We based our first peptide probe design on NPY since it is known to be an excellent substrate of DPP-4.4,26 NPY is a natural 36-amino-acid neuromodulator that is abundant in the brain as well as in the peripheral nervous system.27 NPY can modulate NPY receptors, which have been implicated in many diseases such as metabolic diseases, obesity, pain, cancer, and cardiovascular regulation. Here, NPY is linked to metabolic disease.