5-ALPHA-REDUCTASE TYPE-1 IS LOCALIZED TO THE OUTER NUCLEAR-MEMBRANE

The subcellular distribution of the two isozymes of 5 alpha-reductase has been controversial. To resolve this issue which could provide clue s about the respective functions of the two isozymes, two antisera wer e generated, one which was specific for the Type 1 5 alpha-reductase a nd one which recognized both isozymes. In COS cells transfected separa tely with the Type 1 or Type 2 cDNA, both isozymes were detected on We stern blots at an M(r) of 26 000. Subfractionation of the COS cells re sulted in the partitioning of both isozymes between the crude nuclear and cytosolic fractions, while cytoimmunofluorescence localized both r eductases to the nuclear periphery. In rat liver homogenate, the 5 alp ha-reductase was also detected at M(r) 26 000. The 5 alpha-reductase i mmunoreactivity was increased after castration of the animals with no further effect when castrated animals were treated with androgens. Alt hough the rat liver expresses only the Type 1 5 alpha-reductase, the 5 alpha-reductase was distributed about equally between crude nuclear a nd cytosolic subfractions; this distribution could be shifted to the c ytosolic fractions with harsher homogenization procedures. Further ext ensive subfractionation and extraction studies identified the rat live r Type 1 5 alpha-reductase as an integral membrane protein present in the outer nuclear membrane of the nuclear envelope and in rough endopl asmic reticulum. Thus, the subfractionation and cytoimmunofluorescence studies are consistent with the localization-of the Type 1 5 alpha-re ductase to the outer nuclear membrane of the nuclear envelope which is continuous with and indistinguishable from the endoplasmic reticulum. This study is the first to localize rat liver Type 1 5 alpha-reductas e to the nuclear envelope to which the prostatic 5 alpha-reductase act ivity previously had been localized. We conclude that, contrary to pre vious tissue distribution studies, but consistent with investigations in transfected cells, both isozymes are similarly localized to the nuc lear periphery.