Differential conjugation of polyamines to calf nuclear and nucleolar proteins

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Abstract
Nuclei and nucleoli isolated from calf liver contain acid‐precipitable putrescine, spermidine and spermine conjugates. The polyamines are released upon peptide bond hydrolysis. All of the nuclear putrescine conjugate and a major portion of the polyamine conjugates are localized within the nucleolus. Nuclei and nucleoli also contain, in proportions consistent with the nucleolar/nuclear protein ratio, the putative conjugating enzyme, transglutaminase, as well as amine acceptor substrates to which radiolabeled putrescine can be conjugated by endogenous enzyme. Extraction of the isolated organelles with saline solutions of increasing ionic strength showed a differential distribution of the polyamine derivatives: all the covalently linked putrescine was associated with the less soluble components of the chromatin residue, while the spermidine and spermine conjugates were associated with several salt‐extractable protein fractions as well as tightly bound to the chromatin pellet. Mono‐γ‐glutamyl putrescine was detected after proteolytic digestion of the 600 mM NaCl fraction, further suggesting the enzymatic action of transglutaminase(s) in the conjugation process.