Abnormalities in the microsomal oxidases of the WHO standard reference strain of Musca domestica.

Observations made during biochemical and toxicological studies of the housefly, in which the WHO standard reference (SR) strain was used as a standard, indicated that this strain differs from other strains in certain physiological and toxicological aspects. Experiments are described in which microsomal oxidases prepared from the standard reference strain and several other housefly strains, both susceptible and resistant to insecticides, were compared for heat stability, sensitivity to ionic strength, pH optima, and the spectral characteristics of their cytochrome P-450. In a comparison with 8 strains the SR strain microsomes were more stable at high incubation temperatures (40-47.5 degrees C). Compared to two other strains (NAIDM, susceptible, and Isolan-B, resistant) SR was more stable at low pH and more sensitive to high ionic strength. In spectral comparisons with NAIDM and Isolan-B, SR was found to contain microsomal cytochrome P-450 that differed slightly in its carbon monoxide or octylamine binding spectra. The results suggest that this strain may not provide a suitable reference in studies involving the microsomal oxidases. However, these apparent differences should not reduce the value of the SR stain as a reference in insecticide resistance studies.