Within the first 30 sec after intravenous injection of [32P]Pi the P-flux was measured within the mononueleotide pool of the normal mouse liver. It is only in this short time interval that the dynamic phase of tracer kinetics is met, permitting proper calculations of flux rates of the mononucleotides with high turnover.The simplified model applied for the calculation of the stated flux rates describes fairly adequately the main pathways within the complex network of possible mononucleotide interconversions.The P-flux from the ,8-P of ADP to the p-P of ATP, catalyzed preferentially by oxidative and substrate phosphorylation amounts to 33 pmoles per g fresh liver per min. 12, 0.7, 0.3 and 0.01 pmoles y-P of ATP are transfered per g per min to the ,8-P of ADP, UDP, GDP and CDP, respectively, via the group nucleoside monophosphate kinases. These flux rates are 1-2 orders of magnitude less than expected from the kinetics behaviour of the involved enzymes in vitro. The reverse reaction of the nucleoside monophosphate kinases is probably small. 9 , 4 and 0.5 pmoles of the y-P of ATP are transferred per g per min to the y-P of GTP, UTP and CTP via nucleoside diphosphate kinase.I n a preceding paper [l] the turnover of glycolytic metabolites and mononucleotides was examined in the normal mouse liver 30-60sec after intravenous injection of carrier-free [32P]Pi. After this labelling time, however, the specific radioactivities of different P-positions of the mononucleotides were partly in the prestationary phase of tracer kinetics [2], and thus did not permit proper calculations of all the corresponding flux rates [l]. Therefore, the specific radioactivities of the ,8-and y -P positions of ATP, ADP, GTP, CTP and UTP were determined 10-30 sec after tracer injection. Evaluation of these data provides quantitative information on the main energy generating and consuming reactions in the liver under conditions in vivo.
MATERIALS AND METHODSThe methods used for the intravenous administration of carrier-free [32P]Pi, the preparation of the livers and the determination of the specific radioactivities of the mononueleotides were exactly as described previously [3] with one exception. The frozen liver was deproteinized with 10 Ol0 trichloraeetie acid [3]) of the first separation procedure containing mainly fructose 1,6-biphosphate, phosphoenolpyruvate, CTP, ATP and UDP were treated with charcoal according to [3]. The charcoal eluent containing the nucleotides was concentrated in vacuo.