Prenylated proteins contain one of two isoprenoid lipids, either the 15-carbon farnesyl or the 20-carbon geranylgeranyl, covalently attached to cysteine residues at or near their C terminus. PCYOX1 is capable of cleaving the thioether bond of prenylcysteines. The enzyme was isolated from bovine brain membranes and exhibits an apparent molecular mass of 63 kDa. This is a specific enzyme involved in prenylcysteine metabolism in mammalian cells, most likely comprising the final step in the degradation of prenylated proteins.