167 D-XYLULOSE-INDUCED DEPLETION OF ATP AND Pi AND INCREASE IN PRPP IN ISOLATED RAT HEPATOCYTES

Abstract

Xylitol provokes hepatic ATP catabolism by trapping Pi as glycerol 3-P (Woods & Krebs, Biochem. J. 134, 437-43, 1973). The dependence of the latter upon an increase in the NADH/NAD+ ratio, due to oxidation of xylitol to D-xylulose (D-Xyl), raised the question whether D-Xyl depletes ATP. Incubation of hepatocytes with 5 mM D-Xyl decreased ATP by 80 % within 5 min and by 30 % with 5 mM xylitol. Intracellular Pi decreased by 66 % within 5 min with both compounds, but reincreased 3-fold faster and reached supraphysiological levels with D-Xyl. D-Xyl was used at the rate of 5 Mmol/min/g of cells, i.e. 3-fold faster than xylitol. With D-Xyl, glycerol 3-P was barely modified but Xyl 5-P increased from 0.02 to 1.4 μ mol/g of cells. The main cause of the ATP− and Pi-depleting effect of D-Xyl was an accumulation of sedoheptulose 7-P which increased from 0.3 to 8 μ mol/g after 10 min. Ribose 5-P increased from 0.03 to 0.50 μmol/g with D-Xyl, and to 0.16 μmol/g with xylitol. Ribose 1-P also accumulated, albeit extracellulary, because of release of purine nucleoside phosphorylase from damaged cells, acting on inosine which diffused out. D-Xyl increased PRPP from a basal value of 6 ± 1 nmol/g to 509 ±72, as compared to 42 ± 11 with xylitol. It is concluded that the markedly higher PRPP level reached with D-Xyl results from a higher activity of PRPP synthetase, owing to a more marked increase of intracellular Pi and of ribose 5-P, combined with other, still unknown factors.