Monitoring lipid peroxidation by breath analysis: Endogenous hydrocarbons and their metabolic elimination

Abstract

Monitoring of ethane and pentane in breath as a noninvasive method to measure lipid peroxidation is performed by an increasing number of laboratories. The alkanes are generated through peroxidative breakdown of polyunsaturated fatty acids. Fasted Sprague-Dawley rats exhale these hydrocarbons at a rate of approximately 1.7 nmol/kg·hr. Through an improved analytical procedure other volatile hydrocarbons could be detected in the breath of the animals, i.e., ethene (1.1 nmol/kg·hr), propane (0.7 nmol/kg·hr), n-butane (0.7 nmol/kg·hr), iso-pentane, and iso-butene. The exhaled hydrocarbons accumulate in the atmosphere if an animal is confined in a closed system. However, the increase is not linear but a steady-state concentration is approached, presumably due to reuptake and subsequent hepatic metabolism. Hydrocarbons are oxidized by the hepatic monooxygenases, at rates increasing with their molecular masses. The same has been found for the elimination of ethane, propane, butane, pentane, and iso-butene by a rat from closed system. Metabolism is inhibited by a variety of substances, e.g., dithiocarb, ethanol, and tetrahydrofurane. Therefore, the increased release of hydrocarbons into the gas phase, after treatment with compounds suspected to induce lipid peroxidation, may merely result from decreased metabolism. Especially if the hydrocarbon exhalation is only moderately elevated as after ethanol administration, this aspect must be taken into account. Similarly, peroxidizing microsomes show elevated hydrocarbon output when treated with carbon monoxide in order to block metabolism. This study provides a procedure for discrimination of inhibitory and peroxidative action which is of particular importance if the increase over control levels is only moderate. In addition, it may serve as monitor for the metabolic capacity of a laboratory animal.