A HIGH ABUNDANCE MIDREGION SPECIES OF PARATHYROID HORMONE-RELATED PROTEIN - IMMUNOLOGICAL AND CHROMATOGRAPHIC CHARACTERIZATION IN PLASMA

The widespread expression of the gene for PTH-related protein (PTHrP) and the high interspecies conservation of the primary sequence of even the non-PTH-like portion of the protein argue for a vital role(s) for PTHrP in normal physiology. Emerging evidence suggests that PTHrP may be processed into smaller bioactive peptides, but the circulating for ms of PTHrP are not well characterized. We have measured plasma concen trations in well defined patient groups using a RIA directed toward mi dregion PTHrP-(37-74), compared midregion concentrations to amino-term inal and carboxy-terminal PTHrP concentrations in the same patients, a nd further defined the components of midregion PTHrP immunoactivity by high pressure liquid chromatography. Patients with humoral hypercalce mia of malignancy (HHM) had concentrations of PTHrP-(37-74) immunoacti vity of 90 +/- 10 pmol/L (mean +/- SEM), 9-fold higher than PTHrP-(1-7 4) immunoactivity and about 3-fold higher than PTHrP-(109-138) immunoa ctivity. There was no consistent elevation of midregion PTHrP in patie nts with local osteolytic hypercalcemia, hyperparathyroidism, or renal failure, but discrimination of these groups from HHM was less complet e using PTHrP-(37-74) than using PTHrP-(1-74) immunoactivity. By rever se phase high pressure liquid chromatography, plasma PTHrP-(37-74) imm unoactivity in patients with HHM was resolved into three components: 1 ) a major peak coeluting with that found in medium conditioned by cell s transfected with human PTHrP-(1-141), which we have previously seque nced and found to represent a midregion peptide beginning at residue 3 8; 2) a minor peak with both PTHrP-(37-74) and -(1-74) immunoactivity; and 3) another minor peak with PTHrP(37-74), but no PTHrP-(1-74), imm unoactivity. In conclusion, the predominant circulating form of PTHrP in patients with HHM is a midregion species similar or identical to th e peptide beginning at residue 38, which has been shown to be a secret ory form of PTHrP.