T. Youdale

Increases in rat liver cyclic AMP concentrations prior to the initiation of DNA synthesis following partial hepatectomy or hormone infusion

Abstract In rat liver following partial hepatectomy an unprecedented biphasic increase in cyclic AMP concentration was observed. After a lag period of 1 1 2 hr , the cyclic AMP content rose to a peak at 2 1 2 hr . The cyclic AMP level fell to normal at 8 hr, and then rose again to a second peak at 12 hr. DNA synthesis started at 18 hr, by which time the cyclic AMP level was approaching control values. A biphasic increase in cyclic AMP concentration was also observed to precede the initiation of DNA synthesis in intact liver following the infusion of a mixture of tri-iodothyronine, amino acids, glucagon and heparin. The relationship between these biphasic changes in cyclic AMP levels and the initiation of DNA synthesis in liver is discussed.

Separation of a subunit necessary for CDP reductase from other ribonucleotide reductase activities of regenerating rat liver

Abstract Purification of ribonucleotide reductase from regenerating rat liver using dATP Sepharose chromatography isolated a subunit of the enzyme which was specific for the reduction of CDP. Activities for the other ribonucleotide diphosphates showed differing distribution in the various fractions suggesting different forms of the enzyme for each ribonucleotide diphosphate.

tsLA23-NRK cells need pp60v-src protein-tyrosine kinase activity in G2 phase to initiate mitosis in serum-free medium

Lowering the temperature from 41 to 36 degrees C stimulates quiescent tsLA23-NRK rat cells (infected with the tsLA23 mutant of the Rous sarcoma virus) in serum-free medium to resume cycling and initiate DNA replication by reactivating the tsLA23-RSVs abnormally thermolabile pp60v-src protein-tyrosine kinase. Inactivating the enzyme in these pp60v-src-stimulated cells by again raising the temperature to 41 degrees C after the cells had initiated DNA replication did not prevent the completion of DNA replication and entry into the G2 phase, but it stopped the initiation of mitosis. Adding serum at the time of the temperature increase replaced the lost pp60v-src activity and the cells were able to continue to mitosis. The G2-arrested cells at 41 degrees C were able to initiate mitosis when pp60v-src was reactivated again by lowering the temperature to 36 degrees C. These observations suggest that protein-tyrosine kinase activity is needed to initiate mitosis and that the tsLA23-NRK cell is a good model for studying the function of this kinase activity in the initiation of mitosis.