Steroid 21-hydroxylase activity in equine ovarian follicles evidenced by isotope dilution-mass spectrometry.

The research shows that the horse ovarian follicle can contribute to the biosynthesis of 11-deoxycorticosterone (steroid hormone) via a 21-hydroxylase enzyme, a process generally associated with the adrenal glands.

Research Objectives and Methods

• The purpose of this study was to investigate if there is any steroid 21-hydroxylase activity in equine ovarian follicles. Steroid 21-hydroxylase is an enzyme that leads to the production of steroid hormones.
• This activity was investigated in the microsome-enriched fraction of follicular linings from horse ovaries using gas chromatography-mass spectrometry, which is a technique to identify different substances within a test sample.
• The resultant 21-hydroxylated metabolites were quantified using isotope dilution with deuterated analogues. Isotope dilution is a method used to determine the quantity of a chemical substance.

Findings

• The two most prevalent potential substrates for follicular steroid 21-hydroxylase are progesterone and 17-hydroxyprogesterone.
• These two substrates were converted to 11-deoxycorticosterone and 11-deoxycortisol respectively. These conversions indicate the presence of 21-hydroxylase activity.
• The research revealed that the conversion of progesterone to 11-deoxycorticosterone showed a higher specific activity and had a lower apparent Km value compared to the conversion of 17-hydroxyprogesterone to 11-deoxycortisol. Km value is a measure of the affinity of an enzyme for its substrate; lower values imply higher affinity for the substrate.
• The study also found that the conversion of progesterone to 11-deoxycorticosterone was inhibited by 17-hydroxyprogesterone and pregnenolone.

Significance and Implications

• This study suggests that the ovarian follicle in horses is an extra-adrenal site for the biosynthesis of 11-deoxycorticosterone, a process typically attributed to the adrenal glands.
• This finding can influence the understanding of steroid hormone biosynthesis in horses and can provide valuable insights into equine reproductive biology. It could also shed light on potential new directions for investigating adrenal gland function and its disorders.