Adenine salvage enzymes and intracellular nucleotide triphosphate content in Physarum flavicomum amoebae during...

The specific activity of adenine phosphoribosyltransferase (APRT) (EC 2.4.2.7) and adenosine phosphorylase (EC 2.4.2.-), two enzymes involved in the utilization of exogenous adenine, was measured in extracts of myxamoebae-swarm cells of Physarum flavicomum undergoing growth, microcyst formation (control), and during adenine inhibition of encystment. Both enzymes showed a higher specific activity in adenine-inhibited cells (AIC) compared to normal control (NC) or growing cells (GC). These experiments revealed that the specific activity of APRT was 7.1-, 5.3-, and 1.7-fold higher than that of adenosine phosphorylase in AIC, GC, and NC, respectively. This suggests a predominant role for the enzyme APRT in the salvage of adenine in this organism. The major route for the utilization of adenine thus seems to be by its direct conversion to AMP rather than via its riboside adenosine. HPLC analysis of the ribonucleotide triphosphates in cell extracts of GC, NC, and AIC revealed a 2.6- and a 3.3-fold increase in the ATP and GTP content, respectively, in the AIC compared with the NC cells. The ATP content in the GC was higher by a factor of 2.2 compared with the NC cells, while the GTP content in the GC was only 0.6 times that in the NC cells. UTP levels in AIC and GC were 1.3- and 1.4-fold higher than in the NC cells. In contrast, the CTP level in AIC was lower than in NC cells and was not detectable in the growing cells.