Metabolism of Parathyroid Hormone by Isolated Rat Kupffer Cells and Hepatocytes

Data from several laboratories indicate that hepatic mechanisms may have a distinctive role in the metabolism of intact hormone after secretion, a process that accounts, at least partly, for the heterogeneity of circulating parathyroid hormone. Accordingly, we studied the proteolysis of intact hormone by isolated rat Kupffer cells and hepatocytes. Kupffer cells (10(6) cells/ml) and hepatocytes (10(7) cells/ml) were incubated with unlabeled and (125)I-labeled bovine parathyroid hormone at 37 degrees C for periods ranging up to 2 h. When incubated with Kupffer cells, intact hormone disappeared with a t((1/2)) of 12+/-4 min. Radio-immunoassays using sequence-specific antisera showed that the dominant hormonal fragments recovered in the medium have an apparent molecular weight of approximately 6,000, lack amino-terminal antigenic determinants, and react in assays that specifically recognize determinants in the carboxy-terminal portion of the intact hormone. Amino-terminal fragments also were detected in high concentrations, particularly after short incubation periods. Radioiodinated fragments resulting from incubation of (125)I-labeled bovine parathyroid hormone with Kupffer cells had the same apparent size as fragments derived from the metabolism of unlabeled, intact hormone; when analyzed by Edman degradation, positions 34 and 37 of the intact hormone sequence were the amino-terminal amino acids of these dominant carboxy-terminal fragments. Hepatocytes did not hydrolyze the hormone. Thus, metabolism of parathyroid hormone by Kupffer cells results in the appearance of fragments in the media that are immunochemically indistinguishable from, and chemically identical with, those found in plasma when intact hormone is injected intravenously. This indicates that the proteolysis observed in vitro accurately reflects the metabolism of the hormone in vivo. The detection of amino-terminal fragments in concentrations nearly equal to those of carboxy-terminal fragments indicates that cleavage of intact hormone is, initially, by an endopeptidase(s). Kupffer cells may be a source from which specific protease(s) that hydrolyze parathyroid hormone can be characterized, particularly in terms of enzymic specificity and requirements for inhibition. Detailed analysis of the cellular and molecular events during incubation of parathyroid hormone with these cells may help to clarify the biologic significance of the peripheral metabolism of the hormone.