Phosphohydrolases on the cell surface of BHK cells: loss during long term culture

Enzymatic activities affecting exogenous nicotinamide adenine dinucleotide in human skin fibroblasts

The fate of nicotinamide adenine dinucleotide (NAD), AMP, and ADP-ribose supplied to intact human skin fibroblasts was monitored, and the concentrations of intra- and extracellular pyridine and purine compounds were determined by HPLC analysis. Two enzymatic activities affecting extracellular NAD were detected on the plasma membrane, one hydrolyzing the pyrophosphoric bond and yielding nicotinamide mononucleotide (nucleotide pyrophosphatase) and the other cleaving the glycoside link and releasing nicotinamide (NAD-glycohydrolase). No AMP or ADP-ribose was found in the extracellular medium of cells incubated with NAD, the former being completely catabolized to hypoxanthine and the latter degraded to adenine and hypoxanthine.

5'-Nucleotide phosphodiesterase and alkaline phosphatase in tumor cells: evidence for existence of novel species in the cytosol

Characteristics of 5'-nucleotide phosphodiesterase (phosphodiesterase I, EC 3.1.4.1) and alkaline phosphatase (EC 3.1.3.1) activities in tumor cell lines of human and murine origin were examined. Of the 15 cell lines tested, 5'-nucleotide phosphodiesterase activity in 13 cell lines and alkaline phosphatase activity in 10 cell lines were inhibited by N-ethylmaleimide and activated by dithiothreitol (N-ethylmaleimide-sensitive), and suggested to be SH-enzymes. In contrast, the two phosphohydrolases from normal tissues were inactivated by dithiothreitol, but not by N-ethylmaleimide (dithiothreitol-sensitive). There was only one tumor cell line in which both activities were dithiothreitol-sensitive. Human hepatoma PLC/PRF/5 cells appear to possess both types of 5'-nucleotide phosphodiesterase and alkaline phosphatase, and the subcellular distribution of these enzymes in this cell line was investigated. Dithiothreitol-sensitive 5'-nucleotide phosphodiesterase and alkaline phosphatase of PLC/PRF/5 cells were localized in the plasma membrane as in normal tissues, but N-ethylmaleimide-sensitive phosphohydrolases were soluble cytosolic proteins. N-Ethylmaleimide-sensitive 5'-nucleotide phosphodiesterase and alkaline phosphatase activities from other cell lines were also recovered in the cytosol. Molecular masses of cytosolic N-ethylmaleimide-sensitive phosphohydrolases were apparently smaller than their membrane-bound dithiothreitol-sensitive counterparts, as judged from gel filtration. It was concluded that many tumor cell lines lack plasma membrane 5'-nucleotide phosphodiesterase and alkaline phosphatase, but express enzymes with similar activities in the cytosol, with properties clearly distinguishable from enzymes so far characterized.

A pyrophosphatase which degrades NAD+ is located on the external surface of cultured fibroblasts: evidence that NAD+ is not extruded during treatment with N-methyl-N'-nitro-N-nitrosoguanidine

The Harvey sarcoma virus-transformed derivative of normal rat kidney cells contains a very large membrane-associated NAD+ pyrophosphatase activity. Exogenous NAD+ was metabolized into NMN, demonstrating that the enzyme was located, at least in part, on the external surface of the cell. This external location of the pyrophosphatase was used to evaluate possible extrusion of NAD+ during treatment with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG); i.e., extruded NAD+ would be degraded to NMN. NAD+ levels quickly decreased following MNNG addition. The nicotinamide moiety of degraded NAD+ was accounted for as nicotinamide in the culture medium, and no NMN was detectable. Thus, NAD+ was not extruded during MNNG treatment. Additional studies were done to determine which enzyme system was responsible for the rapid fall in NAD+ in these cells. MNNG treatment increased (ADP-rib)n synthetase activity; moreover, this increase in activity was sufficient to account for the loss in intracellular NAD+. Also, the decrease in NAD+ during treatment with MNNG was prevented by 3-aminobenzamide, an agent which inhibited (ADP-rib)n synthetase but not NAD+ glycohydrolase or pyrophosphatase. MNNG had no effect on NAD+ glycohydrolase or pyrophosphatase activities. The results are consistent with the proposal that NAD+ is not extruded but rather is rapidly metabolized by the nuclear (ADP-rib)n synthetase during MNNG treatment.