Inorganic Polyphosphate in Eukaryotes: Enzymes, Metabolism and Function

Inorganic polyphosphates (polyP) are linear polymers of orthophosphate (Pi) residues linked by high-energy phosphoanhydride bonds. These polymers are widely distributed in nature, from archaebacteria, eubacteria, fungi, algae, and protozoa to higher plants and animals (for reviews, see Kulaev 1979; Wood and Clark 1988; Kornberg 1994, 1995). PolyP molecules are stable in neutral aqueous solutions, but are hydrolyzed by heat-treatment, and under acidic or alkaline conditions. The chain length of polyP may range from 3 to more than 1000 Pi residues; it can be analyzed on urea/polyacrylamide gels (Clark and Wood 1987; Lorenz et al. 1994a). In contrast to the linear polymer, branched inorganic polyP has not yet been detected in living organisms (Kulaev 1979); for the occurrence of cyclic metaphosphates, see Chapter 5. Until recently, the occurrence and metabolism of polyP have been studied mainly in microorganisms (Kulaev 1979; Kulaev and Vagabov 1983), although significant amounts of these polymers are also present in mammalian cells and tissues (Kumble and Kornberg 1995; Lorenz et al. 1995, 1997a-c). They could be even detected in human cells (Pisoni and Lindley 1992; Kumble and Kornberg 1995; Lorenz et al. 1997a-c). In general, the polyP content in higher eukaryotes is much lower than in bacteria and yeast. Moreover, the amount and chain length of polyP may vary within the same species depending on several factors, including nutrition and growth phase. Some organisms (e.g., yeasts) are able to accumulate large amounts of polyP, up to 10% of the dry cell mass and more (Kulaev 1979). In view of the ubiquity of polyP, it has been concluded that these polymers have a multiplicity of biological functions.