preprohormone

A preprohormone is the very first draft of a peptide hormone, fresh off the ribosome and not yet ready for use. The prefix pre- refers to an extra stretch at the front called a signal peptide — a kind of address label that directs the new chain into the cell's secretory machinery. Once it has done its job, this label is clipped off.

The processing follows a tidy sequence. The ribosome makes the preprohormone; the signal peptide steers the chain into the endoplasmic reticulum and is then removed, leaving a prohormone; further enzymatic cleavage inside vesicles converts the prohormone into the mature, active hormone, which is stored until secretion.

Preproinsulin illustrates the whole pathway: preproinsulin loses its signal peptide to become proinsulin, which is then cut into insulin and C-peptide. Understanding these steps explains why peptide hormones can be made, stored, and activated with such precise control.