Phosphorylation of parathyroid hormone by human and bovine parathyroid glands

Human and bovine parathyroid gland slices were incubated in vitro for varying time periods with inorganic 32P and [35S]methionine or [3H]serine. Tissue was then extracted with aqueous medium, and parathyroid hormone (PTH) purified. Incorporated 32P was found to coelute with immunoreactive PTH in multiple chromatographic systems, and a peak of phosphorylated material could be resolved from the nonphosphorylated hormone by reversed phase high pressure liquid chromatography. The amino acid composition of both the phosphorylated and the nonphosphorylated entities conformed to that of the major glandular species of PTH, and phosphorylated hormone accounted for 10-20% of the total. A time course revealed slow incorporation of 32P into hormone, and after a 4-h preincubation with inorganic 32P, co-elution of 32P with both PTH and its precursor was observed. Phosphoserine was identified in purified PTH labeled with [3H]serine. Additionally dilute acid hydrolysis of PTH, labeled with [35S]methionine and containing 32P, generated an 35S-labeled fragment with which 32P co-chromatographed. The results are consistent with in vitro phosphorylation of PTH on serine residues within the NH2-terminal region of the hormone by both human and bovine glands and suggest that phosphorylation of the prohormone occurs as well.