Enzyme-generated intermediates derived from 4-androstene-3,6,17-trione and 1,4,6-androstatriene-3,17-dione cause a...

Area of Science:

• Biochemistry
• Endocrinology

Background:

• Human placental aromatase is crucial for estrogen biosynthesis.
• Steroids like 4-androstene-3,6,17-trione and 1,4,6-androstatriene-3,17-dione have been identified as competitive inhibitors.
• The precise mechanism of inhibition by these steroids requires further elucidation.

Purpose of the Study:

• To investigate the kinetic evidence for the mechanism of inhibition of human placental aromatase by 4-androstene-3,6,17-trione and 1,4,6-androstatriene-3,17-dione.
• To determine if the inhibition is competitive or time-dependent and involves enzyme-generated intermediates.

Main Methods:

• Enzyme kinetics studies were performed using human placental aromatase.
• The binding affinity (Ki) and rate constants for activity decrease were determined for both steroids.
• Time-dependent inactivation assays were conducted.

Main Results:

• 4-androstene-3,6,17-trione exhibited an apparent Ki of 0.43 microM and a pseudo-first order rate constant of 4.03x10(-3)sec(-1) for activity decrease.
• 1,4,6-androstatriene-3,17-dione showed an apparent Ki of 0.18 microM and a pseudo-first order rate constant of 1.10x10(-3)sec(-1) for activity decrease.
• These kinetic parameters indicate a time-dependent, irreversible inactivation mechanism.

Conclusions:

• The potent inhibition of estrogen biosynthesis by these steroids is primarily due to a time-dependent decrease in human placental aromatase activity.
• Enzyme-generated intermediates from the parent steroids are responsible for this irreversible inactivation.
• The findings suggest a mechanism beyond simple competitive inhibition for these compounds.