Page 226 - The Toxicology of Fishes

206 The Toxicology of Fishes

ATP Taurine
OH CoASH O HN CH 2CH 2SO 3H

O S–CoA O
Ligase
Benzoic acid Aminoacyl Benzoyltaurine
transferase
FIGURE 4.20 The pathway of conjugation of carboxylic acids in fish, illustrated with benzoic acid. The benzoic acid
forms a coenzyme A thioester, in a reaction catalyzed by acyl–CoA ligase. The benzoyl–coenzyme A thioester then reacts
with taurine, catalyzed by aminoacyl transferase, to give benzoyltaurine.

mitochondria. Two enzymes are involved, a carboligase that catalyzes the formation of an acyl-coenzyme
A (CoA) intermediate and an aminoacyltransferase that catalyzes formation of the amide bond (Vessey,
1996). Figure 4.20 shows this pathway with benzoic acid as the substrate and benzoyltaurine as the
product. Studies of the first step of this reaction—formation of the acyl-coenzyme A—have not been
carried out in fish. The second step, catalyzed by acyl-CoA–taurine aminoacyltransferase, is located in
the mitochondrial matrix in those fish that have been studied, and kidney had higher activity than liver
(James, 1976). The activity has been studied with phenylacetyl-coenzyme A as the substrate and taurine
as the cosubstrate. Even when the incubations were supplemented with glycine, no glycine conjugates
were found.

Enzymes Specificity, Regulation, and Inhibition
Taurine conjugation is generally considered to be a detoxication pathway. Taurine conjugates are acidic
and exist largely in the anionic form at physiological pH. It may be expected that they will therefore be
readily excreted in urine. The taurine conjugate of benzoic acid was not, however, as good a substrate
for the southern flounder renal organic acid transporter as hippuric acid (benzoylglycine), even though
no hippurate was formed in flounder given benzoate (James and Pritchard, 1987). It was, nevertheless,
excreted more readily than benzoic acid. Xenobiotic substrates for this pathway include 2,4-dichlorophe-
noxyacetic acid (2,4-D), 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), the herbicide triclopyr, and p-amino-
benzoic acid, a metabolite of the fish anesthetic tricaine methane sulfonate. Endogenous substrates
include bile acids, benzoic acid, and phenylacetic acid, the latter being metabolites of aromatic amino
acids. Southern flounder have been shown to excrete taurine conjugates of benzoic acid (James and
Pritchard, 1987), 2,4-D, and 2,4,5-T (James, unpublished data). Dogfish shark excreted 2,4,5-trichlo-
rophenoxyacetyltaurine in urine following a dose of 2,4,5-T (Guarino et al., 1977). Winter flounder
excreted phenylacetyltaurine and 2,4-D-taurine after doses of phenylacetic acid and 2,4-D, respectively
(James and Bend, 1976; Pritchard and James, 1979). Channel catfish excreted benzoyltaurine in urine
after administration of benzoic acid (Plakas and James, 1990). Triclopyr has not been examined in fish,
but a crustacean, the crayfish, retained triclopyr as the taurine conjugate in hepatopancreas and slowly
excreted the conjugate (Barron et al., 1991). No studies have been performed on the regulation of this
pathway in fish. Inhibitors and activators of the pathway are not known.

Acetylation
Overview
This pathway utilizes acetyl-CoA to donate an acetyl group to an amino group in a xenobiotic. Xenobiotic
substrates include arylamines, aliphatic amines, and hydrazines. The reaction pathway with 2-amino-
fluorene as the substrate is shown in Figure 4.21. The enzymology of N-acetylation has been extensively
studied in mammals, but few studies of acetylation in fish have been performed, beyond demonstration
of the pathway with model substrates (de Wolf et al., 1993; Gregus et al., 1983). Acetylation is also an
important terminal step in the formation of mercapturic acid metabolites from glutathione conjugates
(Figure 4.11). It is thought, however, that the enzyme that acetylates cysteine conjugates is different
from the N-acetyltransferase that acetylates xenobiotics containing amino groups that are not part of an
amino acid.