Role of thyroid peroxidase (tpo) in the iodination of thyroglobulin

1441

Cl3 AN ALTERED DIRECTS

THYROGLOBULIN

GOITER OF AFRIKANDER CATTLE V.P.N. Tassi, A. POLYPEPTIDES. ed OncoR. Di Lauro and C.G. Alvino*, Centro di Endocrinoloqia

THE SYNTHESIS

Nazzaro, loqia Sperimentale

mPNA IN CONGENITAL

OF TWO ABNORMAL

de1 C.N.R.,

I-80131 Naples,

Italy.

Since we still lack a basic understanding of the genetic lesion underlying the biochemical defect in this goiter, we focused our attention whether

or not these abnormal

polypeptides

were encoded

by the same mRNA.

In vitro translation of congenital goiter tissue mRNA directs the synthe-sis of two (M,% 250,000 anda75,000, respectively) abnormal thyroqlobulin-like lowing:

polypeptides.

Common

features

1) both are immunoprecipitated

antibody;

2) the signal

sequence

of these polypeptides by purified,

is present

chains and 3) their synthesis

is apparently

lecular

the goiter

weight mRNA. However,

are the fol-

anti-thyroqlobulin

on their nascent directed

polypeptide

by the same high mo-

thyroqlobulin

mRNA shows, by

Northern blot analysis (using thyroglobulin cDNA clones covering both 3' and 5' regions of the qenome), a slightly reduced size (~30s) as compared to the mRNA from normal, to 15-fold

decrease

mRNA concentration tic defect

bovine,

(as assessed was observed.

in congenital

goiter

thyroid

tissue

(33s). Furthermore,

by dot blot hybridization) Thus,

it can be concluded

of Afrikander

cattle

that the qene-

results

thesis of an altered mRNA which is present in decreased des two abnormal thyroqlobulin-like polypeptides.

a lo-

in relative in the syn-

amount and enco-

Cl4 ROLE OF THYROID PEROXIDASE (TPO) IN THE IODINATION OF THYROGLOBULIN. A. Taurog* and M. Dorris, Univ. of Texas Health Science Ctr, Dallas, TX 7523F When qoiter Tg is iodinated chemically with Is- or enzymatically with TPO to comparable levels of iodination exceeding about 15 atoms of I per molecule, the enzymatically iodinated Tg displays a markedly greater content of It is well established that TPO catalyzes the coupling reaction T4. between DIT residues in Tg (Lamas, Dorris and Taurog, Endocrinol. 90:1417, 1972), and this is undoubtedly a major factor in the observed increase in Another possible mechanism, which has generally been assumed, is that T4. the enzyme-catalyzed reaction preferentially selects the more hormonogenic tyrosyl residues of Tg for iodination. While this is a reasonable assumpTo explore this possibility we iodinated tion, it lacks firm evidence. human goiter Tg at matched low levels of iodination (about 8 atoms I/molecule) both chemically and enzymatically, and we determined the initial iodoamino acid distribution and the increase in T4 and Ts that occurred on incubation of the iodinated Tg with TPO + glucose-glucose oxidase. In several experiments we observed a significantly greater increase in T4 and Ts with the enzymatically iodinated Tg. This was more pronounced in the presence of 1 uM free DIT. Enzymatic iodination also favored the formation of DIT in goiter Tg. This did not seem to be attributable to a difference in mechanism between enzymatic and chemical iodination because favored formation of DIT was not observed when tyrosine was iodinated chemically and enzymatically. Our results support the view that TPO-catalyzed iodination of Tg displays some degree of selectivity toward hormonogenic residues, tyrosyl compared to chemical iodination. Peptide mapping experiments are in progress to examine this question further.