Pineal serotonin N-acetyltransferase: Expression cloning and molecular analysis

Serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, EC 2.3.1.87; AA-NAT) is the penultimate enzyme in melatonin synthesis (serotonin → N-acetylserotonin → melatonin). It plays a key role in vertebrate circadian biology by generating the large day/night rhythm in circulating melatonin. The nocturnal increase in AA-NAT activity not only elevates melatonin production -10-fold but also causes a concurrent 5-fold decrease in pineal serotonin. Here we describe the expression cloning of AA-NAT from the ovine pineal gland. The ovine AA-NAT gene is strongly expressed in the daytime pineal gland and in the retina at 1/4 of the pineal levels. In addition, AA-NAT is expressed at lower levels in the pituitary gland and some brain regions. This raises the possibility that AA-NAT could influence levels of serotonin and other amine substrates in these extrapineal sites, where it might also initiate melatonin synthesis. The abundance of AA-NAT mRNA in the sheep pineal gland increases at night less than 2-fold relative to daytime values. In contrast, in the rat pineal gland AA-NAT mRNA is nearly undetectable during the day and increases >150-fold at night. This indicates that the relative importance of transcriptional and posttranscriptional mechanisms in regulating AA-NAT activity varies between species. Sequence and motif analyses indicate there are no reported orthologs of AA-NAT and that it represents a new gciK family within a large superfamily of acetyltransferases.