Role of adenosine deaminase, ecto-(5'-nucleotidase) and ecto-(non-specific phosphatase) in cyanide-induced adenosine monophosphate catabolism in rat polymorphonuclear leucocytes

Abstract

The role of adenosine deaminase (EC 3.5.4.4), ecto-(5''-nucleotidase) (EC 3.1.3.5) and ecto-(non-specific phosphatase) in CN-induced catabolism of adenine nucleotides in intact rat polymorphonuclear leukocytes was investigated by inhibiting the enzymes in situ. KCN (10 mM for 90 min) induced a 20-30% fall in ATP concentration accompanied by an approximately equimolar increase in hypoxanthine. ADP, AMP and adenosine concentrations were unchanged, and IMP and inosine remained undetectable (< 0.05 nmol/107 cells). Cells remained 98% intact, as judged by loss of the cytoplasmic enzyme lactate dehydrogenase (EC 1.1.1.27). Pentostatin (30 .mu.M), a specific inhibitor of adenosine deaminase, completely inhibited hypoxanthine production from exogenous adenosine (55 .mu.M) but did not block CN-induced hypoxanthine production or cause adenosine accumulation in intact cells. This implied that IMP rather than adenosine was an intermediate in AMP breakdown in response to cyanide. Antibodies raised against purified plasma-membrane 5''-nucleotidase inhibited the ecto-(5''-nucleotidase) by 95-98%. Non-specific phosphatases were blocked by 10 mM-sodium .beta.-glycerophosphate. These 2 agents together blocked hypoxanthine production from exogenous AMP and IMP (200 .mu.M) by more than 90% but had no effect on production from endogenous substrates. Ectophosphatases apparently do not participate in CN-induced catabolism of intracellular AMP in rat polymorphonuclear leukocytes. A minor IMPase, not inhibited by antiserum, was detected in the soluble fraction of disrupted cells.