Characteristics of iodothyronine tyrosyl ring deiodination by rat cerebral cortical microsomes.

We studied the biochemical properties of tyrosyl ring (5-) deiodination of L-T3 and L-T4 by cerebrocortical microsomes from euthyroid rats. Incubations contained radioiodinated L-T3) or L-T4 and dithiothreitol (DTT), and products were analyzed by paper chromatography. The pH maximum was 7.5 for 5-deiodination of both T3 and T4. Increasing the DTT concentration from 5 to 200 mM caused a progressive increase in the 5- deiodination rate of 2 nM T3, but at 500 mM DTT the rate decreased. When three DTT concentrations (5.25, 10.1, and 20.1 mM) were used, double reciprocal plots of 5-deiodination rate as a function of T3 concentration (from 0.3–40 nM) showed a sequential type kinetic pattern, with a limiting Kn of 1.7 nM L-T3. Cerebral microsomes from 4-day-old rats had an apparent Km for L-T3 similar to that of cerebrocortical microsomes from adult rats, but the neonatal tissues had a 3.5- to 10-fold higher apparent maximum velocity. At 50 mM DTT, L-T3 and L-T4 each inhibited 5-deiodination of the other. The ap...

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