Monoamine Oxidase A Mediates Iodotyrosine Formation Induced by Monoamines in Bovine Thyroid Particulate Fraction

Summary

Monoamines are able to increase the thyroid iodine organification in vitro. A predominance of the A form of monoamine oxidase (MAO) has been previously demonstrated to exist in bovine thyroid tissue.

In the present study we have investigated the form of MAO that could be involved in the iodotyrosine formation induced by tyramine, 5-hydroxytryptamine (5-HT) and β-phenylethylamine (PEA) in a bovine thyroid subcellular fraction. The relative capacity of these monoamines to generate H₂O₂ and to incorporate iodine into tyrosine has also been studied.

The MAO A inhibitor clorgyline (10^-9 M) produced a strong inhibition on the iodotyrosine formation induced by tyramine, 5-HT and PEA. In contrast, only a slight reduction was observed with deprenyl as MAO B inhibitor.

Among the three monoamines, tyramine produced the highest H₂O₂ generation and iodotyrosine formation. The lowest Km value obtained was for 5-HT and the highest for PEA. Regarding the Vmax, the lowest value was for 5-HT and the highest for tyramine. The amount of iodine incorporated to tyrosine was not equivalent to the H₂O₂ generated by the monoamines nor to that exogenously added.

Our results indicate that in bovine thyroid tissue mainly the A form of MAO is involved in the monoamine metabolism.

Key words

Iodotyrosine Formation - Thyroid Monoamine Oxidase (MAO) - MAO A-Clorgyline - Deprenyl - Monoamines