AJP - Endocrinology and Metabolism, Vol 242, Issue 3 178-E183, Copyright © 1982 by American Physiological Society
20 alpha-Hydroxysteroid dehydrogenase and 17 beta-estradiol dehydrogenase localize in cytosol of human term placenta
R. C. Strickler and B. Tobias
The 20 alpha-hydroxysteroid dehydrogenase activity in human term placenta
has been localized by different investigators to nuclear, mitochondrial,
microsomal, and cytosolic subcellular fractions. Furthermore, in the
cytosol, 20 alpha-hydroxysteroid dehydrogenase activity may be a second
function of the enzyme that mediates 17 beta-estradiol dehydrogenase
activity. To search for a unique 20 alpha-hydroxysteroid dehydrogenase,
human placental villous tissue, homogenized in three different buffer
systems, was fractionated by differential centrifugation, and the 17 beta-
and 20 alpha-activities were measured by radioisotope conversion assay. The
enrichment and purity of the subcellular fractions were shown by marker
enzyme assays and electron microscopy studies. Under all experimental
conditions, 20 alpha-hydroxysteroid dehydrogenase activity was identified
only in the 105,000 g placental cytosol: intact, osmotically ruptured, and
acetone-extracted mitochondria, nuclei, and microsomes did not convert
progesterone to 20 alpha-dihydroprogesterone. Furthermore, because 17
beta-estradiol dehydrogenase activity was in large part soluble in the
cytosol, these localization studies are consistent with the hypothesis that
the 20 alpha- and 17 beta-oxidoreductase activities in human placenta
reside on one soluble protein.
