Differences in [Mg 2+] and [Ca 2+] dependence of amino acid incorporation by free and membrane-bound polyribosomes isolated from liver, and an effect of the hepatocarcinogen dimethylnitrosamine

Abstract

Free and membrane-bound polyribosomes were isolated from livers of normal rats and rats which had received a single intravenous injection of 50 mg dimethylnitrosamine (DMNA) per kg 5 h previously. Amino acid incorporation by these preparations was studied in the presence of 2–8 mM Mg 2+ plus 0–5 mM Ca 2+. It was found that: 1. 1. In sustaining amino acid incorporation certain combinations of [Mg 2+] plus [Ca 2+] were superior to any [Mg 2+] tested alone. The breakdown of free polyribosomes during amino acid incorporation in vitro at [Mg 2+] below 7 mM was counteracted by Ca 2+. 2. 2. Both the [Mg 2+] and the [Mg 2+] plus [Ca 2+] dependence of amino acid incorporation differed for free and membrane-bound polyribosomes from normal liver. Amino acid incorporation by membrane-bound polyribosomes from normal and dimethylnitrosamine-treated liver also showed a different [Mg 2+] plus [Ca 2+] dependence. 3. 3. These differences were imposed upon the bound polyribosomes by the microsomal membranes, since polyribosomes freed from membranes by deoxycholate behaved as free polyribosomes, whereas free polyribosomes added to ‘inactivated’ membrane-bound polyribosomes behaved as freshly isolated membrane-bound polyribosomes in their divalent cation dependence. Since the membranes did not act by concentrating Mg 2+ during isolation or by changing the medium used for amino acid incorporation, it was concluded that the binding between membranes and polyribosomes imparts upon the latter a diminished requirement for divalent cations functioning in amino acid incorporation. This effect was more outspoken for the microsomal membrane of dimethylnitrosamine-treated than of normal liver.