Inhibition of day time, but not isoproterenol-stimulated pineal N-acetyltransferase activity by an unidentified pineal compound.

Day time rat pineal N-acetyltransferase (P-NAT) activity is higher when homogenate used for incubation is diluted. Samples containing homogenate equivalent to 1/80 pineal gland had highest enzyme activity when compared with samples that contained homogenate equivalent to 1/40 and 1/20 pineal gland, respectively. Thus P-NAT activity is inhibited by a compound present in the pineal gland of the rat. This inhibition was not found in pineal glands of animals that had been injected with isoproterenol. Furthermore day time P-NAT activity is inhibited by excess of substrate, whereas even at 10 mmol/l of substrate isoproterenol stimulated P-NAT activity is not. It is suggested that these two phenomena are involved in the physiologic regulation of the circadian changes of P-NAT activity.