu derlemede, öncelikle organofosfatların genel özelliklerine, etki mekanizmalarına ve zehirlenmelere değinilmiş, ardından PON1’in organofosfat metabolizmasındaki rolüne ve organofosfat zehirlenmelerinde antidot olarak kullanılabilmesine yönelik araştırmalardaki son gelişmeler üzerinde durulmuştur.

Anahtar Kelimeler

• Antidot, insektisit, organofosfat, paraoksonaz, pestisit, PON1

Organophosphate pesticide poisonings and the role of serum paraoxonase 1 PON1 enzyme in organophosphate metabolism

Abstract

According to data from the World Health Organization, nearly 3 million pesticide poisonings occur per year worldwide; 220,000 of these poisonings result in death. Organophosphates, one of the most frequently used pesticides in recent years, are chemicals that effect the nervous system. Organophosphates lead to neurotoxicity by suppressing the enzyme acetylcholinesterase. Paraoxonase 1 PON 1; EC 3.1.8.1 is a calcium dependent glycoprotein enzyme that is found on the high density lipoprotein HDL in serum. Although PON1 activity has been detected in several organs of mammals, paraoxonase activity is close to zero in birds, fish and insects. Among the chemicals that are hydrolized by PON1; the toxic oxon metabolites of organophosphate inseciticides, which are the active forms, such as parathion, diazinon and chlorpyrifos; nerve gases such as sarin and soman; aromatic esters such as phenyl acetate; several aromatic and aliphatic lactones such as homogentisic acid lactone, dihydrocoumarin and homocysteine thiolactone and cyclic carbonates are included. Earlier studies on the toxicology of organophosphate pesticides revealed that having low serum PON1 activity increases the senstivity to the acute effects of organophosphate compounds. Following determination of the sequence of the gene encoding PON1, identification of polymorphisms that lead to differences among people in the activity and level of expression of the enzyme, led to the idea that people with low PON1 activity may be more sensitive to organophosphate poisoning. In studies carried out in recent years, it was shown that injection of purified PON1 to animals that are exposed to organophosphate pesticides, and thus increasing serum PON1 levels artificially, it was possible to reduce the toxic effects of certain organophosphates like chlorpyrifos and diazinon; but this application was observed to be ineffective against parathion exposure. Even though recognized as the enzyme that hydrolyzes paraoxon, paraoxonase activity of PON1 is a bit weak. In studies carried out in recent years, it was observed that injection of purified PON1 protects against chlorpyrifos and diazinon but was not effective against parathion exposure. In order to be used as a prophylactic agent against organophosphate exposure, PON1’s catalytic efficiency has to be increased and the enzyme has to be obtained in adequate amounts. In recent years, using protein engineering methods, changes were made to some amino acids of PON1, these PON1 variants were expressed in bacterial systems at sufficient levels and increase in enzyme activity against some organophosphates were obtained. In this review, first, general aspects of organophosphates, action mechanism and poisonings are addressed; then, the role of PON1 in organophosphate metabolism and recent advances in the research intended for improving PON1 so that it can be used as an antidote in organophosphate poisonings are empasized

Keywords

• Antidote, insecticide, organophosphate, paraoxonase, pesticide, PON1, poisoning

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