These considerations apply to all cycloalkane derivatives, including steroids.
However, the chair form of a ring is inherently more stable than the boat form.
Moreover, the fused-ring nature of the system lends it a very considerable rigidity, and
cis–transisomerization would necessitate the breaking and formation of covalent
bonds. Therefore, steroid substituents maintain their conformation at room temperature,
whereas cyclohexane substituents usually do not. Steroids are classified according to
their substituents in addition to their occurrence.
5.5 STEROID HORMONES: STEROID BIOSYNTHESIS
The biosynthesis of steroids is complex, as one would expect since all the compounds
in this group must be derived from a single precursor (cholesterol (5.4)). The primary
source of all the compounds involved in steroid synthesis is acetate, in the form of
acetyl-coenzyme A. Cholesterol, besides being ingested in food, is synthesized in large
amounts, and an adult human contains about 250 g of cholesterol. In contrast, the
steroid hormones are produced at the milligram level or lower.
The regulation of steroid biosynthesis is achieved by an intricate network of peptide
hormones. The pituitary-based production of these hormones is under neuroendocrine
influence, starting with the hypothalamus in the CNS, which itself is under dopaminer-
gic control. The hypothalamus produces a number of small peptide hormones that act
as the releasing factors for a second series of peptide hormones synthesized in the ante-
rior pituitary gland. Among these hormones,adrenocorticotropic hormone(ACTH)
regulates corticosteroid synthesis in the adrenal cortex, whereas luteinizing hormone
HORMONES AND THEIR RECEPTORS 315