Similarity of the clearance rates of free alpha-subunit and alpha-subunit dissociated from intact human chorionic gonadotropin, despite differences in sialic acid contents.

ABSTRACT

It is well known that the MCR of proteins can be influenced by their carbohydrate structure; i.e. the presence of terminal galactose on proteins results in uptake by hepatic receptors for galactose-terminated glycoproteins. Thus, a protein with its galactose residues covered or removed exhibits a far longer half life in plasma than one with its galactose residues exposed. The free alpha-subunit of human CG (hCG) has been shown to have a different carbohydrate composition than does the alpha-subunit dissociated from the intact hormone. In our laboratory, analysis of alpha-subunits isolated from pregnancy urine indicated that the alpha-subunit dissociated from hCG (hCG alpha) contains primarily monosialylated oligosaccharides, whereas the free alpha-subunit contains more than one sialic acid per oligosaccharide. This difference in the degree of sialylation prompted us to examine the clearance rates of these two subunits. Accordingly, free alpha and hCG alpha were purified by affinity chromatography and labeled with 125I. The labeled subunits were injected iv into rats and serum samples were removed at various time intervals over a 2-h period. The amount of [125I]alpha-subunit remaining in the serum was determined by immunoprecipitation using an antiserum to hCG alpha. The disappearance curves for the two subunits were indistinguishable and could be analyzed by a biexponential model. The t 1/2 of the faster component was 5 min, while the t 1/2 of the slower component was 74 min. In order to determine whether or not terminal galactose was present on either the hCG alpha or the free alpha-subunit, the labeled molecules were subjected to lectin column chromatography using Ricin or peanut lectin linked to agarose. Both of these lectins bind galactose but have different specificities with respect to the penultimate sugar. Both subunit preparations contained only minor amounts of material which could bind or either lectin. However, after desialylation, both hCG alpha and free alpha bound extensively to Ricin, indicating the presence of penultimate galactose residues in both. We conclude that terminal galactose residues are not present on the oligosaccharides of either hCG alpha or free alpha-subunits, and that the difference observed in the sialic acid contents of the two subunits does not affect their rates of clearance.