Which type of cholesterol is the primary substrate for progesterone synthesis?

The primary substrate for progesterone synthesis is cholesterol, and while free cholesterol can be derived from various sources in the body, Low-Density Lipoprotein (LDL) is particularly significant in this context. LDL particles transport cholesterol to peripheral tissues, including the ovaries, where steroids like progesterone are produced.

In the process of steroidogenesis, LDL is taken up by cells through receptor-mediated endocytosis, which facilitates the availability of cholesterol for conversion into progesterone. Once inside the cells, cholesterol is transported to the mitochondria, where it undergoes enzymatic transformations to ultimately produce progesterone.

The other choices do not serve as the primary substrate for progesterone synthesis. High-Density Lipoprotein (HDL) primarily functions in reverse cholesterol transport, bringing cholesterol back to the liver, while Very Low-Density Lipoprotein (VLDL) is more involved in triglyceride transport rather than directly contributing to steroid synthesis. Triglycerides, though a form of stored energy, do not provide the cholesterol necessary for progesterone production. Thus, LDL is the most relevant type of cholesterol when discussing the primary substrate for progesterone synthesis.