Thyroid hormones and brain monoamine oxidase activity: an in vitro study

In vitro experiments were made to study the effect of L-thyroxine and L-triiodothyronine upon the activity of whole rat brain monoamine oxidase. Using tryptamine as the substrate, evidence was obtained for the presence of a preformed endogenous inhibitor of monoamine oxidase. The activity of the inhibitor was suppressed by the thyroid hormones so as to increase monoamine oxidase activity and indoleacetic acid formation. This effect of L-thyroxine and L-triiodothyronine was concentration related, temperature dependent, reversible, stereochemically independent (obtained with D and L forms), and was critically dependent upon pre-incubation of the hormones with the homogenate prior to substrate addition. Additionally, the effect of the thyroid hormones was absent in a washed crude preparation of brain mitochondria but could be restored by replacing certain of the removed cellular components. No evidence of inhibitor activity was found using kynuramine as the substrate. Neither L-thyroxine nor L-triiodothyronine influenced product formation (4-hydroxyquinoline). However, mixed substrate experiments revealed that tryptamine could inhibit kynuramine deamination but the extent of this inhibition was not modified by prior incubation with the thyroid hormones. Thus, L-thyroxine and L-triiodothyronine appear to modulate the activity of a tryptamine sensitive monoamine oxidase which is not involved with the deamination of kynuramine. Studies made in whole tissue homogenates from liver, kidney and heart revealed an organ-related selectivity in the ability of the thyroid hormones to enhance indoleacetic acid production from tryptamine; cardiac monoamine oxidase activity being hardly affected. Thus, the effectiveness of thyroid hormones upo- monoamine oxidase activity is characteristic of the organ. From studies made with brain, it was shown that the activity of the thyroid hormones toward monoamine oxidase activity did not vary with a progressive increase in animal weight and thus appears age-independent. The present study has described, for the first time, (1) the presence of a tissue inhibitor or inhibitors for a tryptamine-sensitive monoamine oxidase and (2) the suppression of inhibitor activity by the thyroid hormones. These new in vitro findings may have relevance to thyroid control mechanisms on monoamine oxidase activity in vivo.