The Effect of Inhibitors of Protein Synthesis on Cholesterol Side‐Chain Cleavage in the Mitochondria of Luteinized Rat Ovaries

Conversion of cholesterol to pregnenolone, termed cholesterol side‐chain cleavage, has been studied in isolated mitochondria from luteinized rat ovaries. Cholesterol in these mitochondria in the presence of cyanoketone is converted to one product pregnenolone on addition of a suitable electron donor. The percentage conversion of total mitochondrial cholesterol to pregnenolone is comparable to that measured by the percentage conversion of [4‐14C]cholesterol to [4‐14C]pregnenolone when the label has been preincubated at 29°C for 10 min with the mitochondria. In mitochondrial incubations the depletion of endogenous cholesterol follows the production of pregnenolone. The initial rate of production of pregnenolone (0.8 nmol × mg protein−1× min−1) was maintained in incubations for 5–6 min. Addition of free cholesterol to incubations allowed the production of pregnenolone to continue at the initial rate for at least 30 min. Treatment of rats with cycloheximide 20 min prior to killing, caused a 58% inhibition of luteal mitochondrial cholesterol side‐chain cleavage and a small but significant rise in the cholesterol content of isolated mitochondria. The cycloheximide treatment did not affect the total luteal mitochondrial cytochrome P‐450 but caused a 56% decrease in the inverted type I pregnenolone‐binding spectrum, suggesting cholesterol binding to cytochrome P‐450 had been reduced. Rats treated with chloramphenicol 3 and 10 h before killing exhibited no differences in total luteal mitochondrial cytochrome P‐450 or mitochondrial cholesterol metabolism when contrasted with saline‐injected animals. Puromycin was shown to bind to the rat luteal mitochondrial cytochrome P‐450 to produce a type I1 difference spectrum and to inhibit luteal mitochondrial cholesterol side‐chain cleavage in vitro. The results are discussed in terms of a protein which is rapidly turning over and which may be synthesised extra‐mitochondrially and in some way affect the binding of cholesterol to luteal mitochondrial cytochrome P‐450.