Enzymes guanosine 5 -tetraphosphate

Quite a different form of exopolyphosphatase was purified from the vacuolar sap of S. cerevisie (Andreeva et al., 1998b). Its molecular mass determined by gel filtration was 245 kDa. This exopolyphosphatase hydrolysed PolyP3 only slightly, and its specific activity increased with the increase in PolyP chain length (Table 6.6). It was unable to hydrolyse adenosine- and guanosine-tetraphosphates and was insensitive to antibodies inhibiting the low-molecular-mass exopolyPase of the cytosol (Table 6.4). This enzyme was stimulated by divalent metal cations to a much lesser extent than 40 kDa exopolyphosphatase (Table 6.5) and was inhibited by EDTA (Table 6.4). The inhibitory effect of EDTA is explained by the binding of Co2+, which is the best activator of the vacuolar exopolyphosphatase at 0.1 mM. 

The enzyme splitting both adenosine-tetraphosphate and guanosine-tetraphosphate was purified to homogeneity from yellow lupin seeds (Guranowski et al., 1997). The polypeptide of 25 kDa catalysed the hydrolysis of nucleoside-5 -tetraphosphate to nucleoside triphosphate and P , and hydrolysed PolyP3, but neither pyrophosphate nor PolyPs The divalent carions Mg2+, Co2+, Ni2+ or Mn2+ were required for the reaction. 

---