The electron transfer is one of the most common and fundamental chemical reactions. Conversion of cholesterol into pregnenolone is an initial step in the metabolism of all steroids. This reaction is catalyzed by side-chain cleavage cytochrome P450 and requires three molecules of oxygen and six electrons. The electrons are transferred to the cytochrome by a small acidic protein, adrenodoxin, from a flavoprotein, adrenodoxin reductase. Three proteins represent the steroid hydroxylase system in the mammalian mitochondria. This monograph probes a widely discussed shuttle model for electron transfer by bovine adrenodoxin using an alternative approach of photo-induced reduction of the protein by a ruthenium(II) bipyridyl complex. Results provide advantage and importance of this method in studying the mechanism of electron transfer in the system.