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Interestingly enough, the effect of the digitalis glycosides is inhibited by a high concentration of potassium in the incubation medium and is enhanced by the absence of potassium ( Wolff, 1960 ).

B.

Organification of iodine

The precise mechanism for organification of iodine in the thyroid is not as yet completely understood.

However, the formation of organically bound iodine, mainly mono-iodotyrosine, can be accomplished in cell-free systems.

In the absence of additions to the homogenate, the product formed is an iodinated particulate protein ( Fawcett and Kirkwood, 1953 ; ;

Taurog, Potter and Chaikoff, 1955 ; ;

Taurog, Potter, Tong, and Chaikoff, 1956 ; ;

Serif and Kirkwood, 1958 ; ;

De Groot and Carvalho, 1960 ).

This iodoprotein does not appear to be the same as what is normally present in the thyroid, and there is no evidence so far that thyroglobulin can be iodinated in vitro by cell-free systems.

In addition, the iodoamino acid formed in largest quantity in the intact thyroid is di-iodotyrosine.

If tyrosine and a system generating hydrogen peroxide are added to a cell-free homogenate of the thyroid, large quantities of free mono-iodotyrosine can be formed ( Alexander, 1959 ).

It is not clear whether this system bears any resemblance to the in vivo iodinating mechanism, and a system generating peroxide has not been identified in thyroid tissue.

On chemical grounds it seems most likely that iodide is first converted to Af and then to Af as the active iodinating species.

In the thyroid gland it appears that proteins ( chiefly thyroglobulin ) are iodinated and that free tyrosine and thyronine are not iodinated.

Iodination of tyrosine, however, is not enough for the synthesis of hormone.

The mono- and di-iodotyrosine must be coupled to form tri-iodothyronine and thyroxine.

The mechanism of this coupling has been studied in some detail with non-enzymatic systems in vitro and can be simulated by certain di-iodotyrosine analogues ( Pitt-Rivers and James, 1958 ).

There is so far no evidence to indicate conclusively that this coupling is under enzymatic control.