The presence and activity of enzymes can be used to determine the health or metabolic state of a cell.  Enzymes are also markers for the cell type since the occurrence and activity of certain enzymes is frequently characteristic of a particular cell.  For instance, the activity of certain enzymes can often be used to distinguish cells of bacterial, plant or animal origin, or to distinguish the identity of tissue from which the enzyme originates. 
Detection of the presence and activity of enzymes can be facilitated by substrates that are converted by the enzyme of interest to a product that has at least one property that can be measured.  These reporter molecules include fluorescent and chromogenic substrates.  Fluorescent substrates have been preferable because, in many cases, they have a very high sensitivity and may permit measurements in living single cells with high spatial and temporal resolution.  Chromogenic substrates can be very specific but often lack a high degree of resolution. 
One family of enzymes useful for measuring the activity of living cells or in extracts of cells is the Cytochrome P450 family.  Cytochrome P450s (CYP450s) are a large family of heme-containing enzymes that, in addition to the endogenous role in cell proliferation and development, includes many catalysts for detoxification and activation of lipophilic xenobiotics including therapeutic drugs, chemical carcinogens and environmental toxins.  In some cases the metabolite(s) is more toxic than the parent compound.  However, in other cases, metabolism of a therapeutic compound reduces the bioavailablity of the compound, lowering efficacy.  This family of genes and the polymorphisms within the family play important roles in the interindividual variation in drug metabolism, occurrence and severity of side effects and therapeutic failures. 
Hundreds of cytochrome P450s have been identified in diverse organisms including bacteria, fungi, plants, and animals (18).  All CYP450s use a heme cofactor and share structural attributes.  Most CYP450s are 400 to 530 amino acids in length.  The secondary structure of the enzyme is about 70% alpha-helical and about 22% beta-sheet.  The region around the heme-binding site in the C-terminal part of the protein is conserved among cytochrome P450s.  A ten amino acid signature sequence in this heme iron ligand region has been identified which includes a conserved cysteine residue involved in binding the heme iron in the fifth coordination site.  In eukaryotic CYP450s, a membrane-spanning region is usually found in the first 15-20 amino acids of the protein, generally consisting of approximately 15 hydrophobic residues followed by a positively charged residue. ( 18, 19.) 
Some of the genes encoding CYP450s are inducible at the transcription level by the compounds they metabolize (1,2).  The genes encoding CYP450s have been divided into families based on homology of deduced amino acid sequences (3).  All mammals share at least 14 CYP450 families but most drug metabolism is catalyzed by only three families: CYP1, CYP2 and CYP3.  Most of the P450 catalyzed drug metabolism in humans takes place in the liver and is accounted for by about 13 enzymes: CYP1A1, CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C18, CYP2C19, CYP2D6, CYP2E1, CYP3A4, CYP3A5 and CYP3A7 (4). 
Because of the central role CYP450s play in drug clearance, toxicity and drug-drug interactions, CYP450s make useful targets for narrowing the field of compounds that should be moved forward in the drug development process (5,8).  Furthermore, knowledge of CYP450/drug interactions can be predictive of drug disposition in a patient.  There is a need for screening assays that can be used in high throughput mode.  Compounds with properties that change in an easily detectable way upon oxidation by a CYP450 are useful as probes in high throughput assays for detecting effects on CYP450 activity (6,7).  There is also need for a method for analyzing metabolic activity in cells under physiological conditions, using a substrate that is specific for CYP450 isozymes and yields products that are easily detectable.  The signal should be detectable in cell-free extracts of cells and in living cells and the assay should have a low background signal. 
Finally, there is a need to protect luciferase activity from its inhibitor inorganic pyrophosphate.  Although the inventors do not intend to limit the source of pyrophosphate, pyrophosphate may be present as a contaminant in orthophosphate salts used in buffers containing a luciferase-based reaction or may be generated as a product of a luciferase reaction with ATP, O2 and luciferin.