Metabolic turnover of sulphated heteroglycans in the gastrointestinal tract, cartilage and skin of the rat

Abstract

Turnover rates of sulphated saccharides in the gastrointestinal tract of the rat were estimated with the aid of [ 35S]sulphate. Two long (16 and 33 days) and three short (3–3.5 days) experiments were made. Entities containing carbohydrate were isolated from the glandular stomach, upper, middle and lower parts of the small intestine and the colon. For comparison, glycosaminoglycans from costal cartilage (chondroitin 4(6)-sulphates) and from skin (dermatan sulphate) were isolated in two of the short experiments. It was noted that, in the gastrointestinal tract, there were large amounts of saccharides which originally belonged to the group of epithelial glycoproteins. These saccharides were composed principally of D-glucosamine, D-galactosamine, L-fucose, D-galactose, sialic acid and sulphate and some also contained D-mannose, D-glucose, L-arabinose and D-xylose, but no uronic acids were detected. The sulphated forms of saccharides were separated from neutral forms by anion-exchange chromatography, and their turnover rates were estimated. The biological half-lives were very short, approximately 14–24 h. The apparent reason for this is that they were secreted into the gut, and in this case the half-life merely reflects the secretion rate of a polysaccharide. In the gastrointestinal tract were also found typical connective tissue polysaccharides, which contain uronic acid, such as hyaluronic acid, heparan sulphate, chondroitin 4-sulphate and dermatan sulphate. The results of the long experiments indicated that the loss of sulphate from the polysaccharides was not constant. The biological half-lives varied considerably, depending on the period of time after sulphate injection during which the estimations were performed. On the other hand, the results of the short experiments indicated that sulphation of polysaccharides begins immediately after the injection of sulphate, and maximal labelling was usually achieved 6–8 h after sulphate injection. Thereafter there were periods of fast decrease in the radioactivity of polysaccharides and periods of slower decrease or even increase in radioactivity. The turnover rates of chondroitin 4-sulphate in costal cartilage and of dermatan sulphate in the skin were also estimated, and similar phenomena were observed. The results were similar in all three experiments. It is difficult to explain these findings. There may be diurnal variations in the rate of biosynthesis of polysaccharides. There may be metabolic heterogenity due to the occurrence of different pools of subjected glycosaminoglycans. Finally the sulphate moieties may metabolise independently of the complete macromolecules to which they are attached.