inducible enzyme

[ĭn-dōōsə-bəl]
n.
  1. An enzyme that is normally present in minute quantities within a cell, but whose concentration increases dramatically when a substrate compound is added.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.