The metabolism of isolated rat-liver nucleoli and other subnuclear fractions. The active site of amino acid incorporation in the nucleus

Abstract

Rat-liver nuclei isolated in 0.25 M-sucrose were disrupted by ultrasonic vibration and subjected to differential centrifuging to isolate nuceloli. The remainder of the disrupted nuclear material was arbitrarily fractionated by centrifuging into 4 sedimentable fractions and a supernatant. Determinations of RNA, DNA, phospholipid and protein, together with staining reactions of the subnuclear fractions, indicate that separation of nucleoli, chromosomal material, a lipid-rich material and nuclear sap was achieved. Although all the subnuclear fractions will incorporate C14 -labelled amino acids into protein and [6-Cl4]orotic acid into RNA without an additional external energy source, the nucleoli and the lipid-rich material have by far the greatest activity. High specific activity of certain oxidative enzyme systems noted in the lipid-rich fractions appear to be absent from nucleoli. It is concluded that the lipid-rich material may be the heterochromatin associated with the nucleoli and that this is the active site for the incorporation of amino acids by rat-liver nuclei.