The catabolism of hyaluronan has been studied by injecting hyaluronan, labelled with 125I-tyramine cellobiose (125I-TC), subcutaneously into the hindpaw of rabbits. Following endocytosis, 125I-TC remains in the cells at the site of uptake, allowing localization of the site of catabolism. At 6 h after subcutaneous injection, 65% of the injected radioactivity was recovered. The skin at the injection site contained 47%, the popliteal gland at the side of injection 10%, and the liver 8% of the injected dose. At 48 h the three organs contained 40% of the injected dose with 17% in the skin, 10% in the lymph node and 13% in the liver. The decline in recovery could be accounted for by urinary excretion of the tracer, implying that some tracer had been released from the cells after endocytosis. Chromatography revealed that over 85% of 125I-TC-hyaluronan in the lymph nodes and liver was of low molecular mass throughout the experiment. In skin, 4% of the injected tracer was recovered with low molecular mass at 6 h, increasing to 12% of injected dose at 24 and 48 h. Thus, a minimum of 12% of the injected tracer was catabolized per 24 h at the skin injection site. If cells in skin are responsible for the subsequent release of tracer, as seen from the decrease in recovery of the injected dose, another 10-15% of the tracer could have been catabolized locally in the skin per day. The major part of the hyaluronan injected in the skin was, however, catabolized by lymphatic removal and subsequent degradation in local lymph nodes and liver.

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