Page 193 - The Biochemistry of Inorganic Polyphosphates

20:32
Char Count= 0
March 9, 2004
WU095-08
WU095/Kulaev
Animals 177
level of longer PolyPs with a chain length of about 700–800 residues (Rodrigues et al.,
2002b). The level of PolyP in both organisms rapidly decreased upon the exposure of
the parasites to a calcium ionphore (ionomycin), to an inhibitor of vacuolar V-ATPase
(bafilomycin A1), or to the alkalization of the medium by NH 4 Cl (Rodrigues et al., 2002b).
Thus, the PolyP level in acidocalcisomes may depend on µH+ on the membrane of these
organelles.
High levels of short-chain (∼ 20 mM) and long-chain (∼ 60 mM) PolyPs were detected
in Leishmania major promastigotes. An exopolyphosphatase has been purified and cloned
from this organism, which resembles the PPX1 enzyme from S. cerevisiae in its properties
and amino acids sequence (Rodrigues et al., 2002a). It was proposed that the characteris-
tics of exopolyphosphatase and PolyP metabolism revealed in pathogenic protozoa might
facilitate the development of novel antiparasitic agents (Rodrigues et al., 2002a).

8.14 Higher Plants

Despite the fact that PolyPs have been found in a wide range of tissues of the higher
plants (see Chapter 3), very little is known about PolyP metabolism in these organisms.
For example, Khomlyak and Grodzinskii (1970, 1972) have shown that 32 P i introduced
into tomato leaves via the steam conductive system is first incorporated into the acid-
soluble PolyP fraction and subsequently into the acid-insoluble one. PolyP-metabolizing
enzymes have been observed in many higher plants (Pierpoint, 1957a,b; Rotenbach and
Hinkelmann, 1954; Jungnickel, 1973) Two different exopolyphosphatases were observed
in plants (Jungnickel, 1973). One of these was a constitutive enzyme, while the other
appeared when the plant was grown under conditions of phosphorus deprivation, i.e. it was
inducible. Some data on the relation of PolyP and nucleic acid metabolisms in plants were
obtained by Schmidt and co-workers (Schmidt and Buban, 1971; Schmidt, 1971, 1972).
It should be noted that the role of phosphate reserve in plants belongs not to PolyP but
to phytin (Ca–Mg salt of inositol phosphate), which is formed in large amounts during the
ripening of seeds, in parallel with the accumulation of reserve substances such as starch and
fats (Sobolev, 1962). Such accumulation and the presence of chlorophyll cause additional
difficulties for the identification and study of PolyPs in plants. However, PolyP is also
present in plants in the sites where large amounts of phytin are accumulated (Asamov and
Valikhanov, 1972; Valikhanov et al., 1980). The amounts of PolyPs normally present in the
tissues of higher plants are very small and may be observed at certain stages of development.
For example, fairly large amounts of PolyPs accumulate at an early stage of the ripening
of cotton seeds (Assamov and Valikhanov, 1972; Valikhanov et al., 1980). At this stage,
PolyPs represented more than 10 % of the total phosphorus of the seeds, exceeding the
phytin phosphorus by more than twofold (Assamov and Valikhanov, 1972).

8.15 Animals

Although the first evidence for the presence of PolyPs in mammalian cells was obtained long
ago (Gabel and Tomas, 1971), the metabolism of this biopolymer in the higher eukaryotes is