Differences in Pregnenolone and Adenosine Triphosphate Syntheses Between Mitochondria from Follicles and Corpora Lutea of Porcine Ovaries

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Abstract
Mitochondria from intact follicles (4-7 mm) and corpora lutea of porcine ovaries were isolated and compared for their abilities to utilize various oxidizable substrates to support ATP and pregnenolone syntheses. The oxidizable substrates studied were isocitrate, citrate, malate and succinate (Kreb''s cycle intermediates), acetyl carnitine and palmitoyl carnitine (representative short and long chain fatty acids, respectively) and pyruvate (an intermediate of glucose metabolism). While there is an overall higher rate of ATP synthesis by luteal mitochondria, the increase in the utilization of certain substrates is especially dramatic. Palmitoyl carnitine and acetyl carnitine support ATP synthesis almost 5-fold better in luteal preparations than in follicular mitochondria (120 vs. 25 nmol/min per mg protein), while isocitrate and citrate are 8 times better (240 vs. 30 nmol/min per mg protein). Fatty acids are poorly utilized by follicular mitochodria to support ATP synthesis, but are good substrates in luteal mitochondria. The inability of follicular mitochondria to utilize isocitrate to support ATP synthesis is not due to a lack of isocitrate dehydrogenase activity, since the activities of both the NAD+-dependent and NADP+-dependent forms in follicular mitochondria are not significantly different than the corresponding activities in luteal mitochondria. With follicular mitochondria there are no significant differences between the rates of pregnenolone synthesis supported by various substrates (approximately 40 pmol/min per mg protein). While the rates of pregnenolone synthesis by luteal mitochondria also do not differ significantly with the various oxidizable substrates, the rates by luteal mitochondria are 5 to 10-fold higher than the corresponding rates by follicular mitochondria. Luteal mitochondria have a greater ability to utilize fatty acids and isocitrate and citrate than do follicular mitochondria. While the rate of ATP synthesis varies with the oxidizable substrate utilized, pregnenolone synthesis requires an oxidizable substrate but does not exhibit any preference.