Immunoreactive parathyroid hormone levels in platyrrhini and catarrhini: A comparative analysis with three different...

Serum concentrations of the hormonal form of vitamin D₃-1,25-dihydroxy-vitamin D₃ [1,25-(OH)₂-D₃]-are elevated in many genera of platyrrhines when compared to catarrhines; this elevation is presumed to result from a decrease in the ability of the target cell receptor effectively to recognize 1,25-(OH)₂-D₃. The activity of the renal 25-hydroxyvitumin D₃-1α-hydroxylase, the mammalian enzyme which synthesizes the majority of the circulating 1,25-(OH)₂-D₃, is accelerated by parathyroid hormone (PTH). In order to determine whether the elevated serum concentrations of 1,25-(OH)₂-D₃ in platyrrhines were the result of relative hyperparathyroidism, we measured serum levels of immunoreactive parathyroid hormone (iPTH) in normocalcemic platyrrhines, catarrhines, and human subjects with assays that recognize different domains of the human PTH molecule. Antisera directed against the biologically active, aminoterminus of PTH yielded comparable mean values for iPTH among three test groups. The mean concentration of iPTH as assessed by a "proximal" midregion assay was significantly reduced in platyrrhine serum when compared to either human or catarrhine serum. A "distal" midregion assay yielded a reduced mean value for iPTH in both platyrrhine and catarrhine serum when compared to human serum. These data suggest that 1) high circulating levels of 1,25-(OH)₂-D₃ in New World primates are not the result of hyperparathyroidism; and 2) structural homology between human and primate PTH diminishes progressively as one moves toward the carboxyterminus of the molecule and is lost more rapidly in the platyrrhine than in the catarrhine hormone.