Since the establishment of their structure, the potential of steroids to act as structural cornerstone for the design of derived drugs became evident. This potential was confirmed by pharmacological studies, which highlighted broad biological effects. Nevertheless, the term steroid is commonly associated with androgenic-anabolic compounds, but the latter compounds comprise only a single, relatively small, class of biologically active steroids. In fact, all steroids share the same three-dimensional backbone. Many of their chemical properties are determined by steric configuration of the steroid nucleus. Despite the high number of chiral centers, steroids occur in nature essentially as a single chiral species. These structural characteristics are closely related with the biological activity of steroids. In this chapter, a review work on different steroids found in natural matrices is presented, aiming to provide a state-of-the-art overview of specific concerns of the biochemistry and physiology of these isoprenoid lipids. The work is organized according to different classes (cholesterol, anabolic steroids, sex steroids, corticosteroids, phytosterols, brassinosteroids, ecdysteroids, among others) and natural sources (animals, plants, fungi) of steroids. Besides structural considerations, the key features leading the information throughout the text include the analytical techniques (chromatography, spectroscopy, hyphenated techniques) used for their identification and/or quantification and regulation of biosynthesis (considering the derivatives from lanosterol and cycloartenol) emphasizing human steroidogenesis. Special attention is paid to the bioactivity of different sterols, focusing on the physiological role of oxidized derivatives of cholesterol and oxysterols in normal and pathological cellular processes, influence of endogenous sex steroids in the regulation of inflammatory processes, anticancer properties, antioxidative effects, and activation of the expression of defense genes.
