N-(Phosphonacetyl)-l-Aspartate, a Potent Transition State Analog Inhibitor of Aspartate Transcarbamylase, Blocks Proliferation of Mammalian Cells in Culture

N-(phosphonacetyl)-l-aspartate (PALA) is an analog of the activated complex for the reaction catalyzed by aspartate transcarbamylase and has previously been shown to be a potent and specific inhibitor of the enzymes from Escherichia coli and mouse spleen. PALA is now shown to be a potent inhibitor of de novo pyrimidine nucleotide biosynthesis in five lines of mammalian cells in culture. When the concentration of PALA in the medium is 10-4 m, simian virus 40-transformed hamster cells do not divide and, within about 2 days, have detached from the culture dishes and appear dead. The toxicity of 10-4 m PALA is prevented completely if 10-4 m uridine is present in the medium, showing that PALA inhibits uridylic acid biosynthesis specifically. The same effect can be observed with the other cell lines. When the external concentration of radioactive PALA is in the range 10-4 to 10-5 m, its concentration in the cytosol is approximately equal to the external concentration. Most of the PALA recovered from cells remains undegraded. In crude cell extracts, the Ki for PALA is about 1 x 10-9 m at 37° and pH 7.5. Addition of 1 x 10-7 m PALA in vitro to an extract of transformed hamster cells inhibits by 60% the incorporation of radioactivity from CO2 into UMP and CMP; approximately 10-6 m PALA is required to give comparable inhibition of incorporation from carbamyl-P. Higher concentrations give virtually complete inhibition in both cases. These results suggest that PALA may find clinical use in suppressing cellular proliferation. A small number of the virus-transformed hamster cells become resistant to 10-4 m PALA in culture and retain resistance after they are grown for many generations in the absence of the drug. Resistance is accompanied by a large increase in aspartate transcarbamylase activity.