Abstract
1. 1. Extracts of whole tissue or isolated nuclei from lactating rat mammary gland that has diminished cell replication capacity were more active than the corresponding extracts of pregnant rat mammary gland that contains actively replicating cells in causing a dose-dependent inhibition of DNA polymerase α in vitro. 2. 2. Purification of the inhibitor from both tissue and nuclear extracts using sequence of Sephacryl S200, DEAE-cellulose and CM52 columns confirmed the above assay results. 3. 3. Using the same assay and purification procedures, both tissue and nuclear extracts from the rapidly growing transplanted R3220AC mammary tumors exhibited very little or no inhibitor activity. 4. 4. The partially purified mammary inhibitor (mol. wt of 155 kD, high A 280nm/A 260nm ratio, heat labile) was equally inhibitory to the purified DNA polymerase α from either R3230AC tumor or calf thymus, and to the nuclear matrix bound DNA polymerase α of R3230AC tumor.
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