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M301698200v1
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Papers In Press, published online ahead of print March 31, 2003
J. Biol. Chem, 10.1074/jbc.M301698200
Submitted on February 18, 2003
Revised on March 28, 2003
Accepted on March 31, 2003

Identification of outer mitochondrial membrane cytochrome b5 as a modulator for androgen synthesis in Leydig cells

Tadashi Ogishima, Jun-ya Kinoshita, Fumiko Mitani, Makoto Suematsu, and Akio Ito

Chemistry Dept., Kyushu University, Fukuoka, Fukuoka 812-8581

Corresponding Author: taogiscc{at}mbox.nc.kyushu-u.ac.jp

Outer mitochondrial membrane cytochrome b5 is an isoform of microsomal membrane cytochrome b5. In rat testes the outer mitochondrial membrane cytochrome b5 is present in both mitochondria and microsomes while microsomal membrane cytochrome b5 is undetectable. Outer mitochondrial membrane cytochrome b5 present in the testis was localized in Leydig cells with cytochrome P-45017alpha which catalyzes androgen genesis therein. We therefore analyzed the functions of outer mitochondrial membrane cytochrome b5 in rat testis microsomes using a proteoliposome system. In a low but physiological concentration of NADPH-cytochrome P-450 reductase and excess amount of progesterone, outer mitochondrial membrane cytochrome b5 stimulated the cytochrome P-45017alpha -catalyzed reactions, 17alpha -hydroxylation and C17-C20 bond cleavage. The effects were different from those by microsomal membrane cytochrome b5; preferential elevation of the 17alpha -hydroxylase activity by outer mitochondrial membrane cytochrome b5 in an amount-dependent manner versus that of the lyase activity by microsomal membrane cytochrome b5 at the low concentration and the inhibition of both activities at the high concentration. At a low concentration of progesterone reflecting a physiological cholesterol supply, outer mitochondrial membrane cytochrome b5 elevated primarily the production of 17alpha -hydroxyprogesterone and then facilitated the conversion of the released intermediate to androstenedione. Thus, we demonstrated that outer mitochondrial membrane cytochrome b5 not microsomal membrane cytochrome b5 functions as an activator for androgengenesis in rat Laydig cells.


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