Page 181 - The Toxicology of Fishes

Biotransformation in Fishes 161

CYP3A56
Killifish
100
CYP3A30
Killifish
98
CYP3A40
Medaka
77
100 CYP3A38
Medaka

CYP3A45
Rainbow trout
100
CYP3A27
Rainbow trout

95 CYP2P3
Killifish
100
CYP2P1
91
Killifish
66 CYP2N2
Killifish
100 CYP2X1
79 Channel catfish

CYP2M1
Rainbow trout
68
CYP2K1
Rainbow trout

CYP1B1
Plaice

CYP1A3
Rainbow trout

CYP1A1
Rainbow trout

FIGURE 4.4 Phylogenic analyses of CYP in fish.

CYP1A—Research on CYPs in fish is more limited than that reported for mammals. Like mammals,
fish CYPs catalyze oxidation of many of the same endogenous substrates and environmental chemicals.
The most studied CYP form in fish is CYP1A. The first fish CYP1A cDNA was cloned by Heilmann
et al. (1988), and sequence analysis showed 57 to 59% and 51 to 53% homology to mammalian CYP1A1
and CYP1A2 genes, respectively. This cDNA hybridized to a 2.8-kb mRNA that was induced 10-fold
by 3-methylcholanthrene (3-MC) in rainbow trout. In fish, this form is generally recognized as CYP1A,
and the terminology is supported because mammalian 1A1 and 1A2 are believed to have diverged 250
MYA by a gene-duplication event (Nebert and Gonzalez, 1987), whereas fish diverged from the mam-
malian line prior to that time. A hybrid CYP1A gene in fish is further suggested because some regions
of the trout CYP1A sequence are identical to all mammalian CYP1A1 but not CYP1A2, while other