Peptide YY (PYY) is a short (36-amino acid) peptide released by cells in the ileum and colon in response to feeding. It is also known as peptide tyrosine tyrosine or pancreatic peptide YY3-36. [1]
Studies have shown an important role of PYY (3-36) in the regulation of appetite control. It is the major circulating form of the peptide, is considered to reduce food intake in humans and rodents via high affinity binding to the auto-inhibitory neuropeptide Y receptor Y2R in the hypothalamic arcuate nucleus (ARC), a brain area responsible for the regulation of food intake. PYY (3-36) may represent a promising tool for the treatment of obesity.
PYY exerts its action through NPY receptors; it inhibits gastric motility and increases water and electrolyte absorption in the colon. [2] PYY may also suppress pancreatic secretion. Several studies have shown acute peripheral administration of PYY3-36 inhibits feeding of rodents and primates. Other studies on Y2R-knockout mice have shown no anorectic effect on them. These findings indicate PYY3-36 has an anorectic (losing appetite) effect, which is suggested to be mediated by Y2R. PYY works by slowing the gastric emptying; hence, it increases efficiency of digestion and nutrient absorption after a meal. Research has also indicated PYY may be useful in removing aluminum accumulated in the brain.

References:
1.Entrez Gene: PYY peptide YY
2.DeGroot, Leslie Jacob (1989). In J. E. McGuigan. Endocrinology. Philadelphia: Saunders. p. 2754.