What is the difference between Cytosol and S9 fraction?

Cytosol and S9 fraction are obtained from tissue homogenate. However, they differ widely in their functionality and thus have distinct biomedical research applications.

Cytosol is the liquid portion of the cell that remains after removing the nucleus, mitochondria, lysosomes, microsomes, and other membrane-bound organelles through ultracentrifugation. It contains soluble proteins, enzymes, metabolites, and cofactors involved in various cellular processes. Cytosolic preparations are commonly used to study phase II drug metabolism, enzyme activity, signal transduction, and intracellular biochemical pathways.

In contrast, the S9 fraction is produced by centrifuging a tissue homogenate at approximately 9,000 × g, leaving behind a supernatant that contains both the cytosolic components and microsomal vesicles. Because it includes enzymes from both cellular compartments, the S9 fraction supports studies of both phase I and phase II metabolism. It contains important drug-metabolizing enzymes such as cytochrome P450s (from microsomes), along with transferases and other soluble enzymes found in the cytosol.

Based on the research purpose and distinct objective, the cytosol or S9 fraction is chosen. For instance, if the purpose is investigation of the soluble enzymes or intracellular metabolic pathways, cytosol is the preferred preparation. However, when researchers need a broader understanding of drug metabolism, including oxidation, reduction, hydrolysis, and conjugation reactions, the S9 fraction offers a more comprehensive in vitro model.

Thus, cytosol represents soluble intracellular fraction, whereas S9 fraction combines cytosolic and microsomal enzymes.