Uterine relaxing action of parathyroid hormone: effect of oxidation and methionine substitution.

The effects of bPTH-(1-34), oxidized bPTH-(1-34),[Nle8,Nle18, Tyr34] bPTH-(1-34) amide, and oxidized [Nle8,Nle18,Tyr34]bPTH-(1-34)amide were tested in an in vitro rat uterine assay. When bPTH-(1-34) was treated with hydrogen peroxide (H2O2), the ability of this peptide to reduce oxytocin-stimulated uterine contraction in vitro was no longer evident. An analogue of bPTH-(1-34), in which the methionines at positions 8 and 18 were replaced with norleucine ([Nle8,Nle18,Tyr34]bPTH-(1-34)amide), was capable of reducing oxytocin-stimulated uterine contraction. However, when the [Nle8,Nle18,Tyr34]bPTH-(1-34)amide was oxidized, it retained the ability to reduce uterine contraction. Since we have previously shown that H2O2 oxidation affected only the methionines, these results suggest that the methionines are not necessary for the uterine activity of bPTH-(1-34). We have previously shown that oxidation of bPTH-(1-34) also destroys its blood vessel relaxing activity but has no effect in the rat or the Japanese quail hypercalcemic assays. These data combined with the results of the present studies suggest that the uterine and vascular smooth muscle relaxing properties of bPTH-(1-34) may require the same structural conformation and that this conformation is different from that required for the hypercalcemic action of the peptide.