Subcellular localization of steroid hormone metabolism in rat liver

The metabolism of 4-[4-¹⁴C]androstene-3,17-dione was studied in smooth- and rough-surfaced microsomes, plasma membranes, mitochondria, Golgi apparatus, and lysosomes isolated from rat liver. 5α-Reductase, 17β-hydroxysteroid reductase, 16α-, 6β-, and 7α-hydroxylase activities were measurable in all isolated subcellular fractions except in lysosomes which did not metabolize 4-androstene-3,17-dione. Steroid metabolism patterns in smooth and rough microsomes were very similar. The extent of microsomal contamination in the plasma membrane, Golgi apparatus, and mitochondrial and lysosomal fractions was measured both by ultrastructural (morphometric) analysis and by distribution analysis of "marker enzymes" and was less than 15, 3, 16, and 3%, respectively. Whereas it could not be excluded that the metabolisms of steroids in the Golgi apparatus enriched fraction occurred due to microsomal contamination, the data indicate that the steroid metabolism in isolated plasma membranes and mitochondria occurred due to the presence of steroid reducing and hydroxylating enzyme systems in these cell organelles. Indirectly, such a similarity in enzymatic makeup between different cell organelles would tend to support "the membrane flow hypothesis.".