Microsomes

Microsomes are vesicular fragments of intracellular membranes derived from the endoplasmic reticulum that form when cells are disrupted and the homogenate is subjected to differential centrifugation. They appear as closed vesicles that reseal after disruption and can be isolated as a pellet by ultracentrifugation, typically around 100,000 g. Microsomes reflect both rough endoplasmic reticulum, which bear ribosomes on their cytosolic surface, and smooth endoplasmic reticulum, which lack ribosomes.

Composition and enzymes commonly found in microsomes include the enzymes of the endoplasmic reticulum involved in xenobiotic metabolism, most notably the cytochrome P450 family and associated NADPH-cytochrome P450 reductase, along with other ER-resident enzymes and lipid-synthesizing activities. RM and SM fractions differ in ribosome content, and experimental treatments can modulate ribosome association.

Preparation of microsomes typically involves cell disruption by homogenization or sonication, followed by differential centrifugation and an ultracentrifugation step to collect the microsomal pellet. The resulting preparation is enriched in ER membranes and associated enzymes and can be further purified or separated into rough and smooth fractions under suitable conditions.

Applications and limitations: Microsomes are widely used in vitro to study drug metabolism, enzyme kinetics, and the formation of Phase I metabolites, particularly those mediated by cytochrome P450 enzymes. They provide a controllable system for testing enzyme induction or inhibition and for reconstituting metabolic pathways. Limitations include the absence of intact cellular architecture and transport processes, variability between species and tissues, and dependence on cofactors such as NADPH for enzymatic activity.