Reactivation of NAD(H) biosynthetic pathway by exogenous NAD+ in Nil cells severely depleted of NAD(H)

Xanthine oxidase/dehydrogenase activity in intact cultured cells (in situ analysis)

The measured ratio of xanthine oxidase activity to the total activity of xanthine oxidase and dehydrogenase showed higher values in intact cells than when similar cells were homogenized. The total activity was the same for both systems. The xanthine oxidase ratio was 90, 60, 50, 50, 60% in V79, RIF/Ha3, SCC7, KHT intact cells and freshly extracted murine peritoneal macrophages respectively while the corresponding ratios measured were 25, 40, 38, 35, 22% when the cells were lysed by homogenization. Superoxide radical O2-. production by addition of xanthine to intact or homogenized cells to activate intracellular xanthine oxidase was higher in intact than homogenized cells. Homogenization of cells and tissues in the presence of dithioerythritol (DTE) can evidently lead to a considerable under-estimation of the xanthine oxidase ratio. The effect of hypoxia on cells has also been examined.

Poly(ADP-ribose) may signal changing metabolic conditions to the chromatin of mammalian cells

In mammalian cells, NAD+ serves a dual role as a respiratory coenzyme and as a substrate for the posttranslational poly(ADP-ribose) modification of chromatin proteins, catalyzed by the nuclear enzyme poly(ADP-ribose) polymerase [NAD+ ADP-ribosyltransferase, EC 2.4.2.30]. Biological evidence strongly suggests that poly(ADP-ribosyl)ation modulates chromatin functions, although the precise molecular mechanisms involved have not yet been elucidated. Here we describe conditions for the rapid uptake of exogenously supplied NAD+ by living hepatocytes in primary monolayer culture. Raising the intracellular NAD+ concentration by 70% caused a 5-fold increase of chromatin-bound poly(ADP-ribose). We conclude that the constitutive level of posttranslational poly(ADP-ribose) modifications of chromatin proteins in mammalian cells is related to the availability of NAD+, which varies in different physiological and pathological states. We propose that poly-(ADP-ribose) may serve a hitherto unrecognized function by signaling altered metabolic conditions to the chromatin and thus modulate its functions in tune with changing metabolic states.

ADP-ribosylation in cultured cells treated with Clostridium difficile toxin B

In cultured fibroblasts intoxicated with Clostridium difficile toxin B, a radioactive moiety was transferred from [14C-adenosine]NAD, but not from [14C-nicotinamide] NAD, into a cellular protein (MW 90,000). No labeling was detected in toxin-treated cultures not yet showing any toxin-induced cytopathogenic effect, whereas maximal labeling was obtained in cultures with about half of the cells showing a cytopathogenic effect. The radioactivity was removed from the substrate by treatment with snake venom phosphodiesterase. The results suggest that ADP-ribosylation of a cellular protein occurs in toxin B-treated cells and that this reaction may be responsible for development of the cytopathogenic effect.

Hexose transport derepressed and refractory to purine regulation in NAD(H)-Depleted Nil cells

Nil hamster fibroblasts depleted of NAD(H) by growth in medium devoid of nicotinamide (NAm-MEM) exhibit up to 2-3-fold higher rates of glucose transport. Derepression of glucose transport is observed only when Nil cells have become severely depleted of both intracellular NAD(H) and ATP, despite the continued presence of 5.5 mM D-glucose in the growth medium. Neither the initial rate of transport, approximated from 3-O-methylglucose uptake, nor accumulation of D-glucose itself is repressed upon restoring nicotinamide to the medium. Exposure of the cells to NAD+ (10(-5) M), however, leads to a sharp curtailment of transport within 2 to 3 hours. The purines, hypoxanthine and guanine, that sharply reduce glucose transport capacity of normal cells, have no significant effect upon transport activity of NAD(H)-depleted cells.