Cytoplasmic androgen binding protein of rat liver: molecular characterization after photoaffinity labeling and functional correlation with the age-dependent synthesis of .alpha.2u-globulin

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Abstract
The liver of the mature male rat contains a moderate affinity (Kd = 10-8 M), low-capacity, cytoplasmic androgen binding protein (CAB) whose appearance during puberty and disappearance during senescence correlate with the androgen-dependent synthesis of .alpha.2u-globulin. Molecular properties of CAB were examined by photoaffinity labeling with tritiated methyltrienolone (R-1881), a synthetic androgen, and by its localization within the hepatocytes which are competent to produce .alpha.2u-globulin. Photoaffinity labeling of the liver cytosol derived from postpubertal male rats, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography, showed a predominant androgen binding band corresponding to Mr 31 000. This 31-kilodalton (kDa) binding component was conspicuously absent in the liver of androgen-insensitive prepubertal and senescent male rats and in adult male rats treated with estradiol-17.beta.. In addition, unlike the cytoplasmic extract, the nuclear lysate of the male rat heptatocytes did not contain the 31-kDa androgen binder. Disappearance of the 31-kDa androgen binding band from the cytosolic fraction of androgen-insensitive animals was associated with a concomitant appearance of minor androgen binding component of apparent Mr 29000. The livers of postpubertal male rats normally contain two subpopulations of hepatocytes, only one of which is highly active (competent) in .alpha.2u-globulin synthesis. Separation of these two subpopulations through a fluorescence-activated cell sorter followed by whole cell labeling showed more than a 2-fold higher uptake of R-1881 by the competent cells. These results suggest that the 31-kDa androgen binder may play an important role in androgen action and that a conversion of the 31-kDa androgen binding component to a 29-kDa binder may be involved in the regulation of the hepatic androgen sensitivity.

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