THE RELATIONSHIP BETWEEN SUGAR METABOLISM, TRANSPORT AND SUPEROXIDE RADICAL PRODUCTION IN RAT PERITONEAL-MACROPHAGES

Dexamethasone inhibits sugar-dependent phorbol myristate acetate (PMA) -stimulated superoxide production and 2-deoxy-D-glucose (2-dGlc) trans port in rat peritoneal macrophages (Rist, R.J., Jones, G.E. and Naftal in, R.J. (1991) Biochem. J. 278, 119-128; Rist, R.J. and Naftalin, R.J . (1991) Biochem J. 278, 129-135). Here it is shown that with glucose as a substrate, dexamethasone (0.1 muM) acts as a non-competitive inhi bitor of PMA-induced superoxide production; decreasing the maximal rat e of superoxide production (P < 0.001) without altering the K(m). In c ontrast, with 2-dGlc as a substrate, dexamethasone shows competitive i nhibition of PMA-stimulated superoxide production; increasing the K(m) of superoxide production, (P < 0.001) without altering the V(max). Th e maximal rate of PMA-stimulated superoxide production with glucose as substrate is 10-12-fold in excess of the maximal rate with 2-dGlc as substrate. Diphenylene iodonium (DPI) is a non-competitive inhibitor o f PMA-stimulated glucose-dependent superoxide production in macrophage s, (K(i) = 1-5 muM) and significantly reduces the activity of the PMA- induced hexose monophosphate shunt, (HMPS) (P < 0.01). However, DPI (1 muM) has no significant effect on the PMA-induced increase in 2-dGlc uptake, suggesting that the stimulus for HMPS activity and superoxide production is separate from the stimulus for hexose transport. A model is described which explains the observed differences in hexose transp ort and glucose- and 2-dGlc-dependent superoxide production in terms o f the differences in metabolism of the two sugars. Accumulation of fre e 2-dGlc within the cytosol leads to saturation of hexokinase and henc e, the effects of PMA and dexamethasone, which alter the coupling betw een hexokinase and the transporter, arc only observed at low concentra tions of 2-dGlc, where it is accumulated to sub-saturating levels. Sin ce glucose is completely metabolized within the cell, PMA and dexameth asone increase and decrease, respectively, net uptake of sugar and sup eroxide production at all glucose concentrations.