Preliminary characterization of circulating amino- and carboxy-terminal fragments of parathyroid hormone-related peptide in humoral hypercalcemia of malignancy.

PTH-related peptide (PTHrP) immunoreactivity in plasma from six well characterized patients with humoral hypercalcemia of malignancy was characterized by gel filtration chromatography. An immunoradiometric assay directed against the N-terminal 74 amino acids of PTHrP and a RIA directed against the C-terminal region (amino acids 109-138) of the peptide were used to assay column fractions. When examined using acid (pH 5.0) nondenaturing conditions, N-terminal PTHrP immunoreactivity eluted with an apparent M(r) of 30,000-40,000. The apparent M(r) of this PTHrP fragment shifted to approximately 25,000 when gel filtration was performed at pH 9.0. The apparent M(r) shifted further, to approximately 6,500, when chromatographed under denaturing conditions in 4 M guanidine-HCl. Carboxy-terminal PTHrP immunoreactivity in plasma eluted with an M(r) of approximately 12,000 under acid nondenaturing conditions, as did the synthetic C-terminal PTHrP column marker, PTHrP (109-138). Synthetic PTHrP (1-36) and (1-74), and recombinant PTHrP (1-141) as well as native PTHrP species found in milk and keratinocyte-conditioned medium migrated in their expected positions when analyzed under alkaline nondenaturing or under denaturing condition. We conclude that native, synthetic, and recombinant PTHrP peptides may migrate anomalously when examined using gel filtration under nondenaturing conditions, and such studies should be interpreted with caution. Plasma from patients with humoral hypercalcemia of malignancy contains at least two PTHrP species. One native N-terminal fragment appears, as assessed under denaturing conditions, to have an M(r) of approximately 6,500, and to therefore comprise approximately 50-60 amino acids of full-length PTHrP. A second chromatographically and immunologically distinct C-terminal fragment with an M(r) of approximately 12,000 under nondenaturing conditions is also present.