Rat thyroid monoamine oxidase (MAO) is regulated by thyrotropin: evidence that the main form of the enzyme (MAO A) is not directly involved in iodide organification

CABANILLAS ANA M, MASINI-REPISO ANA M, COLEONI ALDO H

JOURNAL OF ENDOCRINOLOGY

The Dorset Press, Dorchester

Lugar: Dorset, Great Britain; Año: 1991 vol. 131 p. 25 - 25

0022-0795

dir> The characteristics and regulation of monoamine oxidase (MAO) were studied in rat thyroid tissue. A measured Michaelis constant (Km) value of 102 mumol/l was similar to the Km values found in other tissues. Maximal velocity (Vmax) was 1.028 nmol/mg protein per min. It is known that MAO is present as two isoenzymes, A and B, which are sensitive to clorgyline and deprenyl respectively. The in-vitro effect of graded concentrations of these selective MAO inhibitors was used to estimate the relative proportion of A and B isoenzymes. Clorgyline strongly decreased thyroid MAO activity at concentrations as low as 1 pmol/l while the effect of deprenyl was observed only at concentrations higher than 10 mumol/l. These results indicated that MAO-A is the main form of the enzyme in the rat thyroid. In-vivo administration of L-thyroxine (5.6-224 nmol/kg) significantly reduced thyroid MAO activity at doses equal to or greater than those which have be