The Biological Synthesis of Cholesterol1

This chapter examines cholesterol metabolism in terms of the molecular events that are responsible for constructing cyclopentanoperhydrophenanthrene derivatives from small molecular precursors. There is good reason for centering a discussion of steroid biogenesis around cholesterol, not only because this substance has received the major share of attention but also because cholesterol synthesis appears to be an integral part of the processes leading to the physiologically active steroids, namely, the bile acids, and the adrenocortical and sex hormones. It seems further than the syntheses of cholesterol, of ergosterol, and of the sterols in higher plants follow the same general plan and branch from each other only a t a relatively late stage. It is largely a matter of definition whether a discussion of the present subject properly belongs to “Vitamins and Hormones.” As a major, and probably obligatory, precursor of progesterone and, by way of progesterone also of the androgens, the estrogens, and the corticoids, cholesterol qualifies at least as a prohormone. Secondly, since cholesterol is known to be obligatory for the growth of representatives of at least two animal phyla, protozoa and insects, it is technically correct to regard cholesterol as a vitamin. It seems clear that the traditional classification of cell constituents on the basis of nutritional or functional parameters is far too narrow, at least for the case of cholesterol.