Human placenta hydrolases active on free ADP-ribose: an ADP-sugar pyrophosphatase and a specific ADP-ribose pyrophosphatase

Free ADP-ribose has a reducing ribose moiety and it is hazardous due to its nonenzymic reactivity toward protein side chains. ADP-ribose hydrolases ar e putative protective agents to avoid the intracellular accumulation of ADP -ribose. In mammalian sources, two types of enzymes with ADP-ribose hydrola se activity are known: (i) highly specific ADP-ribose pyrophosphatases, whi ch in a Mg2+-dependent fashion hydrolyse only ADP-ribose and the nonphysiol ogical analogue IDP-ribose, and (ii) less specific nucleoside diphosphosuga r or diphosphoalcohol (NDP-X) pyrophosphatases, which besides A(I)DP-ribose hydrolyse also some nonreducing NDP-X substrates. So far, of these two enz yme types only the less specific one has been reported in human sources: an ADP-sugar pyrophosphatase purified from erythrocytes or expressed from cDN A clones. Here we report that human placenta extracts contain two ADP-ribos e hydrolases, which were characterised after a near 1000-fold purification. One is an ADP-sugar pyrophosphatase: it hydrolysed ADP-ribose, ADP-glucose and ADP-mannose, but not e.g. UDP-glucose, at similar rates. It resembles the erythrocyte and recombinant enzyme(s), but showed a 5-20-fold lower K-m for ADP-ribose (7 muM). The other enzyme is a highly specific ADP ribose p yrophosphatase (the first of this kind to be reported in humans): it hydrol ysed only ADP-ribose and IDP-ribose at similar rates. with a very low, 0.4 muM K-m for the former. This is a major candidate to control the accumulati on of free ADP-ribose in humans. It remains to be seen whether it belongs t o the 'nudix' protein family, which includes several ADP-ribose hydrolases and other 'housecleaning' enzymes (M.J. Bessman, D.N. Frick, S.F. O'Handley , J. Biol. Chem. 271 (1996) 25059-25062). (C) 2001 Elsevier Science B.V. Al l rights reserved.