degradation of dihydrofolate and tetrahydrofolate.Can. J. Biochem. 43,999-1009Biochem. 43,999- (1970. 7,8-Dihydrofolate has been degraded oxidatively using ferricyaniae under anaerobic conditions. Three reaction pathways have been found for this oxidation, and these pathways can be controlled by variation of the pH and by the nature of the buffer anion. At pH 9, treatment s f dihydrofolate with two equivalents of ferricyanide leads to quantitative formation of folate. At pH 5.6 in morpholinoethanesulfonic acid buffer, the same reactants produce p-arninobenzoylglutarnate, dihydrsxanthogterin, and formaldehyde. Each of these products has been identified, and isolated in high yield. 6-Formyldihydropterin can be isolated by oxidation of dihydrofolate in either phosphate or carboxylic acid buffers. This compound does not appear to be an intermediate in the formation of dihydroxanthopterin under the conditions of our experiments. Tetrahydrofolate was also degraded oxidatively using ferricyanide. Although it is more susceptible to cleavage than is dihydrofolate, tetrahydrofolate gives similar products. In addition, moderate quantities of pterin were isolated from oxidation of tetrahydrofolate at pH 5.6.

ARCHER, Me C., a m SCRIMGEBUR, K. G . Rearrangement of quinonoid dihydropteridines to 7,8dihydropteridines. Can. J, Biochem. 48, 278-287 (1996).The quincpnoid isomer of 6,7-dimethyltetrahydropterin has been prepared in situ from the tetra-Baydropteedn by rapid oxidation with potassium ferricyanide. Measurements of the kinetics sf the rearrangement of the quinonoid dihydropterin to 7,8-dihydropterin show that the reaction is hugercatalyzed. Both the acidic and basic forms of the buffer participate as catalysts, with the relative contribution sf each species toward the rate s f the reaction being dependent on the pKa value sf each buffer. Isotopic experiments have shown that the cleavage of the C-H bond at position 6 is ratelimiting during both the acid-and the base-catalyzed rearrangements.

The oxidation of tetrahydropterins and 7,8-dihydropterins by ferric iron has been examined by both polarographic and spectral techniques. Quinonoid dihydropterin, an intermediate in the oxidation of the tetrahydropterins, is formed rapidly, as is ferrous iron. The quinonoid dihydropterin then rearranges to 7,8-dihydropterin. The rate of oxidation of 7,8-dihydropterins is much slower than that of tetrahydropterins. The reduction potential of the reversible quinonoid dihydropterin – tetrahydropterin couple has been measured and shown to be consistent with the known chemistry of these compounds. Kinetic measurements and structural studies have been used to formulate a mechanism for the oxidation of tetrahydropterins to quinonoid dihydropterins. This mechanism involves abstraction of an electron from position 5, followed by rapid loss of two protons and the other electron.

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