Estrogen biosynthesis. The biosynthesis of estrogen is initiated by the synthesis of Androstenedione,19-carbon steroid hormone, from cholesterol. Then this compound is converted to estrogensEstrone or Estradiol, either immediately or through Testosterone,which may also be derived from cholesterol. Reduction of Androstenedione to Testosterone requires Hydroxysteroid(17-beta) dehydrogenase 2 ( HSD17B2 ) ,  andHydroxysteroid (17-beta) dehydrogenase 3 ( HSD17B3 ) , , . The reverse reaction, oxidation of Testosterone at17-position to form Androstenedione, is catalyzed by monooxygenase CytochromeP450, family 2, subfamily C, polypeptide 19 ( CYP2C19 ) . Androstenedione undergoes a three-step A-ring aromatization to Estronecatalyzed by monooxyganases: aromatase Cytochrome P450, family 19, subfamily A,polypeptide 1 ( CYP19 )  and Cytochrome P450, family 11, subfamilyB, polypeptide 1 ( CYP11B1 ) . The first intermediate reaction isthe formation of 19-Hydroxyandrostenedione , which then isconverted to Androst-4-en-3,17,19-trione , followed by oxidation to Estrone , . Another pathway of Estrone biosynthesis is oxidation of17-alpha-Estradiol by Estradiol 17a-dehydrogenase ( 220.127.116.11 ) . Aromatase CYP19 also catalyzes oxidation of Testosterone toEstradiol , . The first step is formation of19-Hydroxytestosterone , which in turn is oxidized to19-Oxotestosterone , and then to Estradiol ,, similar to Androstenedione catabolism to Estrone. Another steroid substrate, which undergoes a CYP19 -catalyzed three-steparomatization, is 16alpha-Hydroxyandrostenedione , . The final product of that oxidation is 16alpha-Hydroxyestrone. Interconversion of Estradiol and Estrone requires Hydroxysteroid(17-beta) dehydrogenases 1 ( HSD17B1 ) , 7 ( HSD17B7 ) and 8 ( HSD17B8 ) .