S9 fractions, which include both microsomal and cytosolic elements, are obtained from the supernatant following the first low-speed centrifugation of a tissue homogenate. As they house enzymes from both phase I and phase II pathways, S9 fractions yield a nearly comprehensive array of metabolites. They're routinely employed to assess in vitro intrinsic clearance or to identify metabolites, and when studying phase II reactions, it's necessary to supplement the assay with cofactors such as UDPGA and PAPS.
