8‐hydroxydeoxyguanosine as a urinary biomarker of oxidative DNA damage

Abstract

Living organisms are continuously exposed to reactive oxygen species as a consequence of biochemical reactions as well as external factors. Oxidative DNA damage has been implicated in aging, carcinogenesis and other degenerative diseases. The urinary excretion of the DNA repair product 8‐hydroxydeoxyguanosine (8OHdG) has been proposed as a noninvasive biomarker of oxidative DNA damage in humans in vivo. We have developed a three‐dimensional HPLC analysis with electrochemical detection for the analysis of 8OHdG in urine and studied factors affecting the excretion of this biomarker in 83 healthy humans and in various laboratory animals, including dog, pig, and rat. Previously, other groups have used comparable HPLC methods or gas chromatography‐mass spectrometry with selective ion monitoring for measuring the excretion of 8OHdG in humans, rats, mice, and monkeys. In the 169 humans studied so far, the average 8OHdG excretion was 200–300 pmol/kg per 24 h with a sevenfold range, and the coefficient of variation was 30–40%. This excretion corresponds 140–200 oxidative modification of guanine bases per cell per day. Thirty‐two smokers from our study population excreted 50% (31–69%; 95% confidence interval) more 8OHdG than 53 nonsmokers. This indicates a 50% increased rate of oxidative DNA damage from smoking, adding to the other well‐known health hazards of smoking. The biochemical‐physiological basis is unknown but may be related to smoke constituents including or generating reactive oxygen species and/or consuming antioxidants and/or the well‐known enhancing effect of smoking on the metabolic rate. In our 83 healthy subjects the 8OHdG excretion correlated with body composition. Thus, lean and/or male subjects excreted more than obese and/or female subjects, possibly related to differences in metabolic rate. In accordance, the excretion of 8OHdG decreased after calorie restriction, which will cause a decline in the metabolic rate. Across the investigated species, humans, dogs, pigs, and rats, the excretion of 8OHdG correlated with the specific metabolic rate, confirming data from other groups on humans, monkeys, rats, and mice. The excretion of 8OHdG decreased with age in rats in parallel with the decline in metabolic rate with advancing age. The excretion of 8OHdG reflects the formation and repair of only one out of approximately 20 described oxidative DNA modifications. So far, methods are not available for the determination of the corresponding repair products, except 8OHdG and thymidine glycol, in urine. Moreover, the importance in terms of mutagenicity, particularly regarding tumour supressor genes and oncogenes, is mainly documented for 8OHdG in DNA. In mammalian cells an excission repair enzyme complex for 8OHdG has been demonstrated, but whether the main product is 8OHdG or the base is yet unknown. In addition, 8OHdG may derive from DNA during turnover of mitochondria and cells as well as from oxidation in the deoxynucleotide and deoxynucleoside pools that provide building blocks for new DNA. Thus, the excretion of 8OHdG will reflect the general average risk of promutagenic oxidative adducts in DNA of all tissues and organs. We suggest that the individual variation in the apparent massive extent of oxidative DNA‐damage in humans predicts the rate of aging and the risk of cancer as well as other degenerative diseases. The use of 8OHdG and similar urinary biomarker of oxidative DNA damage offers a valuable tool for testing such hypotheses in humans.