Phosphorylation of parathyroid secretory protein.

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Abstract
The phosphorylation of proteins released into the medium of bovine parathyroid gland slices or isolated cells incubated with 32Pi was investigated. The primary protein phosphorylated had a MW of 68,000 and coeluted with newly synthesized parathyroid secretory protein (PSP) on Bio-Gel chromatography and on polyacrylamide gel electrophoresis. Isoelectric focusing of double-labeled samples ([35S]methionine and 32Pi) revealed comigration of the 2 radioactive markers at a pH of 4.6, which was similar to that of purified PSP. Phosphorylation of the MW 68,000 protein was also demonstrated in cell homogenates incubated with [.gamma.-32P]ATP; the MW 68,000 protein was the predominant labeled protein. Increasing quantities of Ca, with and without added ethylene glycol bis-(.beta.-aminoethyl ether)N,N,N'',N''-tetraacetic acid, caused a progressive decrease in phosphorylation of the protein. PSP is readily phosphorylated in parathyroid cells, the degree of phosphorylation is inversely proportional to Ca concentration, and PSP is the major phosphorylated protein released from the gland. The relationship of phosphorylation to the potential physiologic importance of PSP remains to be determined.