The 7 cis- or trans-coumaryl permethrate esterases as a possible biochemical marker of pyrethroid resistance in Pediculus humanus capitis

PABLO L. SANTO ORIHUELA, MARÍA I. PICOLLO, EDUARDO N. ZERBA & HÉCTOR MASUH

Buenos Aires

Congreso; International Congress on Phthiraptera; 2006

CIPEIN

Increased hydrolytic metabolism of pyrethroid insecticides has been associated with resistance among Pediculus humanus capitis populations, and particularly with those enzymes that hydrolyze the ester group of the insecticides (esterases). Earlier attempts have been made to synthesize pyrethroid surrogate substrates for determining pyrethroid-cleaving esterase, but often these new substrates vary in structure from the pyrethroids or lack sensitivity. The aim of this work was the synthesis of a new substrate as similar as possible to permethrin and sensitivity enough for studying levels of esterase enzymes in individual head-lice. The cis- and trans-7-hydroxy coumaryl permethrates (7-CP) were synthesized by reacting the corresponding cis- and trans- permetrinic acid chloride with the 7-hydroxicoumarin (7-OHC). The hydrolysis of 7-CP by esterase yields 7-OHC, a fluorescent product which was monitored by spectrofluorometry. Hydrolysis of new substrates by human louse esterase was conducted with permethrin resistant P. humanus capitis (Phc) and susceptible P. humanus humanus (Phh) individual homogenates on 96-well microplates. Fluorescence production of 7-OHC was measured at 440 nm. Histograms of individual activities showed normal distributions, they were different between susceptible and resistant lice. The average activity for the cis- 7-CP was 1.38 pmol/min for Phh and 3.71 for Phc (p<0.05) whereas the trans- isomer showed an activity of 3.28 pmol/min (Phh) and 7.30 pmol/min (Phc) (p<0.01). All the susceptible insects showed activities for the cis- isomer below 1.9 pmol/min and only 8.33 % of resistant insects had activities under this value. With trans- isomer all the susceptible insects had activities lower than 5.3 pmol/min and 12.5 % of resistant lice had activities lower than this number. This increased frequency of individuals with a greater capacity to hydrolyze cis- or trans-7-CP represent a possible biochemical parameter for detecting resistant populations to pyrethroid insecticides.