Interest in the use of the post-mitochondrial supernatant (PM-supernatant) for the study of in vitro amino acid incorporation arose from experiments indicating that the ribosomal aggregation of the PM-supernatant system more nearly represented that of the intact cell than did other ribosomal preparations. The PM-supernatant system was found to be 25 or 50 % more active than the microsomal and ribosomal amino acid incorporation systems. For PM-supernatant in vitro incorporation of l-[14C]valine into acid insoluble material, ATP, GTP, an energy generating system, Mg2+, K+, and a mixture of l-amino acids were necessary. Sephadex G-25 column chromatography of the PM-supernatant, pH 7.4, and presence of glutathione during homogenization of the tissue were necessary for optimal PM-supernatant in vitro incorporation. Under these conditions the rate of l-valine incorporation by the PM-supernatant from 0.17 to 0.20 nmoles/min per mg RNA for a liver from a fed rat. Fasting (20 h) prior to sacrifice, resulted in a 30-40 % decrease in amino acid incorporation by the PM-supernatant as well as a decrease in ribosomal aggregation. The decrease in PM-supernatant incorporation by fasting was found to be associated with both polysomal and cell sap factors.
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