ASPARAGINE-LINKED OLIGOSACCHARIDE STRUCTURES DETERMINE CLEARANCE AND ORGAN DISTRIBUTION OF PITUITARY AND RECOMBINANT THYROTROPIN

The recombinant human TSH (rhTSH) with highly sialylated oligosacchari de chains showed higher in vivo bioactivity and a lower MCR than the p redominantly sulfated pituitary human TSH (phTSH). The aim of the pres ent study was to investigate the role of terminal carbohydrate residue s in organ distribution and metabolic clearance of TSH using an in viv o rat model. The different I-125-labeled TSH preparations with distinc t carbohydrate composition were injected iv. At various time points (5 -180 min) after bolus TSH injection, blood, liver, kidney, spleen, lun g, heart, and thyroid samples were collected. TSH uptake was determine d by trichloroacetic acid precipitation of [I-125]TSH in the organ hom ogenates. The rhTSH (solely sialylated) was distributed predominantly to the kidneys 5, 15, and 30 min after injection. In contrast, phTSH ( sulfated/sialylated) and bovine TSH (bTSH; solely sulfated) were clear ed predominantly by the liver (at 5 min), with a later renal phase of clearance (at 30 min). Asialo-rhTSH was cleared by the liver with only minor involvement of other organs. The early liver uptake (at 5 min) was proportionally highest for the asialo-rhTSH and bTSH preparations and lowest for rhTSH, which correlated inversely with the serum levels and the degree of sialylation. Blockade of the N-acetylgalactosamine (GalNAc) sulfate receptors by injection of bovine LH resulted in a sig nificant decrease in liver uptake of phTSH. Similarly, liver uptake of asialo-rhTSH was significantly inhibited by injection of asialo-fetui n. Thus, phTSH and bTSH preparations containing sulfated oligosacchari de chains are cleared at least in part by the GalNAc sulfate-specific receptors in the liver. In contrast, rhTSH with highly sialylated olig osaccharides in both subunits accumulates predominantly in the kidneys , even at the early phase of clearance, indicating that sialylated gly coprotein hormones escape from specific receptor-mediated clearance me chanisms in the liver. These data indicate that terminal sialic acid a nd GalNAc sulfate residues, each to a different extent, determine glyc oprotein hormone distribution and thereby plasma level, which as we ha ve shown previously is a major factor in determining the in vivo poten cy of TSH.