Page 175 - The Toxicology of Fishes

Biotransformation in Fishes 155

TABLE 4.1
Phase I and Phase II Enzymatic Activities and Cofactors
Enzyme Cofactors
Phase I
Oxidation
Cytochrome P450 Oxygen, NADPH, cytochrome b 5 (optional)
Flavin-containing monooxygenase Oxygen, NADPH
Monoamine oxidase H 2 O
Aldehyde oxidase NAD +
Alcohol dehydrogenase
Aldehyde dehydrogenase
Cyclooxygenase Arachidonic acid, oxygen
Peroxidase (PGH synthetase, lipoxygenase) Peroxide (lipid-OOH or H 2 O 2 )
Reduction
DT diaphorase NAD(P)H
Hydrolysis
Carboxylesterase H 2 O
Epoxide hydrolase
Phase II
UDP-glucuronosyl transferase UDPGA
Sulfotransferase PAPS
Amino acid conjugation Amino acids (taurine, glycine, glutamine)
Glutathione S-transferase Glutathione
Acetylation Acetyl-coenzyme A
Methylation S-Adenosylmethionine

and prostaglandins (Ryan and Levin, 1990). Inducible CYP forms (CYP1 through CYP4) emerged later
in evolution and are primarily involved in the breakdown (catabolism) of endobiotics as well as xeno-
biotics. The following section attempts to update the phylogeny of each CYP family, addresses what is
known regarding specific regulation, and then discusses what has been discovered regarding substrate
specificities, catalytic function, and, when available, physiological role. Some families and isoforms
have been better characterized than others. It is hoped that informational gaps are identified here that
will stimulate further research in underrepresented CYP families and enhance our understanding of this
important superfamily of enzymes.
The CYP superfamily is ancient, with the ancestral gene having existed more than 3.5 billion years ago
(Nelson et al., 1993). Animals, plants, and microorganisms all contain CYP, and in mammals they have
been identified in all tissues that have been examined. The emergence of new CYP genes results from a
sequence of events, including speciation, gene duplication, divergence, and drift as a function of mutation
and fixation all withstanding evolutionary pressures (Nebert et al., 1989; Nelsen, 1999). CYPs are generally
most prevalent in the liver in association with the endoplasmic reticulum or mitochondria (Peter and Coon,
1991). As of 2004, the human genome has 57 putatively functional full-length CYP genes; Fugu rubripes
(pufferfish) (Nelson, 2003) and Danio rerio (zebrafish) genomes have 54 and at least 81, respectively.
CYP sequences that have been reported to the P450 nomenclature committee are listed on the cytochrome
P450 homepage (http://drnelson.utmem.edu/CytochromeP450.html). The CYPs considered in the same
family display more than 40% amino acid sequence similarity, and those within a subfamily are more
than 55% similar (Nelson et al., 1993). Nomenclature has been standardized so CYP indicates the gene,
followed by an Arabic numeral for the gene family, a capital letter for the subfamily, and an Arabic numeral
for the specific subfamily member; for example CYP1A1 is responsible for the metabolic activation of
benzo(a)pyrene (BaP) in most species, including mammals. The microsomal CYPs responsible for oxi-
dation or metabolism of steroids and xenobiotic metabolism are located in families one through four.
The overall CYP-mediated reaction takes the form of:
+
RH + O + NADPH + H → ROH + H O + NADP +
2 2