The Effects of Luteinizing Hormone in vivo on Cholesterol Side-Chain-Cleavage Activity in Mitochondria of the Rat Corpus Luteum

Luteinizing hormone is known to stimulate the production of progesterone in the corpus luteum of several species (Mason et al., 1962; Major et al., 1967; Armstrong, 1968; Collett et al., 1973). The rate-limiting step of steroidogenesis in steroid-hormoneproducing tissues is thought to be cholesterol side-chain cleavage (Stone & Hechter, 1954; Hall & Young, 1968). It was therefore decided to measure the rate of cholesterol side-chain cleavage in rat luteal mitochondria and to investigate parameters affecting this rate such as trophic hormones. Immature female rats were superovulated by the method of Parlow (1958). These animals were treated with 20pg of luteinizing hormone (NIH-LH-S18), given as a subcutaneous injection in 0.2ml of 0.9% NaCl I h before their ovaries were removed. Control animals received injections of 0.2ml of 0.9 % NaCI. Mitochondria were prepared from the ovaries of these animals (Arthur & Boyd, 1974). Cholesterol, cytochrome P-450 and cholesterol side-chain-cleavage activity were measured in the mitochondria as described previously (Arthur & Boyd, 1974). The results of such an experiment are shown in Fig. 1. Treatment with luteinizing hormone decreased the cholesterol side-