Camp. Biochem. Physiol. Vol. 104C, No. 3, pp. 387-388, 1993 Printed in Great Britain 0306-4492/93 $6.00 + 0.00 0 1993 Pergamon Press Ltd SPECIES DIFFERENCE IN PLASMA ANTIOXIDANT ACTIVITY N. KOCAK-TOKER, D. AYRIBAS and M. UYSAL University of Istanbul, Istanbul Faculty of Medicine, Department of Biochemistry, Capa, 34390, Istanbul, Turkey (Received 30 November 1992; accepted for publication 20 January 1993) Abstract-l. Plasma antioxidant activity was determined in rats, guinea pigs, rabbits and humans. 2. Low levels of antioxidant activity were observed in rats and guinea pigs. Both species showed high susceptibility to lipid peroxidation in red blood cells. 3. In rabbit and human plasma antioxidant activity was high. In these species, susceptibility to lipid peroxidation was low. INTRODUCTION Free radical-mediated lipid peroxidation is considered a general threat to all aerobic organisms (Halliwell et al., 1986; Frei et al., 1988). The antioxidant systems which are diverse and complex in nature are known to differ from species to species (Cranfield et al., 1979; Igarashi et al., 1983). Plasma is shown to possess powerful antioxidant activity, which is related to the function of plasma proteins (Cranfield et al., 1979; Barber et al., 1961; Stocks et al., 1974b; Stocker and Frei, 1991). In this study the antioxidant activity of plasma is assessed in different species like rabbit, guinea pig, rat and human. Red blood cells respond to oxidative stress con- ditions by their antioxidant mechanisms (Stocker and Frei, 1991), thus it seems to be interesting to study the susceptibility of red blood cells to lipid peroxidation in relation to antioxidant capacity of plasma. plasma to inhibit malondialdehyde (MDA) pro- duction generated from a standard ox brain hom- ogenate by spontaneous autoxidation is determined in this test. To prepare standard ox brain homogen- ate, a fresh ox brain was homogenized in four times its weight of ice-cold phosphate-buffered saline. The homogenate was centrifuged for 15 min at 1000 g and the supernatant was stored at -20°C as stock hom- ogenate. A sample of stock homogenate was thawed at room temperature and dilqted with three times its volume of phosphate-buffered saline. To 5 ml of dilute homogenate, 50~1 of plasma or phosphate- buffered saline were added. For zero-time MDA determinations, aliquots were removed immediately. The tubes were put into 37°C water-bath for incu- bation of 60 min. MDA was determined with thiobar- bituric acid (TBA) reaction. The antioxidant activity of plasma was calculated according to the following equation: Antioxidant activity = 1 - nmol MDA/ml(test) - nmol MDA/ml(O time) nmol MDA/ml(control) - nmol MDA/ml(O time) x 100 MATERIALS AND METHODS In this study four different species as human, rabbit, rat and guinea pig were investigated. Healthy young men aged between 18-38 years were selected. New Zealand White rabbits weighing 2.5-3 kg, Al- bino guinea pigs weighing 400-600 g and Wistar rats 150-170 g body weight were used. Each group con- sisted of 12 members. The animals were freely given water and commercial laboratory chow. They were obtained from the Center for Experimental Medical Research and Application (DETAM), of Istanbul University. The antioxidant activity of plasma was assessed by the method of Stocks et al., 1974a. The capacity of Plasma lipid peroxide levels were also determined (Kamal et al., 1989). The susceptibility of red blood cells to lipid peroxidation was determined with the method of Stocks et al., 1972. The composition of the incubation mixture was 6.5 mM H,Oz, 1.6 mM sodium azide and red blood cell suspension in phosphate-buffered saline (14 mg hemoglobin per ml incubation mixture). TBA-reac- tive material was assayed by measurement of MDA production during 2 hr incubation period at 37°C. Values were expressed as nanomoles of MDA per gram hemoglobin. Hemoglobin concentration of red blood cell suspension was measured by Drabkin’s reagent. 387