Steroid interactions with cytochrome P-450 from testis microsomes

Area of Science:

• Biochemistry
• Endocrinology
• Molecular Biology

Background:

• Cytochrome P-450 enzymes in gonadal microsomes are crucial for steroid hormone synthesis, specifically 17 alpha-hydroxylase and 17,20-lyase activities.
• Steroid substrate interaction with cytochrome P-450 causes measurable spectral shifts, indicating binding events.

Purpose of the Study:

• To investigate the substrate binding specificities of testis microsomal cytochrome P-450.
• To determine the structural features of steroids that influence their interaction with testis microsomal cytochrome P-450.

Main Methods:

• Difference spectroscopy was used to measure spectral shifts upon steroid substrate binding.
• Various C19 and C21 steroids, including modified testosterone and pregnane derivatives, were tested for binding.

Main Results:

• Testis microsomal cytochrome P-450 preferentially binds C21 steroids, contrasting with placental cytochrome P-450's affinity for C19 steroids.
• Specific modifications to testosterone, such as 17 alpha-methyl, 17 beta-acetate, or 17 beta-benzoate groups, enabled binding.
• Multiple oxygen functions on C21 steroids, like those in cortisol, prevented binding.
• At least one oxygen function is necessary for binding, as demonstrated by the binding of 20-deoxypregnenolone and the lack of binding for 5 alpha- and 5 beta-pregnane.

Conclusions:

• Testis microsomal cytochrome P-450 exhibits distinct substrate binding preferences compared to placental cytochrome P-450.
• Steroid substrate binding to testis microsomal cytochrome P-450 is influenced by specific structural features, including the presence and position of oxygen functions.
• Factors beyond simple hydrophobic interactions govern the binding of steroid substrates to testis microsomal cytochrome P-450.