From The Collaborative International Dictionary of English v.0.48:
proenzyme \pro"en*zyme\, n. [.] (Bioch.)
Any one of a class of proteins that are converted, in the
normal course of cellular metabolism, into one or more active
enzymes; also called zymogen. The conversion usually is due
to a specific cleavage of a peptide bond by another enzyme,
or may be due to acid.
[PJC]
From The Collaborative International Dictionary of English v.0.48:
Zymogen \Zym"o*gen\, n. [Zyme + -gen.] (Physiol. Chem.)
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.
[1913 Webster]
The pancreas contains but little ready-made ferment,
though there is present in it a body, zymogen, which
gives birth to the ferment. --Foster.
[1913 Webster]