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Duality of serotonin-N-acetyltransferase in the
gilthead seabream (Sparus aurata): molecular cloning and
characterization of recombinant
enzymes.
Zilberman-Peled B, Benhar I, Coon SL, Ron
B, Gothilf Y.
Department of Zoology, George S. Wise Faculty
of Life Sciences, Tel Aviv University, Tel Aviv,
Israel.
Serotonin-N-acetyltransferase
(arylalkylamine-N-acetyltransferase, AANAT) is the key enzyme in the
biosynthesis of melatonin in the pineal gland and retinal
photoreceptors. Rhythmic AANAT activity drives rhythmic melatonin
production in these tissues. The presence of two AANATs, AANAT1 and
AANAT2, has been previously demonstrated in three fresh water teleosts.
This duality, the result of early gene duplication, is unique to teleost
species. In this study, the cDNAs encoding for AANAT1 and AANAT2 were
cloned from a marine fish, the gilthead seabream (sb, Sparus aurata).
Northern blot hybridization analysis indicates that sbAANAT1 and
sbAANAT2 are exclusively expressed in the retina and pineal gland,
respectively. Bacterially expressed recombinant sbAANATs exhibit
differential enzyme kinetics. Recombinant retinal sbAANAT1 has
relatively high substrate affinity and low activity rate; it is
inhibited by high substrate and product concentrations. In contrast,
recombinant pineal sbAANAT2 exhibits low substrate affinity and high
activity rate and is not inhibited by substrates or products. The two
recombinant enzymes also exhibit differential substrate preference.
Retinal sbAANAT1 acetylates a range of arylalkylamines while pineal
sbAANAT2 preferentially acetylates indoleethylamines, especially
serotonin. The different spatial expression patterns, enzyme kinetics,
and substrate preferences of the two sbAANATs support the hypothesis
that, as a consequence of gene duplication, teleosts have acquired two
AANATs with different functions. Pineal AANAT2 specializes in the
production of large amounts of melatonin that is released into the
circulation and exerts an endocrine role. Retinal AANAT1, on the other
hand, is involved in producing low levels of melatonin that execute a
paracrine function. In addition, retinal AANAT1 may carry out an as yet
unknown function that involves acetylation of arylalkylamines other than
serotonin. Copyright 2004 Elsevier Inc.
PMID: 15302263 [PubMed -
indexed for MEDLINE]
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