Activity, stereospecificity, and stereoselectivity of microsomal enzymes in dependence on storage and freezing of rat liver samples

Liver microsomes are now one of the most widely used in vitro test systems for biotransformation studies of drugs, toxins, and other xenobiotics. The standard procedure of preparation of microsomes from fresh liver taken imme diately after death of the animal is impossible in experiments with liver s amples from human or wild animals and the choice of a relatively optimal wa y of liver storage is necessary in these cases. We studied the possibility of using the stereoselectivity and stereospecificity of biotransformation e nzymes for evaluation of the changes in enzyme function dependent on tissue handling. Activity, stereospecificity, and stereoselectivity of several en zymes in microsomes prepared from fresh liver, frozen liver in liquid nitro gen, or ice-cooled liver were compared. The effect of storage period (2, 3, 5 h) on these parameters were also tested. Both freezing and cooling of li ver change the native function of enzyme systems and could result in incorr ect stereospecificity data for the microsomal, metabolism. All parameters o bserved also differ in their dependence on period of ice cooled storage. As it is difficult to hold strictly to the same storage period, we recommend freezing liver in liquid nitrogen if the storage of liver is necessary. In projects comparing enzyme activities in human and laboratory animals the sa me freezing procedure of liver should be maintained before preparation of m icrosomes from all species. Chirality 12:649-653, 2000. (C) 2000 Wiley-Liss , Inc.