The secretion of parathormone fragments 3484 and 3784 by dispersed porcine parathyroid cells

Dispersed porcine parathyroid cells were incubated with radioactive amino acid at 0.5 and 3.0 nw calcium, after which radioactive proteins of the medium were analyzed. In addition to parathormone and secretory protein-I, a carboxylterminal immunoactive fragment peak was detected that could be separated from intact hormone by differential precipitation with 825% trichloroacetic acid. The components of this radioactive peak were analyzed for molecular size, for peptides resulting from tryptic digestion, and for amino acid sequences by Edman degradation. The results showed that two components derived from parathormone were present: fragment 3784 and fragment 3484, in a ratio of about 2:1. These fragments were secreted rather than produced in the medium from secreted hormone, since under the incubation conditions used, there was little loss of exogenous parathormone incubated with the cells and medium. However, omission of serum from the incubation medium (or, more specifically, α²-macroglobulin) led to substantial degradation of hormone and the hormone fragments 3484 and 37-84 and to the production of different immunoactive fragments. In the presence of serum there were about 1.5 mol fragment secreted/mol hormone, an amount that accounted for much of the previously described intracellular losses of parathormone. In addition, the ratio of fragment to hormone was slightly larger at the higher calcium concentration, in keeping with the reported greater intracellular hormone loss under these conditions.