Heterogeneity of thyroxine binding by serum albumins in normal subjects and patients with familial dysalbuminemic hyperthyroxinemia.

The nature and properties of the T4-binding albumins in the sera of normal subjects and patients with the syndrome of familial dysalbuminemic hyperthyroxinemia (FDH) were investigated by means of isoelectric focusing in polyacrylamide gels. Albumins isolated from normal sera and sera of patients with FDH displayed multiple protein bands, with isoelectric points between 4.65 and 5.75, and protein patterns were the same in the two groups. In normal albumin, [125I]T4 was consistently localized in two bands, termed B1 and B2, whose isoelectric points (pIs) were 5.44 +/- 0.03 and 5.31 +/- 0.02 (mean +/- SE), respectively. Occasionally, binding of far smaller proportions of [125I]T4 was seen in two additional bands of lower pI (5.22 +/- 0.03 and 4.97 +/- 0.07), termed B3 and B4, respectively. In FDH albumin, [125I]T4 was also localized in four bands whose PIs were almost identical to those of B1-B4 in normal albumin. In FDH albumin, however, bands corresponding to B1 and B2 bound only a minor fraction of [125I]T4, the great majority being bound by bands corresponding to B3 and B4, especially the latter. Identity of the four T4-binding albumins in normal and FDH albumin was suggested by their virtually identical pIs in both types of albumin, by the emergence or intensification of the B3 band in both after extraction of endogenous T4, and by the finding that an 125I-labeled derivative of T4 used in a commercial one-step assay for serum free T4 was bound by the B4 band in both normal and FDH albumin, though more intensely in the latter. Treatment of FDH albumin with increasing concentrations of dithiothreitol (DTT; 0.1-5.0 mM) caused a progressive loss of [125I]T4 binding by B3 and B4, leaving [125I]T4 bound by B1 or B2, and had little effect on the binding of [125I]T4 by B1 and B2 in normal albumin. These effects of DTT appeared to correlate well with earlier dialysis studies at physiological pH which revealed that the same concentrations of DTT decreased or abolished the high affinity binding of T4 in FDH albumin but had little effect on the residual lower affinity binding of T4 in FDH albumin or that characteristic of normal albumin. These findings suggest that T4 binding patterns evident during isoelectric focusing of albumin at low pH have relevance to binding of T4 by albumins at physiological pH.(ABSTRACT TRUNCATED AT 400 WORDS)