Page 191 - The Biochemistry of Inorganic Polyphosphates

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Protozoa 175
utilized the accumulated PolyPs for photophosphorylation by shortening the PolyP chains
(Watanabe et al., 1988). During P i uptake, Heterosigma akashiro required Mn , which
2+
was excreted from cells after the PolyP pool had been saturated (Watanabe et al., 1989).
8.12.5 Changes in Polyphosphate Content
under Stress Conditions

The ammonium-induced cytoplasmic alkalization in the unicellular algae Dunaliella salina
resulted in degradation of long-chain PolyPs to PolyP 3 (Pick et al., 1990; Bental et al., 1990;
Pick and Wess, 1991). The hydrolysis was shown to correlate with the recovery of cytoplas-
mic pH and might provide the ‘pH-stat’ mechanism to counterbalance the alkaline stress.
A decrease of the PolyP level in Ulva lactuca was observed at high external nitrate
concentrations during cultivation in continuous light (Lundberg et al., 1989). These authors
assumed that either nitrate might inhibit the P i uptake or, in the case where nitrate was the
only nitrogen source, the energy of the PolyP could be used for nitrate uptake and reduction
(Lundberg et al., 1989).
The alga Phaerodactilum tricornutum was found to respond to hyperosmotic stress
by a marked elongation of PolyP and a decrease in its total amount, while exposure to
hypoosmotic stress resulted in a higher content of shorter PolyPs and an increased total
PolyPs content (Leitao et al., 1995). It is probable that, such variations might allow the
adjustment of the intracellular osmotic pressure to an extracellular one.
In conclusion, it should be said that the PolyP-metabolizing enzymes in algae have been
little studied. An activity, which transferred P i from PolyP to ADP, was observed in cell-free
extracts from Chlorella (Iwamura and Kuwashima, 1964) and Acetabularia (Rubtsov and
Kulaev, 1977). Exopolyphosphatase activity was found in Acetabularia (Rubtsov and
Kulaev, 1977), while polyphosphate glucokinase activity was not found in algae (Uryson
and Kulaev, 1970).
It is clear, however, that the intracellular concentration of ATP and P i , as well as the P i
level in the culture medium, exert a regulatory effect on the biosynthesis and degradation
of PolyPs in algae such as heterotrophs (Curnutt and Schmidt, 1964a,b; Miyachi, 1961;
Miyachi and Miyachi, 1961; Miyachi and Tamiya, 1961; Miyachi et al., 1964; Rubtsov
and Kulaev, 1977; Kuhl, 1960, 1962, 1976). There is little doubt that PolyP metabolism in
algae is strongly influenced by light (Miyachi and Miyachi, 1961; Miyachi et al., 1964) and
oxygen concentration in the medium (Ullrich, 1970).

8.13 Protozoa

PolyP in protozoa was found long ago (Ebel et al., 1958b; Mattenheimer, 1958; Janakidevi
et al., 1965; Rosenberg, 1966). Its metabolism was studied with the purpose of searching
for specific biochemical peculiarities of parasitic representatives of this taxon, which could
offer prospects for drug development.
The intracellular levels of short- and long-chain PolyPs were measured by means
of polyphosphate glucokinase assay in Leishmania major promastigotes incubated in a