17β-Estradiol hydroxylation catalyzed by human cytochrome P450 1B1


The 4-hydroxy metabolite of 17β-estradiol (E2) has been implicated in the carcinogenicity of this hormone. Previous studies showed that aryl hydrocarbon-receptor agonists induced a cytochrome P450 that catalyzed the 4- hydroxylation of E2. This activity was associated with human P450 1B1. To determine the relationship of the human P450 1B1 gene product and E2 4- hydroxylation, the protein was expressed in Saccharomyces cerevisiae. Microsomes from the transformed yeast catalyzed the 4- and 2-hydroxylation of E2 with K(m) values of 0.71 and 0.78 μM and turnover numbers of 1.39 and 0.27 nmol product min-1 · nmol P450-1, respectively. Treatment of MCF-7 human breast cancer cells with the aryl hydrocarbon-receptor ligand indolo[3,2-b]carbazole resulted in a concentration-dependent increase in P450 1B1 and P450 1A1 mRNA levels, and caused increased rates of 2-, 4-, 6α-, and 15α-hydroxylation of E2. At an E2 concentration of 10 nM, the increased rates of 2- and 4-hydroxylation were approximately equal, emphasizing the significance of the low K(m) P450 1B1-component of E2 metabolism. These studies demonstrate that human P450 1B1 a catalytically efficient E2 4- hydroxylase that is likely to participate in endocrine regulation and the toxicity of estrogens.

Publication Title

Proceedings of the National Academy of Sciences of the United States of America