Methodical and theoretical aspects of the isolation of transciptionally active rat liver nuclei with special reference to the effect of in vivo administration ofethyl- o-p- chlorp henoxyisobutyrate (Clofibrate) on the rate of transcription of the peroxisomal fatty acyl~oA oxidase and enoyl-CoA hydratase/3-h ydroxyacul-CoA dehydrogenase genes in isolated rat live r nuclei