Hydrolysis of malathion by ali-esterases in vitro and in vivo.

The structure of malathion, S-[1,2-di(ethoxycarbonyl)ethyl] 00-dimethyl phosphorodithioate, has suggested to many investigators (e.g. Murphy & Dubois, 1957; Seume & O'Brien, 1960) that hydrolases may be involved in the synergism observed by Frawley, Fuyat, Hagan, Blake & Fitzhugh (1957) between malathion and O-ethyl O-p-nitrophenyl phenyl phosphorothioate. On the basis of their known substrate specificities, ali-esterases (carboxylic ester hydrolase, EC 3.1.1.1), cholinesterases and lipases might be expected to contribute to the enzymic hydrolysis of the ethyl esters of malathion. By the same criteria (Main, 1960), serum Aesterases were not considered as likely possibilities. The A-esterases of other tissues were not investigated. Ali-esterases (Myers & Mendel, 1953), also known as 'non-specific esterases' and 'liver esterases', are B-type esterases as defined by Aldridge (1953). The terms 'non-specific esterases' and 'liver esterases' are misleading, since ali-esterases exhibit a definite if broad specificity pattern and occur in many tissues besides the liver. They are probably identical with the B-esterases of the liver and serum and, as will be shown, hydrolyse both aromatic and aliphatic uncharged carboxylic esters. Unlike cholinesterases, they readily hydrolyse simple aliphatic esters such as ethyl butyrate and do not hydrolyse acetylcholine and similar substrates. Serum and liver cholinesterases, like ali-esterases, readily hydrolyse glycerol triesters, such as tributyrin (Strelitz, 1944), and aromatic esters such as phenyl butyrate and o-nitrophenyl butyrate (Main, Miles & Braid, 1961). Lipases appear to work best on emulsified substrates (Aldridge, 1954), whereas ali-esterases hydrolyse substrates in the aqueous phase. Both readily hydrolyse simple aliphatic esters. Cook, Blake & Yip (1958) and Yip & Cook (1959) have observed enzymic hydrolysis of the ethyl esters of malathion by rat-liver homogenates and by extracts of rat-liver acetone-dried powders. However, the authors used crude preparations and did not establish the number and identity of the esterases contributing to their 'nslathionase' activity, nor did they place the hydrolysis on a quantitative basis. Yip & Cook (1959) and, later, Murphy, Anderson & Dubois (1959) have speculated that the inhibition of 'malathionase' might be involved in the synergisms observed between malathion and such compounds as O-ethyl O-pnitrophenyl phenyl phosphorothioate, parathion and tri-o-tolyl phosphate, but it was not always clear what type of hydrolase the authors had in mind when they referred to 'malathionase'. In the present work the contribution of aliesterases, cholinesterases and lipases to the hydrolysis of malathion was investigated. Since considerable overlapping of substrate-specificity patterns occurs between these classes of esterases it seemed desirable to examine the action of each esterase on malathion with preparations essentially free from other esterases. Differentiation in crude tissue preparations was then placed on a more secure basis. For this purpose horse serum cholinesterase and liver ali-esterase from rat and human were partially purified. In addition, a commercial preparation of pancreatic lipase was tested. On the basis of the results obtained in vitro, the hydrolysis of malathion under physiological conditions was investigated.