Peptide hormones or protein hormones are hormones whose molecules are peptides, or proteins, respectively. The latter have longer amino acids chain lengths than the former. These hormones have an effect on the endocrine system of animals, including humans. Most hormones can be classified as either amino acid–based hormones (amine, peptide, or protein) or steroid hormones. The former are water-soluble and act on the surface of target cells via second messengers; the latter, being lipid-soluble, move through the plasma membranes of target cells (both cytoplasmic  and nuclear) to act within their nuclei. The four most biologically active peptides are:

i.                    Insulin

ii.                  Oxytocin

iii.                Vasopressin

iv.                Angiotensin II

Where is insulin produced? 

Pancreas

Insulin is a hormone made by an organ located behind the stomach called the pancreas. There are specialized areas within the pancreas called islets of Langerhans (the term insulin comes from the Latin insula that means island). The islets of Langerhans are made up of different type of cells that make hormones, the commonest ones are the beta cells, which produce insulin.