The growth of the eucaryotic microorganism Dictyostelium discoideum in liquid culture was completely inhibited by the
aspartic acid analog
hadacidin (N-formylhydroxyamino-
acetic acid). Growth arrest occurred both in chemically defined medium and in complex growth medium containing
aspartic acid and
AMP precursors such as
adenine and
adenosine. Although these compounds could not overcome the effect of
hadacidin, growth was restored if cells were washed and resuspended in fresh growth medium. Additional experiments showed that D. discoideum contains
adenylosuccinate synthetase, the
enzyme which catalyzes the synthesis of
adenylosuccinate from
IMP,
aspartic acid, and
GTP in the de novo biosynthesis of
purines. A partially purified preparation of this
enzyme was obtained, and the effect of
hadacidin on its activity was studied. We found that maximum inhibition of the D. discoideum activity occurs at a ratio of
aspartic acid to
hadacidin of 5:1, suggesting that the affinity of the
drug for this
enzyme is less than for the
enzyme from rabbit muscle and plants but greater than for that from Escherichia coli. The effect of the
drug can be overcome by a 10-fold excess of
aspartic acid, suggesting that the
drug acts as a competitive inhibitor. A comparison of the
adenylosuccinate synthetase activity levels at various stages of growth showed that its specific activity decreases about 60% as cells enter the stationary growth phase, and decreases about 75% after
starvation for 2 h. Further studies showed that in cells treated with
hadacidin the rate of uptake of exogenous nutrients is reduced about 75% and that these cells are more resistant to
rupture by osmotic shock. While the results of this study are consistent with the proposal that growth arrest is contingent upon inhibition of
adenylosuccinate synthetase activity, they also suggest that, as a consequence of this inhibition, some physiological properties of the cell have been altered.