Synthesis And Biochemistry Of Ascorbic Acid Analogues

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One explanation for selective in vivo toxicity of ascorbic acid to cancer cells is that ascorbic acid serves as a pro-drug for preferential steady state formation of ascorbate radical and hydrogen peroxide in the extravascular space compared to blood. To test this hyptothesis, a procedure was developed for direct measurement of the relationship between ascorbic acid concentration and hydrogen peroxide in extracellular fluid using a synthetic boronate fluorophore peroxyanthone, coupled to catalase controls. The peroxyxanthone, prepared using a modification of a recent literature report, reacts with hydrogen peroxide through the usual ?ate? mechanism to produce a fluorescent molecule. Based on the unique chemistry of boron compounds, the reaction is specific for hydrogen peroxide. Rats were administered intravenous, oral, or intraperitoneal ascorbic acid using typical human pharmacologic doses (0.25-0.5 mg/g). In extracellular fluid obtained by microdialysis and in blood, ascorbic acid was measured by HPLC with coulometric electrochemical detection and ascorbate radical was measured by electron paramagnetic resonance. Upon intravenous injection, ascorbic acid concentrations in blood and extracellular fluid increased from baselines of 50-100 uM to peaks of approximately 8 mM, and intraperitoneal injections produced peak ascorbic acid concentrations approaching 3 mM in blood and extracellular fluid. Ascorbate radical concentrations in extracellular fluid were as much as 9 fold higher than those in blood, even though ascorbic acid concentrations in both compartments were equivalent. In extracellular fluid, ascorbate radical concentrations as high as 250 nM occurred and were an exponential function of ascorbic acid concentrations. Hydrogen peroxide in extracellular fluid was detected when ascorbate radical concentrations in extracellular fluid exceeded 100 nM, and this occurred only with parenteral administration. Hydrogen peroxide concentrations of 20-30 uM in extracellular fluid corresponded to the highest ascorbate radical concentrations of approximately 250 nM. Taken together, these data validate the hypothesis that pharmacologic ascorbic acid is a pro-drug for preferential steady state formation of ascorbate radical and hydrogen peroxide in the extravascular space but not blood. We are continuing synthetic work to prepare new flavonoid analogues to study sturtural parameters of these inhibitors of ascorbate and glucose transport. Inculded will be potential affinity labels for the transport protein(s). Effects of fluorine substitution biolgical begavior of the flavonoids also will be examined. A series of fluorinated chalcones have been prepared and will be cyclized under conditions that produce the flavonoid structures.