Page 101 - The Biochemistry of Inorganic Polyphosphates

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Enzymes of polyphosphate degradation 85
It was demonstrated that the intestinal isoform of alkaline phosphatase from calf was
able to degrade PolyPs with a wide range of chain lengths, in addition to PP i (Lorenz and
Schr¨oder, 2001). The enzyme splits P i from PolyP in a processive manner. The pH optimum
is in the alkaline range. Divalent cations are not required for catalytic activity but instead
inhibit PolyP degradation. The rate of hydrolysis of short-chain PolyPs is comparable with
thatofthestandardalkalinephosphatasesubstrate,i.e.p-nitrophenylphosphate.Thespecific
activity of the enzyme decreases with increasing chain length of the polymer, both in the
alkaline and neutral pH ranges. The K m of the enzyme also decreases with increasing chain
length. The mammalian tissue non-specific isoform of alkaline phosphatase was not able to
hydrolyse PolyP under the conditions applied, while the placental-type alkaline phosphatase
displayed PolyP-degrading activity (Lorenz and Schr¨oder, 2001).
Therefore, the exopolyphosphatases are quite different both in various organisms and
in different cell compartments of the same organism. This suggests their possible different
functions in cells.

6.2.4 Adenosine–Tetraphosphate Phosphohydrolase
(EC 3.6.1.14)

adenosine–5 -tetraphosphate + H 2 O −−→ ATP + phosphate (6.13)
In yeast, adenosine–tetraphosphate phosphohydrolase and guanosine–tetraphosphate phos-
phohydrolase activities are an inherent property of exopolyphosphatase PPX1. It was
demonstrated both with the cytosol preparations, where this enzyme is localized
(Kulakovskaya et al., 1997), and with a purified PPX1 preparation (Guranowskiet al., 1998).
Exopolyphosphatase PPX1 of the cytosol of S. cerevisiae is able to hydrolyse adenosine–

5 -tetraphosphate and guanosine–5 -tetraphosphate about twice more actively than PolyP 15 ,

with an apparent K m value of 80–100 µM (Kulakovskaya et al., 1997). Thus, in yeast PPX1
may link the metabolism of PolyP and some nucleoside polyphosphates.
The enzyme splitting both adenosine–tetraphosphate and guanosine–tetraphosphate was
purified to homogeneity from yellow lupin seeds (Guranowski et al., 1997). The polypep-

tide of ∼ 25 kDa catalysed the hydrolysis of nucleoside–5 -tetraphosphate to nucleoside
triphosphate and P i , and hydrolysed PolyP 3 , but neither pyrophosphate nor PolyP 4 . The
2+
2+
divalent carions Mg ,Co ,Ni 2+ or Mn 2+ were required for the reaction.
The enzyme with adenosine–tetraphoshatase activity was obtained earlier from rabbit
muscle (Small and Cooper, 1966). This enzyme had an effect on inosine tetraphosphate and
tripolyphosphate but showed little or no activity with other nucleotides or PolyPs.

6.2.5 Triphosphatase (Tripolyphosphatase, EC 3.6.1.25)

Only earlier data (Meyerhof et al., 1953; Kornberg, 1957b) are available for trimetaphos-
phate hydrolase (EC 3.6.1.2) catalysing the following reaction:

cyclotriphosphate + H 2 O −−→ PolyP 3 (6.14)