Active variants of human parathyroid hormone (1-34) with multiple amino acid substitutions

We used site-directed mutagenesis to construct 55 single-site variants of r hPTH, a recombinantly-expressed form of human parathyroid hormone (1-34) co ntaining three amino acid changes compared to the natural sequence (ML8, ML 18 and FY34). We identified several mutations, at residues Lys(13), Glu(19) , Val(21), Glu(22), Lys(27) and Asp(30), that increase biological activity by up to 2.5-fold, as measured by stimulation of adenylate cyclase activity in rat UMR-106 cells. We constructed a series of 15 variants in which two to eight substitutions at these positions were combined, and found that the mutations behaved additively, leading to peptides with significantly enhan ced potency. The most active combination variant, with six substitutions (K S13, ES19, VQ21, ES22, KQ27 and DN30), is 15 times more active than the par ent molecule. However, the extent to which such combinations increase the a ctivity of the peptide depends critically on the identity of the residues a t positions 8 and 18. We constructed two of the combination variants in a v ariety of sequence backgrounds containing different combinations of leucine , methionine and norleucine at positions 8 and 18. Enhancements in potency were significantly reduced when Met or Nle was present at either of these p ositions, both in UMR-106 cells and human SaOS-2 cells. A corresponding non -additivity was observed in direct measurements of receptor binding affinit y on UMR-106 cells. These results suggest that interactions, either direct or indirect, between certain PTH side chains prevent these mutations from b ehaving in an additive manner. (C) 2000 Elsevier Science Ireland Ltd. All r ights reserved.