Biochemical analysis of ecto-nucleotide pyrophosphatase phosphodiesterase activity in brain membranes indicates involvement of NPP1 isoenzyme in extracellular hydrolysis of diadenosine polyphosphates in central nervous system

Synatptosomes and plasma membranes obtained from rat brain display ectoenzymatic hydrolytic activity responsible for hydrolysis of the neurotransmitter/neuroregulatory nucleotides diadenosine polyphosphates. Intact synaptosomes and plasma and synaptic membranes isolated by sucrose gradient ultracentrifugation from several brain regions (hypothalamus, hippocampus, temporal cortex, frontal cortex striatum and cerebellum) degraded the fluorogenic substrates diethenoadenosine polyphosphates up to ethenoadenosine as by-product. Purified ectoenzyme cleaved substrates always releasing the mononucleotide moieties ethenoadenosine 5'-monophosphate and the corresponding ethenoadenosine (n - 1) 5'-phosphate. Ectoenzymatic hydrolysis reached maximal activity at pH 9.0 (pH range 6.5-9.0) and was activated by Ca2+ and Mg2+ ions, with maximal effects around 2.0 mM cation. EDTA drastically reduced activity and Zn2+ was required for enzyme reactivation. Hydrolysis of substrates followed hyperbolic kinetics with Km values in the 3-10 mu M range. Diadenosine polyphosphates and heparin behaved as competitive inhibitors in the enzymatic hydrolysis of diethenoadenosine polyphosphates and AMP, ATP, alpha,beta-methyleneADP, ADP beta S ATP gamma S, beta,-gamma-methyleneATP, suramin and diethyl pyrocarbonate were also inhibitors. Ectoenzymatic activity shared the typical characteristics of members of the e to-nucleotide pyrophosphatase/phosphodiesterase (E-NPP) family and inhibition data suggest that NPP1 ectoenzyme is involved in the cleavage, of extracellular diadenosine polyphosphates in brain. Synaptic membranes from cerebellum, hypothalamus and hippocampus presented the highest activities and no activity differences were observed between young and aged animals. However, plasma membranes showed a more homogeneous distribution of ectoenzymatic activity but a general increase was detected in aged animals. Enhancement of ectoenzymatic diadenosine polyphosphate cleaving activity found in plasma membranes from old animals could play a deleterious role in aged brain by limiting neuroprotective effects reported for extracellular diadenosine tetraphosphate. (c) 2006 Elsevier Ltd. All rights reserved.