5′-Nucleotidase activity and adenosine formation in stimulated, hypoxic and underperfused rat heart

Abstract

Changes in 5'-nucleotidase activity were calculated on the basis of alterations in ATP, ADP, phosphocreatine, Pi, Mg2+, IMP and AMP, determined by using 31P n.m.r. spectroscopy and h.p.l.c., during isoprenaline infusion, graded hypoxia and graded underperfusion in isolated rat heart. Calculated activity changes were compared with the total efflux of purines (adenosine + inosine + hypoxanthine) in order to assess the involvement of various 5'-nucleotidases in formation of adenosine. Purine efflux exhibited an exponential relation with cytosolic [AMP] during isoprenaline infusion and hypoxia (r = 0.92 and 0.95 respectively), supporting allosteric activation of 5'-nucleotidase under these conditions. Purine efflux displayed a linear relation with cytosolic [AMP] during graded ischaemia (r = 0.96), supporting substrate regulation in the ischaemic heart. The calculated activities of membrane-bound ecto-5'-nucleotidase were similar to the observed relations between purine efflux and cytosolic [AMP] in all hearts. The calculated activities of the ATP-activated cytosolic and lysosomal enzymes and of the ATP-inhibited cytosolic 5'-nucleotidase could not explain the observed release of purines under the conditions examined. These results indicate that the kinetic characteristics of the membrane-bound ecto-enzyme are consistent with an important role in the formation of extracellular adenosine, whereas the characteristics of the other 5'-nucleotidases are inconsistent with roles in adenosine formation under the conditions of the present study.

Surface expression, polarization, and functional significance of CD73 in human intestinal epithelia.

“…CD73 is a purine salvage enzyme located on the external surface membrane of a wide range of cell types (21,(38)(39)(40). CD73 serves a regulatory role in neurotransmitter release (40), functions to control blood flow (39) and may play a role in the immune system response (34).…”

“…CD73 serves a regulatory role in neurotransmitter release (40), functions to control blood flow (39) and may play a role in the immune system response (34). The cDNA has been cloned from human placenta and bovine and rat liver.…”

Acute adenosinergic cardioprotection in ischemic-reperfused hearts

“…Consistent with the substratedependent control of adenosine formation, numerous studies document correlations between free cytosolic [5Ј-AMP] and myocardial adenosine levels (103-107, 116, 144). However, adenosine formation generally increases much more than 5Ј-AMP (48, 90,107,116), indicating formation via additional paths, allosteric activation of 5Ј-nucleotidase (e.g., by Mg 2ϩ and ADP) (105), and/or inhibition of adenosine catabolism.…”

“…Changes in these modulators during ischemia-hypoxia may be sufficient to amplify the effects of the increased substrate (105), contributing to nonlinear relationships between adenosine generation and [5Ј-AMP]. However, more compelling evidence demonstrates that modulation of adenosine salvage via adenosine kinase is the primary mechanism by which formation is markedly enhanced during periods of deenergization.…”

Adenosine downregulates DPPIV on HT-29 colon cancer cells by stimulating protein tyrosine phosphatase(s) and reducing ERK1/2 activity via a novel pathway

“…Adenosine, a purine nucleoside known best for its role in energy metabolism, is produced at increased levels in the extracellular fluid of solid tumors (8). This is a consequence of changes in the rates of production and removal of adenosine that are characteristic of hypoxic tissues like those in the poorly vascularized tumor (15,31,77). The adenosine that is produced is likely to have widespread actions within the tumor, including suppression of the cell-mediated immune response (33,34,40,43,44), promotion of angiogenic activity (4), stimulation of the motility of tumor cells (82), and stimulation of tumor cell growth (47,48).…”