prohormone

A prohormone is a hormone in waiting — an inactive precursor that must be trimmed before it can do its job. Think of it as a tool shipped with a safety cap: the molecule already contains the working part, but a piece must be cut away to switch it on. Cells make and store hormones in this safe, inactive form and activate them only when needed.

Activation happens by enzymatic cleavage: specific enzymes snip the prohormone at precise points, releasing the mature, active hormone. The classic example is proinsulin, which is cut to yield active insulin plus a leftover fragment called C-peptide. This cutting step is a control point — it lets the cell store a ready supply that can be activated quickly.

The word is also used more loosely for substances the body converts into an active hormone elsewhere, such as vitamin D being processed in the liver and kidney into active calcitriol. In both senses the theme is the same: the secreted or supplied molecule is not yet the final messenger.