Comparison of the in vitro conversion of estradiol-17β to estrone of normal and neoplastic human breast tissue

Abstract

Specific activity of 17β-hydroxysteroid dehydrogenase (17β-HSD) was measured in 48 tissue specimens of human female breast cancer and, in addition, 48 nonmalignant tissue specimens obtained in each case from the same cancer-bearing breast. In all cases the nonmalignant tissue showed greater conversion of estradiol-17β into estrone than the neoplastic tissues. In normal human breast tissue of premenopausal women specific enzyme activity depended on the phase of the menstrual cycle: the highest values of 17β-HSD activity were found in the early secretory phase. To determine the intracellular distribution of the 17β-HSD, purified microsomes, mitochondria, peroxysomes, lysosomes, nuclei and cytosol fractions were prepared. The purity of each fraction was monitored by marker enzymes. It was found that the 17β-HSD was mainly located in mitochondria and microsomes. Furthermore it could be demonstrated that the microsomal enzyme was bound tightly to the membranes of the endoplasmic reticulum, while the mitochondrial 17β-HSD was mainly associated with the outer membranes of the organelle. Kinetic parameters ( K m -values, coenzyme requirements and maximal velocities) of a cytoplasmic, nuclear, mitochondrial and microsomal 17β-HSD of normal and neoplastic human mammary tissue were compared. Maximal velocity was highest in enzyme preparations of normal mammary tissue obtained from premenopausal women in the early secretory phase. K m values were nearly identical in normal and neoplastic mammary tissue preparations (approx. 1 × 10 −6 M). NAD was more efficient than NADP as a cofactor. For the conversion of estradiol to estrone the optimum temperature was approximately 40°C and the optimum pH 9.S. For the reduction of estrone the optimum pH was 6.5. Sulphydryl groups were shown to be essential for catalysis.