Isolation and characterization of diadenosine 5',5'''-P1,P4-tetraphosphate pyrophosphohydrolase from Physarum polycephalum

Abstract

A new enzyme that hydrolyzes diadenosine 5'',5''''''-P1,P4-tetraphosphate has been purified by a factor of 250 from the acellular slime mold P. polycephalum. Activity was assayed radioisotopically with [3H]Ap4A. Isolation of the enzyme was facilitated by dye-ligand chromatography. The enzyme symmetrically hydrolyzes Ap4A to ADP and exhibits biphasic kinetics for the substrate with values for the apparent Km of 2.6 .mu.M and 37 .mu.M. The 2 values of Vmax differ by a factor of 10. Mg2+, Ca2+ and other divalent cations inhibit the activity with 40-80% inhibition occurring at 0.5 mM. Mg2+, at 0.5 mM, decreases both values of Vmax by 50%, decreases the low Km value by .apprx. 30% and increases the high Km value by .apprx. 100%. (Ethylenedinitrilo)tetraacetic acid (EDTA) and [ethylenebis(oxyethylenenitrilo)]tetraacetic acid (EGTA), at 10 mM, inhibit the activity by 50%. ADP, ATP Ap4 and Gp4 are equipotent inhibitors, with 50% inhibition occurring at 30 .mu.M. AMP is a relatively weak inhibitor. The MW of the enzyme is 26,000 on the basis of elution of activity from a calibrated Sephadex G-75 column.