Intramolecular cyclization in irradiated nucleic acids: correlation between high-performance liquid chromatography and an immunochemical assay for 8,5'-cycloadenosine in irradiated poly(A).

A correlation between high-performance liquid chromatography (HPLC) analysis and an in situ enzyme-linked immunosorbent assay (ELISA) for 8,5'-cycloadenosine formation in irradiated poly(A) has been established. The correlation shows that the ELISA precisely reflects changes in the combined yield of R- and S-8,5'-cycloadenosine but that a correction factor must be applied to the ELISA values for accuracy. The HPLC analysis reveals that the intramolecular cyclization proceeds stereoselectively in irradiated poly(A) to preferentially produce the R isomer at pH 7.0 which is similar to the result for irradiated adenosine but in contrast to the result for 5'-AMP where the S isomer predominates at neutral pH. The HPLC analysis shows that two events originating in hydroxyl radical attack at the sugar phosphate backbone in poly(A); that is, adenine release and 8,5'-cycloadenosine formation have somewhat different dose-yield responses. The formation of 8-hydroxyadenosine was detected in the HPLC chromatograms of poly(A) irradiated under N2O at neutral pH, and the yield of this compound was similar to the yield observed in 5'-AMP or adenosine irradiated under similar conditions.

Stereoselective intramolecular cyclization in irradiated nucleic acids: R- and S-8,5'-cycloadenosine in polyadenylic acid.

The yields of R- and S-8,5'-cycloadenosines have been measured in poly A irradiated with gamma rays in the absence of oxygen. High performance liquid chromatographic analysis of the nucleoside analogues obtained by hydrolysis of the irradiated poly A shows that the R isomer predominates to the extent of 2.5-fold at doses of ionizing radiation in the range of 0-400 Gy.

Reductive activation of nitroaromatics and enzyme inhibition: misonidazole and xanthine oxidase.

Reductive activation of misonidazole and misondiazole acetate, a simple derivative, in the presence of xanthine oxidase causes inactivation of the enzyme. The inactivation is not accompanied by binding of the misonidazole to the enzyme. The nitroreductase activity of xanthine oxidase is inhibited as measured by the reduction of 3,5-dinitrobenzonitrile (DNBN). Cysteine does not appear to protect against the enzyme inactivation by misonidazole but, by itself, cysteine has a strong stimulating effect on the reduction of DNBN. The possible significance of these reactions to the toxicity of misonidazole are discussed.

Structure-function dependence and allopurinol inhibition of radiosensitizer/nitroreductase interaction: approaches to improving therapeutic ratios.

Normal tissue toxicity of nitroaromatic radiosensitizers may originate in radiosensitizer/nitroreductase interaction. A study of two mammalian cell nitroreductases, xanthine oxidase and NADH cytochrome c reductase, shows that the efficiency of electron transfer is dependent on sensitizer electron affinity and not lipid solubility. Misonidazole and its demethylated metabolite (RO-05-9963), for example, are equally efficient as electron acceptors from xanthine oxidase. The only exception to the electron affinity correlation is m-nitrobenzamidine hydrochloride (MNBAM) which results because MNBAM inhibits electron donation to xanthine oxidase from its cofactor, xanthine. Allopurinol inhibits electron transfer and might be a useful adjuvant to the use of radiosensitizers. Evidence that allopurinol interacts with nitroreductases in vivo is deduced from the observation that allopurinol significantly alters the serum lifetimes in mice of misonidazole and RO-05-9963.