Steroids and Related Isoprenoids

The principal nonsaponifiable lipids of mammalian cells are derived from multiple units of the five-carbon hydrocarbon isoprene (2-methyl-1,3-butadiene) (Fig. 1A), and may consequently be termed isoprenoids. These compounds are further subcategorized as terpenes and sterozds (for reviews, see Heftmann, 1969; Brown and Goldstein, 1980; Poulter and Rilling, 1981). Steroids have in common a 19-carbon saturated tetracyclic structure known as perhydrocyclopentanophenanthrene, which may thus be considered the basic steroid nucleus (Fig. 1B). Terpenes consist of either linear or cyclic arrangements of multiple isoprene units; since the steroid nucleus is derived from cyclization of the linear terpene squalene, it is evident that steroids, in fact, represent a subclass of terpenoids. Cholesterol is quantitatively the major steroid synthesized within neural tissue (Kabara, 1973; Bhat and Volpe, 1983), and methodologies relating to the metabolism of cholesterol will consequently be a major focus of this review.