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All adrenal steroidogenesis begins with cholesterol. The cholesterol has two origins: (1) uptake from low-density lipoproteins (LDL) by specific LDL receptors on the surfaces of adrenal gland cells and (2) de novo synthesis of cholesterol within the adrenal cortex from acetyl CoA. The structure of cholesterol is shown in Figure . Cholesterol has a four ring structure called the steroid nucleus that is common to all steroid hormones. These steroid hormones possess a wide variety of activities that are due to surprisingly small variations in their structure. The classification of the steroid hormones is based not only on their activities but also on their structure, as shown in Figure .
Biosynthesis of steroid hormones requires a battery of oxidative enzymes located in both mitochondria and endoplasmic reticulum. The rate-limiting step in this process is the transport of free cholesterol from the cytoplasm into mitochondria. Within mitochondria, cholesterol is converted to pregnenolone by an enzyme in the inner membrane called CYP11A1. Pregnenolone itself is not a hormone, but is the immediate precursor for the synthesis of all of the steroid hormones. The following table delineates the enzymes required to synthesize the major classes of steroid hormones.