The metabolism of labeled parathyroid hormone. III. Studies in rats.

A series of experiments indicated that 125I-parathyroid hormone administered to the rat i.v. was deposited very rapidly in three organ systems only, bone, kidney and the liver. As indicated by increasing solubility in trichloracetic acid, the hormone seemed to be rapidly metabolized at the sites of deposition. The experiments were in part repeated employing quantitative extraction and gel filtration procedures. It was found that intact hormone predominated (82%) in the circulation but was cleared very rapidly: 91, 97, and 99% having left by 10, 30 and 60 minutes respectively after injection. The intact hormone was found at 10 minutes to be principally in kidney, bone, and liver. Although there was evidence of fragmentation preferentially to polypeptides of molecular weights of 6000, 4500 and 2500 these cannot be claimed to represent real molecular entities. By one hour, only a few per cent of the injected hormone remained intact, the bulk having been degraded to fragments of small molecular weight.