Relationship of oligosaccharide modification to the cause of serum thyroxine-binding globulin excess

T₄-binding globulin (TBG), a glycoprotein with four N-glycosyl complex oligosaccharide chains, exhibits sialic acid-dependent microheterogeneity on isoelectric focusing (IEF). Increasing the sialic acid content of TBG increases its anodal IEF mobility and decreases its rate of in vivo degradation, a mechanism for the elevation of serum TBG levels in pregnancy. In this study, the structure of oligosaccharides in TBG from subjects with various conditions associated with TBG excess was determined by measuring the proportion that bound to Concanavalin-A (Con-A). Since oligosaccharides with three or more branches (antennae) attached to the trimannosyl core are excluded from binding to Con-A, the percentage of serum TBG not bound to Con-A (% peak A) represented the portion of TBG molecules with three or more antennae in all oligosaccharide chains interacting with Con-A. Peak A contained the most anodal IEF bands, while the Con-A-bound TBG (peak B) contained the cathodal bands. Serum samples from 10 normal men and 10 premenopausal women did not significantly differ in terms of TBG levels, % peak A, or IEF mobility and were combined as a single group (normal). Eight subjects with elevated serum TBG levels due to inherited TBG excess [62.0 ± 10.1 (±SD) mg/L] or 2 receiving 5-fluorouracil treatment (26.2 and 31.3 mg/L) compared to 20 normal (14.7 ± 3.3 mg/L) had % peak A values and IEF mobility similar to those in normal subjects. On the other hand, high serum TBG levels in 8 women during the third trimester of pregnancy (39.2 ± 5.3 mg/L), 2 women taking oral contraceptives (25.7 and 27.0 mg/L), and 3 women with acute hepatitis (34.8 ± 4.8 mg/L) were associated with significant elevations of % peak A values (5.68 ± 1.73%, 3.31% and 2.41%, and 3.25 ± 0.78%, respectively) compared to those in normal subjects (1.33 ± 0.40%), as well as increased anodal mobility on IEF. Treatment of a man for 3 days with ethinyl estradiol produced similar changes. Using data from densitometry measurements of IEF bands of TBG, the degree of anodal shift was quantitated (anodal index). This index correlated with the % peak A (r = 0.92) in all study subjects. We conclude that increased sites for sialylation, resulting from the increased proportions of triantennary oligosaccharide chains, account for the increased anodal mobility of TBG in hyperestrogenemia and hepatitis. Thus, in these two conditions, a reduced TBG degradation rate resulting from oligosaccharide modification is the likely mechanism of increased serum TBG levels. The lack of change in % peak A values or IEF mobility of TBG from subjects with inherited TBG excess and patients receiving 5-fluorouracil therapy suggests that increased TBG synthesis, rather than changes in oligosaccharide structure or composition, is the likely cause of their elevated serum TBG levels.