Properties of clock-controlled and constitutive N-acetyltransferases from chick pineal cells

The pineal gland synthesizes its hormone melatonin (O-methyl-N-acetylserotonin) from serotonin. Acetyl-CoA: serotonin N-acetyltransferase (SNAT), the enzyme that catalyzes the committed step in this biosynthesis, is largely restricted to the pineal gland and is regulated by adrenergic and circadian mechanisms. Another enzyme, acetyl-CoA: arylamine N-acetyltransferase (ANAT), having an apparently similar activity, is also present in the pineal. This enzyme, however, is not rhythmically regulated. SNAT activity of cultured chick pineal cells was obtained without ANAT after ammonium sulfate precipitation. ANAT activity was retained without SNAT activity after pre-incubation at 37 degrees C. Thus, each enzyme could be examined independently. Overlap in substrate specificity between the two enzymes was minimal. Kinetic analysis of the separated enzyme activities revealed that while SNAT operates via a random or ordered bi bi mechanism, ANAT catalysis occurs through a ping pong bi bi mechanism with substrate inhibition by acetyl-CoA. By size-exclusion chromatography, ANAT was confirmed to be 30-35 kDa, and SNAT was estimated at 15-20 kDa. Taken together, these results indicate that the two enzymes differ in their structure, reactivity, stability, and mechanism of catalysis.