Most hormones are either peptides or proteins. They range in size from small peptides having only three amino acids to small proteins (some of which are glycoproteins). For convenience, it is sometimes referred to all these hormones as peptide hormones. They are water-soluble hormones which consist of a few amino acids that introduce a series of chemical reactions to change the cell's metabolism. Examples of peptide hormones include hormones of the pituitary gland and parathyroid glands.
In many cases, peptides hormones are initially synthesized on the ribosomes of the endocrine cells as larger proteins known as preprohormones, which are then cleaved to prohormones by proteolytic enzymes in the rough endoplasmic reticulum. The prohormone is then packaged into secretory vesicles by the Golgi apparatus. In this process, the prohormone is cleaved to yield the active hormone and other peptide chains found in the prohormone. Therefore, when the cell is stimulated to release the contents of the secretory vesicles by exocytosis, the other peptides are co-secreted with the hormone. In certain cases they, too, may exert hormonal effects. In other words, instead of just one peptide hormone, the cell may be secreting multiple peptide hormones that differ in their effects on target cells.
Many peptides serve as both neurotransmitters (or neuromodulators) and as hormones. For example, most of the hormones secreted by the endocrine glands in the gastrointestinal tract (for example, cholecystokinin) are also produced by neurons in the brain, where they function as neurotransmitters.