Effects of retinal and pineal low molecular weight fractions and antipsychotic drugs on hydroxyindole-O-methyltransferase.

Fractions from bovine retinas and sheep pineals of low molecular weight decrease the activity of purified hydroxyindole-O-methyltransferase (HIOMT). The antipsychotic drugs chlorpromazine and haloperidol show a similar effect under the same experimental conditions. It is suggested that they may imitate the effect of (an) unknown natural suppressor(s) of HIOMT-activity, present in retina and pineal, decreasing the biosynthesis of methoxyindoles.