Identification of specific histidine residues and the carboxyl terminus are essential for serotonin N-acetyltransferase enzymatic activity

Carboxy-terminal mutations of bile acid CoA:N-acyltransferase alter activity and substrate specificity

Bile acid CoA:amino acid N-acyltransferase (BAAT) is the terminal enzyme in the synthesis of bile salts from cholesterol and catalyzes the conjugation of taurine or glycine to bile acid CoA thioesters to form bile acid N-acylamidates. BAAT has a dual localization to the cytosol and peroxisomes, possibly due to an inefficient carboxy-terminal peroxisomal targeting signal (PTS), -serine-glutamine-leucine (-SQL). Mutational analysis was used to define the role of the carboxy terminus in peroxisomal localization and kinetic activity. Amidation activity of BAAT and BAAT lacking the final two amino acids (AAs) (BAAT-S) were similar, whereas the activity of BAAT with a canonical PTS sequence (BAAT-SKL) was increased >2.5-fold. Kinetic analysis of BAAT and BAAT-SKL showed that BAAT-SKL had a lower Km for taurine and glycine as well as a greater Vmax There was no difference in the affinity for cholyl-CoA. In contrast to BAAT, BAAT-SKL forms bile acid N-acylamidates with β-alanine. BAAT-S immunoprecipitated when incubated with peroxisomal biogenesis factor 5 (Pex5) and rabbit anti-Pex5 antibodies; however, deleting the final 12 AAs prevented coimmunoprecipitation with Pex5, indicating the Pex5 interaction involves more than the -SQL sequence. These results indicate that even small changes in the carboxy terminus of BAAT can have significant effects on activity and substrate specificity.

Molecular cloning of a cDNA encoding arylalkylamine N-acetyltransferase from the testicular system of Periplaneta americana: primary protein structure and expression analysis

DNA encoding a fragment of putative arylalkylamine N-acetyltransferase (NAT) of the American cockroach, Periplaneta americana, was amplified by PCR with degenerate primers based on the two peptides previously purified from the testicular system of this species. A full clone was obtained by RACE-PCR. The clone consisted of 89 bp 5'-UTR, 753-bp open reading frame and 712-bp 3'-UTR. The amino acid sequence of 251 residues deduced from this ORF corresponded to the predicted molecular mass of 28.5 kDa. The predicted amino acid sequence had an overall identity of 35% with NAT1 and 27% with NAT2 of Drosophila melanogaster, respectively. Structural analysis revealed that NAT from P. americana contained two motifs characteristic of the NAT superfamily and three conserved regions (C/c-1, D/c-1, D/c-2) distinguishing aaNAT subfamily. Northern blot analysis showed that the mRNA of approximately 1.5 kb was transcribed at a high level in the testicular system, and corresponded to the length of the cDNA, i.e., 1,554 bp. Significant levels of NAT transcript were also detected in the midgut, ovary and the accessory glands and at much lower levels in the fat body and brain. Southern blot analysis suggested the presence of a single copy of the cloned gene in the genome.

Purification of the recombinant human serotonin N-acetyltransferase (EC 2.3.1.87): further characterization of and comparison with AANAT from other species

Melatonin is synthesized by a series of enzymes, the penultimate one, serotonin N-acetyltransferase, catalyzing the limiting reaction. In the present study, we compared the recombinant serotonin N-acetyltransferases from rat, ovine, and human. The human protein is particularly difficult to purify because it interacts strongly with a putative chaperone protein from bacteria whereas the rat and sheep enzymes, which interact less strongly with this protein, have been purified close to homogeneity. We identified the contaminating protein as GroEL, the bacterial equivalent of Hsp60. We present numerous catalytic activities (substrate and cosubstrate specificities as well as inhibitor specificities) measured on the three species enzymes from which we deduced that the presence of the chaperone might partly explain the differences between the various species enzyme characteristics, beside the inter-species ones resulting from sequence differences. Despite several trials reported in the literature, a purification to homogeneity of the human (recombinant) enzyme has never been described. We present a new purification method, by using an original denaturation/renaturation process in which the enzyme is immobilized on an affinity chromatography column. The enzyme is then eluted in an active and pure form (i.e., absence of chaperone). The up-scaled system permitted us to perform the necessary experiments for the measurement of more accurate affinities of human serotonin N-acetyltransferase towards its main natural substrates, showing that only the activity of the enzyme towards phenylethylamine was modified.

An intramolecular disulfide bridge as a catalytic switch for serotonin N-acetyltransferase

Serotonin N-acetyltransferase (EC. 2.3.1.87) (AA-NAT) is a melatonin rhythm-generating enzyme in pineal glands. To establish a melatonin rhythm, AA-NAT activity is precisely regulated through several signaling pathways. Here we show novel regulation of AA-NAT activity, in which an intramolecular disulfide bond may function as a switch for the catalysis. Recombinant AA-NAT activity was irreversibly inhibited by N-ethylmaleimide (NEM) in an acetyl-CoA-protected manner. Oxidized glutathione or dissolved oxygen reversibly inhibited AA-NAT in an acetyl-CoA-protected manner. To identify the cysteine residues responsible for the inhibition, AA-NAT was first oxidized with dissolved oxygen, treated with NEM, reduced with dithiothreitol, and then labeled with [(14)C]NEM. Cys(61) and Cys(177) were specifically labeled in an acetyl-CoA-protected manner. The AA-NAT with the Cys(61) to Ala and Cys(177) to Ala double substitutions (C61A/C177A-AA-NAT) was fully active but did not exhibit sensitivity to either oxidation or NEM, whereas the AA-NATs with only the single substitutions retained about 40% of these sensitivities. An intramolecular disulfide bond between Cys(61) and Cys(177) formed upon oxidation and cleaved upon reduction was identified. Furthermore, C61A/C177A-AA-NAT expressed in COS7 cells was relatively insensitive to H(2)O(2)-evoked oxidative stress, whereas wild-type AA-NAT was strongly inhibited under the same conditions. These results indicate that the formation and cleavage of the disulfide bond between Cys(61) and Cys(177) produce the active and inactive states of AA-NAT. It is possible that intracellular redox conditions regulate AA-NAT activity through switching via an intramolecular disulfide bridge.