Activation of purified cytoplasmic 17 beta-hydroxysteroid dehydrogenase from human placenta by human albumin and globulin

The cytoplasmic 17 beta-hydroxysteroid dehydrogenase of human placenta, purified more than 2500-fold, was activated by small amounts of human albumin and globulin. This activation was dependent on substrate concentration. At 20 microM estradiol (10 X KM) and two different concentrations of enzyme (0.01 and 2 micrograms/ml), the activation was greatest at albumin or globulin concentrations between 0 and 30 micrograms/ml. At "low" concentrations of estradiol (20 nM = 10(-2) X KM) and enzyme (0.01 microgram/ml), maximal activity occurred at approximately 10 micrograms/ml. Higher concentrations of albumin and globulin led to a decline in activity.