The nature of DNA polymerases from Morris hepatomas and ascites hepatomas were compared with those of adult, regenerating, and fetal rat liver. For DNA polymerases from the tissues tested, the ratios of activities for native and heatdenatured DNA were variable over a wide range. Namely, DNA polymerases from regenerating liver, fetal liver, and rapidly growing hepatomas exhibited higher activities for heatdenatured DNA than for native DNA, but normal liver and slowly growing hepatomas utilized either native or heatdenatured DNA equally as primer.

In order to test the hypothesis that DNA polymerase consists of components that have different ratios of activities for native and heat-denatured DNA, the crude extracts of DNA polymerase were submitted to fractionation by several means. Among them, the gel filtration by Sephadex G-100 was most successful; it separated DNA polymerase from normal liver into Peak I, which exhibited higher activity for heat-denatured DNA, and Peak II, which was lower in molecular weight than Peak I and utilized only native DNA as a primer. The activities of both peaks in normal liver were nearly equal. In regenerating liver, the activity of Peak I was increased, while that of Peak II revealed no change from that of normal liver. In fetal liver only Peak I was found.

The rapidly growing hepatomas did not exhibit the type found in fetal liver but rather that of regenerating liver. Hepatoma 7316B, which was the most slowly growing among the minimal deviation hepatomas tested, was found to possess the type close to normal liver.

From these results the physiologic roles of DNA polymerase of each peak were discussed.

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This work was supported in part by a grant-in-aid for the Fundamental Scientific Research from the Ministry of Education of Japan.

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