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Neutrophils, in addition to their role in host defense, can cause injury to normal tissues during inflammatory processes. Oxygen radicals and secreted proteases, in particular, are responsible for some aspects of neutrophil-mediated injury to endothelial cells and cardiomyocytes. A variety of neutrophil functions, including adhesion and reactive oxygen species production, are inhibited by adenosine (Ado) (Cronstein, 1991 and Cronstein, et al., 1992). Furthermore, inhibition of neutrophil adhesion by adenosine regulating agents like acadesine and adenosine kinase (AK) inhibitors (Firestein, et al., 1994) appears to be mediated by Ado, since it is reversed by the addition of adenosine deaminase (ADA) or Ado receptor antagonists. Although Ado and adenine nucleotides can be released at inflammatory sites during platelet aggregation or from endothelial cells during ischemic stress conditions, little is known about Ado formation by human neutrophils. To determine if neutrophils can serve as an endogenous Ado source and thereby provide an autocrine stimulus, we evaluated purine metabolism and Ado formation in human neutrophils.

A large number of physiological functions of adenosine including regulation of coronary vasodilation, (1) neurotransmission (2) and immune response (3) have been described and discussed (4–8). However, the mechanism(s) involved in adenosine formation remain controversial. Adenosine found in the extracellular environment can be formed by: a) the intracellular process of AMP dephosphorylation and SAH hydrolysis resulting in adenosine release across the cell membrane; b) extracellular degradation of adenine nucleotides released from the tissues; c) both intracellular and extracellular mechanisms occurring simultaneously.

Little is known about the formation and role of Z-nucleotides (ZMP,ZDP,ZTP) in rat and human cells. In cells having an active purine de novo biosynthesis pathway, ZMP (AICA-riboside monophosphoribonucleoside) is formed naturally as one of the final intermediates in IMP biosynthesis. ZMP, when synthesized by this pathway, is then efficiently formylated by 5-amino-4-imidazole-carboxamide 5′ribonucleotide transformylase (EC 2.1.2.3) and therefore the intracellular level of ZMP is usually very low and is not detectable by HPLC (1–3). ZMP can also be formed when cells with an active or absent de novo purine biosynthesis pathway are exposed to acadesine (AICA-riboside) or RICA-base. ZMP is formed from acadesine after phosphorylation, whereas formation from AICA-base requires the transfer of the phosphoribosyl group from PRPP. It has already been established that adenosine kinase (AK) is responsible for the phosphorylation of acadesine to ZMP, and AK inhibited or AK deficient cells are unable to metabolize acadesine (4–7). ZMP, when formed from acadesine, can be further metabolized by alternative pathways depending on its intracellular concentrations (2). At low ZMP concentrations, metabolism of ZMP follows predominantly the steps of de novo synthesis of IMP and other purines. At intermediate concentrations of ZMP, net retrograde flux through adenylosuccinate lyase results in sZMP (5-amino-4imidazole-N-succinocarboxamide ribonucleotide) formation, while at high ZMP concentrations, accumulation of ZMP, ZDP and ZTP becomes significant.