Effects of oxidation of human parathyroid hormone on its biological activity in continuously infused, thyroparathyroidectomized rats.

The effect of oxidation of human parathyroid hormone 1-34 (hPTH 1-34) on the hormone's biological activity was assessed in vivo using a multiparameter, thyroparathyroidectomized (TPTX) rat model. The peptide was oxidized by treatment with hydrogen peroxide, and the oxidized form (8,18-methionine sulfoxide) was isolated by reverse-phase HPLC. Vitamin D-deficient rats were infused with either intact or oxidized hormone along with a 5 mM calcium chloride solution for 4 or 18 hr. Infusion of nonoxidized hormone (0.1-0.8 nmoles/hr) resulted in dose-dependent increases in serum calcium, decreases in serum phosphate, decreases in urine calcium, increases in urine phosphate and cAMP, and increased renal 1,25-dihydroxyvitamin D3 (1,25 (OH)2D3) production. Oxidized PTH infused at doses up to 0.8 nmole/hr had no effect on any of these parameters. To assess the effect of oxidation on the ability of PTH to inhibit the production of the 24,25-dihydroxyvitamin D3 (24,25(OH)2D3), the infusion protocol was performed in vitamin D-deficient rats repleted with 1,25(OH)2D3 by injection. In these experiments, intact hormone markedly suppressed 24,25(OH)2D3 production, whereas the oxidized form was without effect. We conclude that intact methionine residues at positions 8 and 18 of hPTH 1-34 are necessary for all its major biological actions, including its effect on the renal metabolism of 25-hydroxyvitamin D3(25(OH)D3).