Characterization of pyrimidine metabolism in the cellular slime mold, Dictyostelium discoideum

Abstract

Pyrimidine nucleotides may be provided for cellular growth via a de novo biosynthetic pathway or salvage pathways of interconversion of nucleosides and bases. The de novo biosynthesis in D. discoideum is provided by the common six step pathway which is initiated by a class II carbamoylphosphate synthetase, specific for pyrimidine biosynthesis. This CPSase utilizes L-glutamine as its sole substrate, and shows slight inhibition with both UTP and CTP. The second enzyme in the de novo pathway is an allosterically unregulated aspartate transcarbamoylase. It is an apparently multimeric enzyme of 105,000 daltons. The enzyme demonstrates a lack of response to all tested allosteric effectors and closely resemble the prokaryotic class c ATCase. Additionally the amoeba demonstrate the ability for salvage of some pyrimidine bases and nucleosides. Upon starvation on a solid substratum, axenically grown amoeba begin a concerted developmental program. During development, nucleotide metabolism is restructured. Absolute levels of the ribonucleotide pools drop by as much as 98%. In spite of this, both the adenylate energy charge and GTP/ATP ratio are maintained at high levels for up to 50 hours after the initiation of development. The adenylate energy charge at 50 hours is .74 while the GTP/ATP ratio is .27. This maintenance of metabolic energy requires tight cell-cell contact. Thus, pyrimidine metabolism is maintained through developmental morphogenesis as pools are sustained for the first 12 hours. In spite of the lack of arginine biosynthesis in D. discoideum, the pyrimidine biosynthetic pathway appears to be evolutionarily conserved from bacteria, while the genetic structure of pyrimidine biosynthesis in D. discoideum appears to resemble that of plant systems more closely than of animal cells.