Page 135 - The Biochemistry of Inorganic Polyphosphates

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The functions of polyphosphates in higher eukaryotes 119
Few data are available on the PolyP function in higher plants. PolyPs were first ob-
served in maize roots (Vagabov and Kulaev, 1964), while Niemeyer studied PolyPs and
their interactions with the inositol phosphate pools in plants (Niemeyer, 1975, 1976, 1999;
Niemeyer and Selle, 1989). Some data on the dynamics of PolyPs during the development
of cotton plants were obtained (Valikhanov et al., 1980). It is probable that some plants can
use extracellular PolyP as a phosphorus source (Igamnazarov and Valikhanov, 1980).
Possibly, higher plants possess sufficient amounts of PolyPs only at specific develop-
ment stages and in certain tissues, and investigation of their role is a difficult task. Genetic
methods may be an effective tool for study of the influence of PolyP accumulation on plant
cell metabolism. For example, it was demonstrated that the transformation of potato plants
with the ppk gene from E. coli introduced a new phosphate pool in the chloroplasts of green
tissues. PolyPs accumulated during leaf development from 0.06 mg P per g of dry biomass in
juvenile leaves to 0.83 mg per g of dry biomass in old leaves and had an average chain length
of around 18 residues. Leaves of transgenic plants contained less starch but higher concen-
trations of soluble sugars when compared to control plants (Van Voorthuysen et al., 2000).
As for animals, PolyPs were first found in their cells by Gabel and Thomas (1972). The
role of PolyPs in animal cells is still little studied, but some important facts indicate their
importance in development and regulatory processes.
Relatively high amounts of PolyPs were found in the freshwater sponge Ephydatia
muelleri, particularly in the gemmules (Imsiecke et al., 1996). Germination and hatching
of the gemmules were accompanied by a decrease (by 94 %) in the PolyP level and a
rise in the exopolyphosphatase activity. An increase in the PolyP content and decrease
in exopolyphosphatase activity also occurred during tissue regression, when the hatched
sponges were exposed to polluted river water. Non-ionic organic compounds extracted from
this water were identified as the contaminants responsible for the rise in the PolyP content
of this organism (Imsiecke et al., 1996).
Recently, interest has grown in the study of PolyP functions in the cells of higher animals.
PolyPs (n = 50–800) were found in the tissues of rodents (brain, liver, lungs and kidneys)
and in practically all of the sub-cellular fractions, namely nuclei, mitochondria, plasma
membranes and microsomes (Kumble and Kornberg, 1996). PolyPs were also found in
human blood and bone tissue (Schr¨oder et al., 1999, 2000). Age-dependence studies show
that the amount of PolyP in rat brain increases dramatically after birth (Lorenz et al., 1997b).
The maximal levels were found in 12-month-old animals. Thereafter, the total concentration
of PolyPs decreases to about 50 %. This decrease in the total PolyP concentration is due
to a decrease in the amount of insoluble long-chain PolyPs, as the amount of soluble long-
chain PolyPs does not change significantly with ageing. In rat embryos and newborns,
mainly soluble PolyPs could be detected. In rat liver, the age-dependent changes are less
pronounced. Changes in the PolyP level were accompanied by those in exopolyphosphatase
activity.ThehighestenzymeactivitieswerefoundatlowPolyPlevels.Inductionofapoptosis
resulted in degradation of long PolyP chains to shorter ones, while the total PolyP content
does not change significantly (Lorenz et al., 1997b).
The activity of endopolyphosphatase was also found in animal brain. An endopolyphos-
phatase had been partially purified from rat and bovine brain, where its abundance was
about 10 times higher than that in their other tissues but less than a tenth of that in yeast; the
end product of digestion of a partially purified brain enzyme is tripolyphosphate (Kumble
and Kornberg, 1996).