Properties of the aromatase system associated with the mitochondrial fraction of human placenta.

The aromatase system associated with the mitochondrial fraction of human term placenta, present at 35--50% the specific activity of the microsomal enzyme, is substantially the same as the microsomal enzyme as determined by the following: 1) The rate of aromatization of androstenedione, 19-nortestosterone, and 16alpha-hydroxytestosterone in mitochondria was a nearly constant proportion of the microsomal rate; 2) Sensitivity to carbon monoxide was the same; 3) The magnitude of cytochrome P-450 Type I spectral interactions with androgen substrates was a constant proportion in mitochondria and microsomes; 4) Sensitivity to an antibody raised against hepatic microsomal NADPH-cytochrome c reductase was the same. When inner and outer mitochondrial membrane subfractions were prepared, the predominant aromatase activity was associated with the outer membrane preparation. This aromatase activity could not be accounted for by microsomal contamination as determined by inosine diphosphatase activity, a microsomal marker. After correction, the rate of aromatization in the outer membrane preparation was almost six times that in the inner membranes and three times that of the whole mitochondrial fraction.