Age-associated alteration of DNA polymerase and in vitro DNA synthesis of rat spleen

Examination of the spleens of 3-month- and 25-month-old male Fischer 344 rats revealed three kinds of DNA polymerase (α, β and γ), which can be detected and characterized by their molecular weights, subcellular localizations, chromatographic migrations and template specificities.

Spleen DNA polymerases from both young and old rats are similar in their elution patterns in ion-exchange chromatography and also in their sedimentation rates in sucrose gradient centrifugation. Extracts from the cytoplasmic fraction of the young and old rat spleens contained DNA polymerase α, β and γ in approximately the same quantities. However, polymerases β and γ were found to be significantly decreased in the nuclear and large-particule fractions of the old rat spleens. Partially purified DNA polymerases from young and old rat spleens showed similar heat lability and in vitro replicational infidelity.

When included in a DNA synthesis reaction, nuclei isolated from the spleens of young rats showed a 1.2- to l.5-fold higher dNmp incorporation than did those from old animals. Upon addition of exogenous DNA polymerase to the reaction mixture, the dNMp incorporation by the nuclei was enhanced and conversely became 3- to 4-fold higher in the old than the young. Mild acid treatment of the nuclei further increased the template activity; in this respect, the young appeared to be more responsive than the old. Kinetic analysis revealed that the relatively high template activity of the spleen nuclei from old rats represents increased substrate sites for the exogenous DNA polymerase. This large increase in template activity also occurs in mouse spleen nuclei during the aging process, but not in the nuclei from rat liver or brain, or from mouse liver.

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