The effects of hypo- and hyperthyroidism on nuclear, cytosolic, endoplasmic and mitochondrial fractions of sialoglycoproteins in rabbit hepatocytes.

Enhanced sialylation of glycoproteins occurs during hypo- and hyperthyroidism. The role of sialic acid (SA) in cell membranes is well-standing, but its role in intracellular structures is still under analysis. We wanted to evaluate the influence of thyroid hormones on the sialylation ratio of intracellular proteins presented in cytosolic, mitochondrial, endoplasmic and nuclear fractions of rabbit hepatocytes. Twenty-one New Zealand male rabbits were divided into three groups. Hypothyroidism (N = 7) or hyperthyroidism (N = 7) was induced by adding propylthiouracyl (2 mg/l) or L-thyroxine (12 mg/l), respectively, to the drinking water for four weeks. Results were compared with healthy (euthyroid) control animals (N = 7). After isolation of intracellular fractions, standard SDS-PAGE electrophoresis and transfer onto nitrocellulose membrane were performed. Identification of SA residue was carried out with digoxigenin- labelled lectins: Sambucus nigra agglutinin (SNA) and Maackia amurensis agglutinin (MAA). We noticed significantly higher level of SNA than MAA linkage sialoglycoproteins in all evaluated fractions. The sialylation ratio was significantly lower (P < 0.05) in the nuclear fraction in case of hyperthyroidism (detected with both agglutinins). In contrast to the nuclear fraction the content of SNA-detected sialoglycoproteins was significantly reduced in mitochondrial fraction of hyperthyroid hepatocytes (P < 0.05). Non-significant augmentation of MAA-detected sialoglycoproteins was observed in the mitochondrial fractions in both hypo- and hyperthyroidism. The fluctuations of sialoglycoproteins in endoplasmic fraction were not significant. Our work showed that the subcellular structures are rich in SA residues. Differing effects of thyroid hormones on sialylation ratio suggest an important role for hypo- and hyperthyroidism in sialoglycoprotein metabolism.