Free and membrane-bound ribosomes in normal and methylcholanthrene-treated mouse epidermis

Total and free ribosomes were isolated from postnuclear supernatants of trypsinized normal and 3-methylcholanthrene-treated epidermis from female CD-1 mice. Membrane-bound ribosomes were determined as the calculated difference between total and free ribosomes. The value for total ribosomes from normal epidermis in the resting phase of the hair growth cycle was 0.32 mg. of ribosomal RNA per gm. of epidermis, or 0.07 mg. of ribosomal RNA per mg. of homogenate DNA. About 96 per cent of the ribosomes were free and 4 per cent were membrane-bound. Five days after a single application of 2 mu moles of 3-methylcholanthrene to the dorsal skin in the resting phase of the hair growth cycle, there was epidermal hyperplasia and cellular hypertrophy. Total ribosomes increased 2- to 3-fold, to 0.80 mg. of ribosomal RNA per gm. of epidermis, or 0.21 mg. of ribosomal RNA per mg. of homogenate DNA. This increase was further characterized as proportionate increases of both free and membrane-bound ribosomes.