Peptide hormones are synthesized in endoplasmic reticulum, transferred to the Golgi and packaged into secretory vesicles for export. This is the most commonly used pathway and allows cells to secrete a large amount of hormone over a short period of time. In general, the halflife of circulating peptide hormones is only a few minutes. Steroids Steroids are lipids and, more specifically, derivatives of cholesterol. The cholesterol precursor comes from newly synthesized cholesterol, from cholesterol ester stores within the cell or from uptake of cholesterol-containing low density lipoproteins.
Lipoproteins are most important when steroidogenic cells are chronically stimulated.
The first and rate-limiting step in the synthesis of all steroid hormones is conversion of cholesterol to pregnenolone, which is illustrated here to demonstate the system of numbering rings and carbons for identification of different steroid hormones. Pregnenolone is formed on the inner membrane of mitochondria then shuttled back and forth between mitochondrion and the endoplasmic reticulum for further enzymatic transformations involved in synthesis of derivative steroid hormones. Newly synthesized steroid hormones are rapidly secreted from the cell, with little if any storage . Increases in secretion reflect accelerated rates of synthesis. Following secretion, all steroids bind to some extent to plasma proteins.