Citrulline, a novel compatible solute in drought-tolerant wild watermelon leaves, is an efficient hydroxyl radical scavenger

Drought-tolerant wild watermelon accumulates high levels of citrulline in t he leaves in response to drought conditions. In this work, the hydroxyl rad ical-scavenging activity of citrulline was investigated in vitro. The secon d-order rate constant for the reaction between citrulline and hydroxyl radi cals was found to be 3.9 x 10(9) M-1 s(-1), demonstrating that citrulline i s one of the most efficient scavengers among compatible solutes examined so far. Moreover, citrulline effectively protected DNA and an enzyme from oxi dative injuries. Liquid chromatography-mass spectrometry analysis revealed that at least four major products were formed by the reaction between citru lline and hydroxyl radicals. Activities of metabolic enzymes were not inhib ited by up to 600 mM citrulline, indicating that citrulline does not interf ere with cellular metabolism. We reasoned, from these results, that citrull ine contributes to oxidative stress tolerance under drought conditions as a novel hydroxyl radical scavenger. (C) 2001 Federation of European Biochemi cal Societies. Published by Elsevier Science B.V. All rights reserved.