The inositol pyrophosphate metabolism of Dictyostelium discoideum does not regulate inorganic polyphosphate (polyP) synthesis

Initial studies on the inositol phosphates metabolism were enabled by the social amoeba Dictyostelium discoideum. The abundant amount of inositol hexakisphosphate (IP(6) also known as Phytic acid) present in the amoeba allowed the discovery of the more polar inositol pyrophosphates, IP(7) and IP(8), possessing one or two high energy phosphoanhydride bonds, respectively. Considering the contemporary growing interest in inositol pyrophosphates, it is surprising that in recent years D. discoideum, has contributed little to our understanding of their metabolism and function. This work fulfils this lacuna, by analysing the ip6k, ppip5k and ip6k-ppip5K amoeba null strains using PAGE, (13)C-NMR and CE-MS analysis. Our study reveals an inositol pyrophosphate metabolism more complex than previously thought. The amoeba Ip6k synthesizes the 4/6-IP(7) in contrast to the 5-IP(7) isomer synthesized by the mammalian homologue. The amoeba Ppip5k synthesizes the same 1/3-IP(7) as the mammalian enzyme. In D. discoideum, the ip6k strain possesses residual amounts of IP(7). The residual IP(7) is also present in the ip6k-ppip5K strain, while the ppip5k single mutant shows a decrease in both IP(7) and IP(8) levels. This phenotype is in contrast to the increase in IP(7) observable in the yeast vip1Δ strain. The presence of IP(8) in ppip5k and the presence of IP(7) in ip6k-ppip5K indicate the existence of an additional inositol pyrophosphate synthesizing enzyme. Additionally, we investigated the existence of a metabolic relationship between inositol pyrophosphate synthesis and inorganic polyphosphate (polyP) metabolism as observed in yeast. These studies reveal that contrary to the yeast, Ip6k and Ppip5k do not control polyP cellular level in amoeba.