17β-Hydroxysteroid dehydrogenase (17β-HSD) is present in multiple forms in human breast tissue. One soluble form, with a molecular weight of approximately 35 kDa, was purified to near homogeneity from whole normal breast tissue. This form catalysed the oxidation of oestradiol and the reduction of oestrone, with NADP+ and NADPH as the preferred coenzymes. Three other soluble forms with higher molecular weights (in the range 50–80 kDa) were isolated. They catalysed the oxidation of oestradiol but not the reduction of oestrone, and all of them had properties very different from those of the low molecular weight enzyme.
Activities of 17β-HSD were measured in particulate and soluble fractions from normal breast adipose and non-adipose tissues, and from breast tumours obtained from post-menopausal women, in the oxidative direction with NAD+ and NADP+ as coenzymes and in the reductive direction with NADH and NADPH as coenzymes. Particulate fractions from tumours had much higher oxidative and reductive activities than those from normal tissues. Soluble fractions from tumours had higher oxidative activities than those from the normal tissues but similar reductive activities. The major soluble form of 17β-HSD in adipose tissue was the 35 kDa enzyme which had both oxidative and reductive activities. In contrast, the majority of the soluble activity in non-adipose tissue was due to enzymes, with molecular weights in the range 50–80 kDa, which had oxidative activity only. The soluble fractions of tumours, like those of non-adipose tissue, contained enzymes with molecular weights in the range 50–80 kDa. In addition, they contained a 35 kDa enzyme with properties different from those of the enzyme with the same molecular weight present in adipose tissue.