THE 69-84-AMINO-ACID REGION OF THE PARATHYROID-HORMONE MOLECULE IS ESSENTIAL FOR THE INTERACTION OF THE HORMONE WITH THE BINDING-SITES WITHCARBOXYL-TERMINAL SPECIFICITY

We evaluated the competitive inhibitory effect of intact PTH, the amin o-terminal PTH(1-34) fragment, and a series of truncated carboxyl-term inal PTH fragments on the binding of internally S-35-labeled human PTH (1-84) ([S-35]hPTH(1-84)) to osteoblastic cells (ROS 17/2.8), in order to identify the minimum and critical elements within the PTH molecule for the interaction with the binding sites specific for the carboxyl- terminal region of the hormone. When the aminoterminal region of the P TH molecule was truncated stepwise, hPTH(35-84), hPTH(53-84) and hPTH( 69-84), but not hPTH(70-84), significantly inhibited the [S-35]hPTH(1- 84) binding. On the other hand, the simple deletion of the carboxyl-te rminal glutamine at position 84 of hPTH(53-84) [hPTH(53-83)] resulted in blunting the inhibitory effect of the peptide on the [S-35]hPTH(1-8 4) binding. Furthermore, hPTH(35-84), hPTH(53-84) and hPTH(69-84), but not hPTH(70-84) nor hPTH(53-83), augmented the inhibitory effect of t he amino-terminal PTH fragment [hPTH(1-34)] on the [S-35]hPTH(1-84)bin ding. Of special interest was that the combination of hPTH(1-34) and h PTH(35-84) reproduced the inhibitory effect of unlabeled hPTH(1-84) on the [S-35]hPTH(1-84) binding, on an equimolar basis. The 69-84 region of the PTH molecule thus appears to be crucial for binding to the car boxyl-terminal specific binding sites for PTH in osteoblasts. The inte raction of the amino-terminal and carboxyl-terminal regions of a PTH m olecule with their own respective binding sites seemed to occur in a f airly independent manner.