ORGANIZATION OF THE CYTOCHROME P-450 SYSTEM

PHASE TWO ENZYMES

ANCIENT EVOLUTIONARY ROLE OF CYTOCHROME P-450?

• Removal of reactive oxygen after evolution of photosynthesis

• Co-occurrence with formation of ferric deposits and the generation of O2 by photosynthesis.

• Later superceded by peroxisomes and then mitochondria.

Phase II Enzymes.

• Series of enzymes that are involved in the conjugation of polar substances to the phase 1 products. Can also conjugate various groups to the unreacted xenobiotic.

• Generally make the xenobiotic more water soluble and therefore more readily excreted and less toxic.

• Since it involves covalent linkage, the conjugation requires high energy activated reactants.

Common Phase II DetoxificationCommon Phase II Detoxification Enzymes:Enzymes:

ConjugationConjugation: : withwith glutathioneglutathione byby glutathionglutathion--SS--transferasetransferase withwith glucuronateglucuronate fromfrom UDPGA (UDPGA (UDPUDP--glucuronicglucuronic acidacid)) withwith sulfatesulfate fromfrom PAPS PAPS (Phosphoadenylylsulfate)(Phosphoadenylylsulfate) withwith acetylacetyl groupgroup fromfrom acetylacetyl CoACoA With glucose from glucose transferaseWith glucose from glucose transferase

MethylationMethylation

Glutathione S-Transferases

• Involved in the transfer of the ubiquitous tripeptide glutathione (gamma glutamy-cysteine-glycine) to electrophilic centers of xenobiotics. Conjugation is via the SH of the cyteine group.

• Has aryl, alkyl and epoxide transferse activity

• Found both cytosolically and microsomally and found in liver

Reduced Glutathione (GSH)

Glutamic Acid

Cysteine

Glycine

UDP-Glucoronosyltransferase

• Conjugation of glucoronic acid (carboxylic acid derivative of glucose) to hydroxl and amino groups of xenobiotics.

• Found in the endoplasmic reticulum and therefore in close proximity to the P-450 enzymes.

• Uses a high energy intermediate UDPGA (Uridinediphosphate glucoronic acid) for the conjugation reaction.

High Energy Substrate for UDP-Glucoronosyltransferase

Sulfotransferases

Conjugates sulfate to hydroxl and amino groups of xenobiotics. Competes with UDP-Glucoronosyltransferase

Uses a high energy intermediate 3’ phospho-adenosine-5’-phosphosulfate (PAPS) formed enzymatically by the reaction of sulfate to ATP.

Found in the cytoplasm of the liver and gills. Very abundant in the digestive glands of molluscs.

High Energy Substrate for Sulfotransferases

Epoxide Hydrolases

• Enzyme that adds water to an epoxide group to form a diol.

• The diol is less electrophilic and chemically reactive

• Found primarily in the liver and hepatopancreas and although mostly microsomal there appears to be some cytosolic activity

• An important enzyme in the activation of various poly cyclic hydrocarbons such as benzo(a)pyrene.

Phase 1 and 2 Biotransformation Enzymes in Aquatic Animals

Patterns of Bioaccumulation of Some Organic Contaminants in Marine Organisms

Phase I and Phase II products can be more toxic than the original parent molecule.

MACROMOLECULAR ADDUCT FORMATION IN MARINE INVERTEBRATES