zymogen

noun

1. any of a group of compounds that are inactive precursors of enzymes and require some change (such as the hydrolysis of a fragment that masks an active enzyme) to become active

   noun.substance

   Synonyms proenzyme

   Source: WordNet 3.0

2. A mother substance, or antecedent, of an enzyme or chemical ferment; -- applied to such substances as, not being themselves actual ferments, may by internal changes give rise to a ferment.

   noun

   Source: webster1913