Fenvalerate isomers formation

Finally, the enzymatic nature of CPIA-cholesterol ester formation will be briefly mentioned. None of the enzyme preparations of three known biosynthetic pathways for cholesterol esters, namely, acyl-CoA cholesterol Q-acyltransferase (ACAT), lecithin cholesterol 0-acyltransferase (LCAT), nor cholesterol esterase, was effective in producing CPIA-cholesterol ester from the Ba isomer or CPIA. In contrast, the 9,000 g supernatant or microsomal fractions from liver or kidney homogenate were found to be capable of producing CPIA-cholesterol ester without the addition of any cofactors. As substrate, only the Ba isomer was effective, and none of the 3 other fenvalerate isomers nor free CPIA was effective. The hepatic enzyme preparation also catalyzed hydrolysis of fenvalerate, and in this case all the 4 isomers were utilized as substrates. These facts imply that CPIA-cholesterol ester is formed from the Ba isomer through a transesterification reaction via intermediary acyl-enzyme complex. 

Kaneko, H., M. Matsuo, and J. Miyamoto. 1986. Differential metabolism of fenvalerate and granuloma formation. I. Identification of a cholesterol ester derived from a specific chiral isomer of fenvalerate. Toxicol. Appl. Pharmacol. 83 148-156. 

In addition, three types of lipophilic conjugates have been found in pyrethroid metabolism studies (Fig. 4). They are cholesterol ester (fenvalerate) [15], glyceride (3-PBacid, a common metabolite of several pyrethroids) [16], and bile acid conjugates (fluvalinate) [17]. It is noteworthy that one isomer out of the four chiral isomers of fenvalerate yields a cholesterol ester conjugate from its acid moiety [15]. This chiral-specific formation of the cholesterol ester has been demonstrated to be mediated by transesterification reactions of carboxylesterase(s) in microsomes, not by any of the three known biosynthetic pathways of endogenous cholesterol esters... 

Diastereomeric racemates of a-cyanobenzyl esters or allethronyl esters not only have different tendencies of formation or energy contents, but also different solubilities in the case of the crystallizable isomers. This can be utilized to separate one isomer by its crystallization out of a solution or melt of the mixture [816]. The other one can subsequently be epimerized by basic treatment [817], thus forcing the overall reaction by a second order asymmetric transformation [818] to yield only one racemate, containing the insecticidal isomer, if this is the less soluble one. Unfortunately however, the opposite is true for allethrin [818a], cycloprothrin [819], trans-cypermethrin [820], other analogues of fenvalerate (Table 105) [821] and fluvalinate. 

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