Aromatase Cytochrome P450, The Enzyme Responsible for Estrogen Biosynthesis*

THE biosynthesis of estrogens appears to occur throughout the entire vertebrate phylum including mammals, birds, reptiles, amphibians, teleost and elasmobranch fish, and Agnatha (hagfish and lampreys) (1–3). It has also been described in the protochordate Amphioxus (4). To our knowledge, estrogen biosynthesis has not been reported in nonvertebrate animal phyla. In most vertebrate species that have been examined, aromatase expression occurs in the gonads and in the brain. This is true of the fish and avian species that have been examined as well as most mammals such as rodents. In many species estrogen biosynthesis in the brain has been implicated in sex-related behavior such as mating responses, and frequently a marked sexually dimorphic difference has been demonstrated. This is true, for example, in avian species in which the song of the male is important in courtship behavior (5). In the case of humans and a number of higher primates, there is a more extensive tissue distribution of estrogen biosynthesis, since this also occurs in the placenta of the developing fetus as well as in the adipose tissue of the adult.