Ribeiro J M, Carloto A, Costas M J, Cameselle J C
Unidad de Bioquímica y Biología Molecular, Facultad de Medicina, Universidad de Extremadura, Apartado de Correos 108, E-06080 Badajoz, Spain.
Biochim Biophys Acta. 2001 Apr 3;1526(1):86-94. doi: 10.1016/s0304-4165(01)00113-1.
Free ADP-ribose has a reducing ribose moiety and it is hazardous due to its nonenzymic reactivity toward protein side chains. ADP-ribose hydrolases are putative protective agents to avoid the intracellular accumulation of ADP-ribose. In mammalian sources, two types of enzymes with ADP-ribose hydrolase activity are known: (i) highly specific ADP-ribose pyrophosphatases, which in a Mg(2+)-dependent fashion hydrolyse only ADP-ribose and the nonphysiological analogue IDP-ribose, and (ii) less specific nucleoside diphosphosugar or diphosphoalcohol (NDP-X) pyrophosphatases, which besides A(I)DP-ribose hydrolyse also some nonreducing NDP-X substrates. So far, of these two enzyme types only the less specific one has been reported in human sources: an ADP-sugar pyrophosphatase purified from erythrocytes or expressed from cDNA clones. Here we report that human placenta extracts contain two ADP-ribose 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 microM). The other enzyme is a highly specific ADP-ribose pyrophosphatase (the first of this kind to be reported in humans): it hydrolysed only ADP-ribose and IDP-ribose at similar rates, with a very low, 0.4 microM K(m) for the former. This is a major candidate to control the accumulation of free ADP-ribose in humans. It remains to be seen whether it belongs to 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).
游离的ADP - 核糖具有还原性核糖部分，因其对蛋白质侧链的非酶反应性而具有危险性。ADP - 核糖水解酶被认为是避免ADP - 核糖在细胞内积累的保护剂。在哺乳动物来源中，已知有两种具有ADP - 核糖水解酶活性的酶：（i）高度特异性的ADP - 核糖焦磷酸酶，它以Mg(2+)依赖的方式仅水解ADP - 核糖和非生理性类似物IDP - 核糖；（ii）特异性较低的核苷二磷酸糖或二磷酸醇（NDP - X）焦磷酸酶，除了ADP - 核糖外，还能水解一些非还原性的NDP - X底物。到目前为止，在人类来源中仅报道了这两种酶类型中特异性较低的一种：一种从红细胞中纯化或从cDNA克隆中表达的ADP - 糖焦磷酸酶。在此我们报道，人胎盘提取物含有两种ADP - 核糖水解酶，经过近1000倍的纯化后对其进行了表征。一种是ADP - 糖焦磷酸酶：它以相似的速率水解ADP - 核糖、ADP - 葡萄糖和ADP - 甘露糖，但例如不水解UDP - 葡萄糖。它类似于红细胞和重组酶，但对ADP - 核糖的K(m)值低5 - 20倍（7 microM）。另一种酶是高度特异性的ADP - 核糖焦磷酸酶（这是在人类中报道的第一种此类酶）：它以相似的速率仅水解ADP - 核糖和IDP - 核糖，对前者的K(m)值非常低，为0.4 microM。这是控制人体内游离ADP - 核糖积累的主要候选者。它是否属于“nudix”蛋白家族还有待观察，该家族包括几种ADP - 核糖水解酶和其他“清洁”酶（M.J. Bessman，D.N. Frick，S.F. O'Handley，J. Biol. Chem. 271 (1996) 25059 - 25062）。
