Studies on microsomal metabolizing systems for mutagenesis tests. Stability of fortified fractions

Rat liver S-9 microsomal fractions were fortified with NADP+, G-6-P, and MgCl2 and stored at -80 degrees C and -20 degrees C. At various times aminopyrine demethylase and lipid peroxidation were determined and compared with that of non-fortified fractions. No difference in stability was noted between fortified and not fortified fractions. The known stability pattern of non fortified fractions was confirmed. Lipid peroxidation was found blocked in the fractions stored at -80 degrees C whereas a small release of malonyl dialdehyde proceeded in the frozen state at -20 degrees C, contributing to the loss of activity at this temperature. The use of pre-fortified fractions stored at -80 degrees C is justified.