Comparison of the nocturnal temporal profiles of N-acetyl-transferase and thyroxine 5'-deiodinase in rat pineal.

The nocturnal temporal patterns of pineal N-acetyltransferase (NAT) and type II thyroxine 5'-deiodinase (5'-D) were compared in the same animals. Both NAT and 5'-D had a similar rise to a midnight zenith with the same lag period but 5'-D rose to its peak 1 h before NAT. The temporal profiles of decrease in enzyme activities were quite different, with 5'-D declining long before NAT. Acute light exposure caused a rapid dramatic decrease in NAT but not in 5'-D. These data suggest that different mechanisms are in involved in the decrease of these two enzyme activities. The role of the antecedent rise in 5'-D activity in the nocturnal rise of NAT activity is apparently minor at most, since pretreatment with iopanoic acid completely blocked the nocturnal rise in 5'-D but had no effect on the nocturnal rise in NAT nor on the acute fall in NAT from its midnight zenith after exposure to light.