Abstract
To evaluate quantitatively and qualitatively iodine metabolism and thyroid function of the developing chick embryo, we measured total iodine contents and identified chemical nature of the main iodine components of the egg (white Leghorn) by an autoanalizer for PBI (Technicon) and other analytical methods.
The iodine was predominantly distributed in the yolk (21 μg, 74%) at first, while a small portion (0.9 μg) was found in the white. The contents in the shell (6 pig) and shell membrane (0.4 μg) remained constant during incubation but the latter increased to 1.5 pig on Day 18. After Day 10, when vascularization of the yolk was well developed and the thyroid started to accumulate iodide, yolk iodine decreased abruptly to Day 12 (6.2 pig), then gradually to Day 18 (0.9 μig), while iodine contents in the amniotic and allantoic fluids increased inversely from Day 10 to Macimally Day 15 (14.2 and 8.4 pig, respectively). On Day 18, however, the iodine in the two compartments, particularly in the amnion was reduced (3.9 and 6.9 pig, respectively).
In accordance with those changes, serum iodine in the embryo started to increase gradually from Day 10 (44 μg/ml), tliereafteir vigorously to Day 20 (1, 580 ng/ml). After hatching, serum iodine decreased dramatically probably via urine excretion.
Thyroidal iodine also increased from Day 10 (34 ng/gland) and rapidly from Day 15 (560 ng/gland) to Day 18 (2, 341 ng/gland). Serum T4 level measured by RIA increased gradually to Day 19 (17 ng/ml), whereas T3 showed no marked change.
We then extracted the iodine in the yolk, serum (Day 19) and the two fluids (Day 15) with CHCl3 and MeOH (2:1) and found the iodine more than 80% in MeOH-H2O layer and others were found as protein-bound and lipid-bound iodine. The iodine rich fraction was analyzed by five methods, i) Sephadex G 10 column, ii) Sephadex G 25 column, iii) thin-layer chromatography, iv) cation-exchange chromatography, and v) a fully automated amino acid analyzer. We identified two major components, as iodide and monoiodohistidine (MIH). Besides those, diiodotyrosine was also identified among other minor components. The two major components were identified in amniotic and allantoic fluids as well as in the yolk and serum.
We concluded from the present findings that laying hen ovary concentrates an excess amount of iodine mainly as iodine and MIH into the yolk, and iodide as active source for thyroid hormone synthesis and MIH as an inert iodine compound circulating in compartments of the developing chick embryo. Thus, a role of MIH in the closed environment might be a detoxication mechanism which was subservient to avoid the Wolff-Chaik off efect caused by excess iodide to inhibit thyroid functions.
