Sterols, and Squalene from Biosynthesis of Progesterone, Acetate-12°C and Mevalonate-22°C by the Bovine Corpus Luteum in Vitro*

Although the role of acetate as a substrate for steroid biosynthesis in systems in vitro is now well established (l-4), the role of mevalonate is currently somewhat obscure. A number of conflicting reports on the utilization of the latter substance for steroidogenesis have appeared in the literature. The diversity of steroidogenic tissue preparations that have been tested for their ability to utilize mevalonate includes testicular homogenates (5), testis slices (6), adrenal minces and homogenates (7, 8), fetal adrenal slices (9), adrenal adenoma (lo), and ovarian minces (11). However, in some of these reports the omission of critical data, such as precise radioactivity measurements or criteria of radiochemical purity, makes it difficult to evaluate the consequently ambiguous results. The utilization of mevalonate in the bovine corpus luteum has not previously been rigorously investigated (3). This compartment of the ovary has the experimental advantage of producing only two steroids, the principal one being progesterone, and 20&hydroxy-AJ-pregnen-3-one comprising the minor product. The conversion of labeled cholesterol to progesterone in the corpus luteum has been amply shown through the use of slices (12), homogenates (13), and mitochondrial preparations (14, 15). The utilization of acetate-1-14C as a substrate for squalene, sterol, and progesterone synthesis in this tissue has also been proven (3,4, IS), and the extent of its utilization for steroidogenesis has been shown to be increased in the presence of the pituitary gonadotropin, luteinizing hormone (4). In view of the obligatory role of mevalonate in the biosynthesis of cholesterol from acetate in liver (17), it was of interest to investigate more fully the utilization of mevalonate-2-14C for progesterone biosynthesis in the bovine corpus luteum. The incorporation of mevalonate-2-i% into other lipid constituents was also assessed. Radioactive leucine incorporation into progesterone was investigated because of the possibility of this amino acid acting as a precursor for mevalonate.