NADH oxidase activity of soybean plasma membranes inhibited by submicromolar concentrations of ATP

Evolutionary origins of the purinergic signalling system

Purines appear to be the most primitive and widespread chemical messengers in the animal and plant kingdoms. The evidence for purinergic signalling in plants, invertebrates and lower vertebrates is reviewed. Much is based on pharmacological studies, but important recent studies have utilized the techniques of molecular biology and receptors have been cloned and characterized in primitive invertebrates, including the social amoeba Dictyostelium and the platyhelminth Schistosoma, as well as the green algae Ostreococcus, which resemble P2X receptors identified in mammals. This suggests that contrary to earlier speculations, P2X ion channel receptors appeared early in evolution, while G protein-coupled P1 and P2Y receptors were introduced either at the same time or perhaps even later. The absence of gene coding for P2X receptors in some animal groups [e.g. in some insects, roundworms (Caenorhabditis elegans) and the plant Arabidopsis] in contrast to the potent pharmacological actions of nucleotides in the same species, suggests that novel receptors are still to be discovered.

Investigation of methaemoglobin reduction by extracellular NADH in mammalian erythrocytes

The effect of extracellular NADH on the rate of reduction of nitrite-induced methaemoglobin in erythrocytes from man, cattle, dog, horse, grey kangaroo, pig and sheep was investigated. Extracellular NADH was found to enhance the rate of methaemoglobin reduction in man, dog, pig and kangaroo erythrocytes, but had essentially no effect on the rate of methaemoglobin reduction in erythrocytes from cattle, horse and sheep. In erythrocytes of those animals affected by extracellular NADH the rate of reduction of metHb in the presence of NADH was the same or greater than that observed in the presence of nutrients such as glucose and inosine. The combination of nutrient and NADH produced a more profound increase in the rate of methaemoglobin reduction. The rate of methaemoglobin reduction in all cases was significantly less than that observed with methylene blue, the standard treatment of methaemoglobinaemia. Extracellular NADH was found to indirectly increase the intracellular NADH concentration through displacement of the pseudo-equilibrium of the intracellular LDH reaction and relied upon the presence of sufficient LDH activity released into the extracellular medium through haemolysis. The lack of response of cattle, horse and sheep RBCs to extracellular NADH was found to derive mainly from their low extracellular LDH activity, but also correlated with their lower NADH-methaemoglobin reductase activity compared to the other species.

Periodic fluctuations in oxygen consumption comparing HeLa (cancer) and CHO (non-cancer) cells and response to external NAD(P)+/NAD(P)H

Oxygen consumption in the presence of cyanide was utilized as a measure of plasma membrane electron transport in Chinese hamster ovary (CHO) and human cervical carcinoma (HeLa) cell lines. Both intact cells and isolated plasma membranes carry cyanide-insensitive NADH(P)H oxidases at their external membrane surfaces (designated ECTO-NOX proteins). Regular oscillatory patterns of oxygen consumption with period lengths characteristic of those observed for rates of NADH oxidation by ECTO-NOX proteins were observed to provide evidence for transfer of protons and electrons to reduce oxygen to water. The oscillations plus the resistance to inhibition by cyanide identify the bulk of the oxygen consumption as due to ECTO-NOX proteins. With intact CHO cells, oxygen consumption was enhanced by but not dependent upon external NAD(P)H addition. With intact HeLa cells, oxygen consumption was inhibited by both NADH and NAD+ as was growth. The results suggest that plasma membrane electron transport from internal donors to oxygen as an external acceptor is mediated through ECTO-NOX proteins and that electron transport to molecular oxygen may be differentially affected by external pyridine nucleotides depending on cell type.