The hydrophobic C-terminal sequence of transthyretin affects its catalytic kinetics towards amidated neuropeptide Y

Transthyretin (TTR) is a transporter for thyroid hormone and retinol binding protein that has recently been reported to have proteolytic activity against certain substrates, including amidated neuropeptide Y (NPY). However, the proteolytic activity of TTR towards NPY is not fully understood. Here, we used fluorescence-based assays to determine the catalytic kinetics of human TTR towards human amidated NPY. The Michaelis constant (K_M) and catalytic efficiency (k_cat/K_M) of TTR proteolysis were 15.88 ± 0.44 μm and 687 081 ± 35 692 m ^-1·s^-1, respectively. In addition, we demonstrated an effect of the C-terminal sequence of TTR. When the C-terminal sequence of TTR was made more hydrophobic, the K_M and k_cat/K_M changed to 12.87 ± 0.22 μm and 983 755 ± 18 704 m ^-1·s^-1, respectively. Our results may be useful for the development of TTR as a therapeutic agent with low risk of the undesirable symptoms that develop from amidated NPY, and for further improvement of the k_cat/K_M of TTR.