The ability of breast cancer tissues from postmenopausal women to form catechol estrogens was examined by using a product isolation assay. Initial assays were carried out in the presence of either: (a) NADPH, the co-factor for monooxygenase mediated catecholestrogen (CE) formation or; (b) light-activated Tween 80 (LAT-80), a putative organic hydroperoxide co-factor for peroxidatic activity. Under monooxygenase conditions, CE formation by homogenates of 10 tumors did not exceed that obtained with heat denatured tissue. In contrast, 17 of 20 tumors incubated with LAT-80 synthesized significant amounts of CE (8.5 ± 1.172-hydroxyestradiol [2-OH-E2] and 12.8 ±2.4 nmol/g protein/10 min 4-hydroxyestradiol [4-OH-E2]). Substitution of cumene hydroperoxide, an organic hydroperoxide, for LAT-80 enhanced estrogen 2/4 hydroxylase (E-2/4-H) activity over 200-fold, making it possible to characterize sytematically the peroxidatic activity. The properties of peroxidatic E-2/4-H activity were similar to those of soluble peroxidases isolated from brain, including an acidic pH optimum, localization in the soluble fraction, an apparent Km in the range of 80 μM and an apparent Vmax in the range of 4000 nmol/g/protein/10 min for both 2-and 4-OH-E2. Under optimal assay conditions, peroxidatic E-2/4-H activity was identified in 10 of 13 tumors (2480 ± 580 nmol/g/protein/10 min for 2-OH-E2 and 2790 ± 600 for 4-OH-E2). The level of activity detected suggest a biological relevance for CE formation by breast cancer tissue.
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