Aromatase/CYP19A1

Aromatase/CYP19A1 is a cytochrome P450 enzyme that converts C19 androgens, including androstenedione and testosterone, into C18 estrogens through a three-step reaction requiring O₂, NADPH, and cytochrome P450 reductase^[1]. Mechanistically, CYP19A1 performs two C19-methyl hydroxylation steps followed by steroid A-ring aromatization, a reaction described as unique to aromatase^[1]. CYP19A1 expression is regulated by tissue-specific promoters in ovary, placenta, adipose tissue, brain, and other estrogen-forming sites, while promoter-derived untranslated exons splice into a common coding region that produces the same aromatase protein^[2]. In breast cancer models, aberrant CYP19A1 transcription increases intratumoral estradiol and supports estrogen-dependent tumor growth, making promoter regulation and aromatase activity practical endpoints for mechanistic studies^[3]. Clinically, high CYP19A1 expression associates with poor overall, disease-free, and metastasis-free survival in estrogen receptor-positive breast cancer, but not estrogen receptor-negative disease^[4]. Compared with broader cytochrome P450 enzymes, aromatase shows androgen-specific substrate recognition and catalyzes the estrogen-forming aromatization step^[1]. For experimental applications, anastrozole and letrozole act as reversible nonsteroidal aromatase inhibitors, whereas exemestane functions as an irreversible steroidal aromatase inactivator^[5].