The aim of this study was to investigate the effect of redox potential (Eh) on the growth of the yeast Yarrowia lipoytica in both oxidizing (Eh = +350 mV) and reducing (Eh = - 150 mV) media and its effect on the expression and activity of hydroperoxide lyase (HPL). HPL activity was assayed in media with Eh values ranging from -250 to +720 mV. In order to change the Eh value of the media, reducing agents including dithiotreitol (1 g/L) and hydrogen (4%) as well as oxidants such as potassium ferricyanide (1 g/L) and oxygen (100%), were used. The experimental findings showed that oxidizing conditions, with Eh of +350 mV, were favorable for the growth of the yeast, whereas reducing conditions, with Eh values of - 150 mV, resulted in a higher expression of HPL. In addition, the results showed that the enzymatic activity of the purified HPL was enhanced in the presence of 0.5 mM dithiotreitol but decreased with I mM potassium ferricyanide and bubbling O-2. However, HPL activity increased 1.5 times in the presence of 4% hydrogen with an Eh value of - 170 rnV. (c) 2006 Elsevier B.V. All rights reserved.