Unraveling the aggregation mechanism and bioactivity of Tyr12 nitrated human calcitonin.

The development of therapeutic human calcitonin (hCT) is limited by its irreversible aggregation to form amyloid fibrils. Salmon calcitonin (sCT), a clinical alternative, with only 50 % hCT sequence homology causes severe side effects, making the development of anti-fibrillation hCT analogs more promising in achieving therapeutic goals. It is reported that Tyr12 plays a critical role in inducing hCT fibrils. We also found that tyrosine nitrated hCT (3-NT-hCT) not only inhibits self-assembly of hCT, but also has little effect on its hypocalcaemic potency in vivo. In order to fully elucidate its aggregation features, as well as its role as a therapeutic agent, firstly, we systematically studied the process and mechanism of 3-NT-hCT aggregation and found that tyrosine nitration delayed the aggregation pathways of hCT. Furthermore, we explained the later in terms of structural and charge changes. Secondly, we investigated the bioactivity of 3-NT-hCT based on treating osteoporosis. The development of osteoporosis largely depends on the relative imbalance of osteoclast activity and osteoblast activity. Our results showed that 3-NT-hCT is more effective than hCT in balancing osteoclast and osteoblast activity. These findings suggest that 3-NT-hCT may be a valuable therapeutic substitute for sCT.