Pathways (generalized)

The initial substrates present for production of steroids are present in most cells. Actual production depends on:

• the presence of suitable receptors for tropic molecules like ACTH, FSH and LH
• the presence of specific enzymes within the cytoplasm to produce a given steroid

The pathways of production for a given steroid are often considered as separate but there is a large amount of interconversion between them. For example, the female sex hormone oestradiol can be converted to the male sex hormone testosterone, and vice versa.

Cholesterol is the precursor of all steroids. Within the endocrine gland it is converted by a number of enzymes, including cholesterol desmolase, into pregnenolone. This entails a reduction in the number of carbon atoms from 27 to 21. The removal of carbon atoms from the basic cholesterol structures is a common theme to steroid biosynthesis; other common reactions include the substitution of keto or hydroxyl groups and changes in saturation.

The reaction is rate-limiting for steroid synthesis. Consequently, it is a means of governing flux through the pathway. Hence, for example, it can be under the direct control of ACTH.

Pregnenolone is the precursor for synthesis of either:

• progesterone or
• 17-alpha-hydroxypregnenolone