repressible enzyme

[rĭ-prĕsə-bəl]
n.
  1. An enzyme whose production is generally continuous but can be halted if a particular substance is present in concentrations greater than normal.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.