Recent results revealed that inorganic
polyphosphates (
polyP), being energy-rich linear
polymers of
orthophosphate residues known from bacteria and yeast, also exist in higher eukaryotes. However, the enzymatic basis of their metabolism especially in mammalian cells is still uncertain. Here we demonstrate for the first time that
alkaline phosphatase from calf intestine (CIAP) is able to cleave
polyP molecules up to a chain length of about 800. The
enzyme acts as an
exopolyphosphatase degrading
polyP in a processive manner. The pH optimum is in the alkaline range.
Divalent cations are not required for catalytic activity but inhibit the degradation of
polyP. The rate of hydrolysis of short-chain
polyP by CIAP is comparable to that of the standard
alkaline phosphatase (AP) substrate
p-nitrophenyl phosphate. The specific activity of the
enzyme decreases with increasing chain length of the
polymer both in the alkaline and in the neutral pH range. The K(m) of the
enzyme also decreases with increasing chain length. The mammalian tissue non-specific
isoform of AP was not able to hydrolyze
polyP under the conditions applied while the placental-type AP and the bacterial (Escherichia coli) AP displayed
polyP-degrading activity.