Page 169 - The Biochemistry of Inorganic Polyphosphates

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Figure 8.18 Changes in the degree of polymerization (n) in PolyP fractions during growth of Saccha-
romyces cerevisiae on glucose (Vagabov et al., 1998): (1a) PolyP(I), precipitation by barium salt at pH
8.2; (1b) PolyP(I), precipitation by barium salt at pH 4.5; (2) PolyP(II); (3) PolyP(III); (4) PolyP(IV).

8.10.5 The Effects of P i Limitation and Excess

The P i limitation causes a sharp decline of PolyPs in yeast cells (Liss and Langen, 1962;
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Kulaev and Vagabov, 1983). Under P i limitation, yeast cells have often no P ‘NMR-visible’
PolyPs (Hofeler et al., 1987). When P i -starved yeast cells are placed on a complete medium,
the PolyP content rises sharply, i.e. the so-called ‘phosphate overplus’ (hypercompensation)
effect occurs (Liss and Langen, 1962).
The content (Figure 8.19) and degree of polymerization (Figure 8.20) of PolyPs were
determined in the course of growth of the yeast Saccharomyces cerevisiae in a medium
with glucose, which contained varying P i amounts at a constant level of all necessary
components (Vagabov et al., 2000). After7hof phosphate starvation, the yeast was shown
to use almost the complete phosphate reserve in the form of PolyPs to support its vitality
(Figure 8.19). The PolyP drop was followed by a considerable shortening of the polymer
chain length of acid-soluble (PolyP(I)) and two alkali-soluble (PolyP(III) and PolyP(IV))
fractions (Figure 8.20). Under the same conditions, the content of a salt-soluble fraction
(PolyP(II)) decreased almost 20-fold with a simultaneous increase of the chain length of
nearly twofold.
Re-inoculation of yeast cells after phosphate starvation to a complete P i - and glucose-
containing medium resulted in the accumulation of PolyP within 2 h, mainly in PolyP(III),
and, to a lesser extent, in the PolyP(I), PolyP(II) and PolyP(V) fractions. In the PolyP(IV)
fraction localized on the cell surface, PolyP ‘super-accumulation’ was not detected. Increase
in the PolyP amount in the above fractions turned out not to be accompanied by simultaneous
elongation of their chain lengths and occurred at the lowest level, which is characteristic
of a polymer level for each fraction (Figure 8.20). Further cultivation of the yeast on the
complete medium over 2 h had little or no effect on the PolyP content in the cells but
led to elongation of the PolyP chain, especially in the PolyP(III) and PolyP(IV) fractions.
This phenomenon of considerable elongation of the PolyP chain on the background of