Academia Arena 2014;6(7) http://www.sciencepub.net/academia http://www.sciencepub.net/academia aarenaj@gmail.com 50 Guava (Psidium guajava Linn.) stem bark extracts: Toxicity and Free radical scavenging activity Taiye R. Fasola 1 , Ganiyat K. Oloyede 2 * and Babalola S. Aponjolosun 1 1 Department of Botany, University of Ibadan, Nigeria. 2 Natural products/Medicinal Chemistry Unit, Department of Chemistry, University of Ibadan, gk.oloyede@ui.edu.ng/oloyedegk@gmail.com; Telephone: +234 803 562 2238 *Author to whom correspondence should be addressed. Abstract: Free radicals and reactive oxygen species (ROS) are associated with various physiological and pathological situations. They are capable of damaging DNA, proteins, carbohydrates and lipids which are generally in aerobic organisms. Therefore, this study was aimed at determining the lethal dose (LC 50 ) and investigating the effect of solvents of different polarity on free radical scavenging activity of Nigerian specie of Psidium guajava stem bark using UV-Visible Spectrophotometer. The crude methanol extract of P. guajava (CMEPG) stem bark was partitioned using hexane (HEFPG), ethyl acetate (EAFPG), butanol (BUFPG) and water (AQFPG). Free radical scavenging activities of CMEPG and the partitioned extracts were tested on 1, 1-Diphenyl-2-Picrylhydrazyl radical (DPPH) and hydroxyl radical generated from Hydrogen Peroxide (H 2 O 2 ). Their activities were compared with known antioxidant standards; ascorbic acid (ASCAD), butylatedhydroxyanisole (BHA) and alpha-tocopherol (α- TOCO). The highest percentage inhibition was observed in HEFPG (97.39%) and BUFPG (96.50%) at 1.0 mg/ml in the DPPH test. BUFPG also scavenged hydroxyl radical generated from H 2 O 2 better than CMEPG and other extracts. CMEPG had a percentage inhibition of 98.91% which is comparable to α−TOCO having 99.86% inhibition at 0.1 mg/ml. Brine shrimp lethality test was carried out to investigate the toxicity of P. guajava to lower animals and the result revealed that HEFPG was the most toxic with LC 50 value of 41.7170 µg/ml while AQFPG with LC 50 greater than 1,000 was non-toxic. Thus, Psidium guajava grown in Nigeria is a promising antioxidant agent. [Fasola TR., Oloyede GK. and Aponjolosun BS. Guava (Psidium guajava Linn.) stem bark extracts: Toxicity and Free radical scavenging activity. Academ Arena 2014;6(7):50-58]. (ISSN 1553-992X). http://www.sciencepub.net/academia . 10 Key words: Psidium guajava, polarity, 1, 1-Diphenyl-2-Picrylhydrazyl radical, hydrogen peroxide, Free radicals, toxicity 1. Introduction Plant-derived drugs remain an important resource, especially in developing countries, in combating diseases. Approximately 60-80% of the world’s population still relies on traditional medicines for the treatment of common illnesses (Oliver-Bever, 1986, Burkill, 1997, WHO, 2002). There has also been growing interest in the involvement of reactive oxygen species (ROS) in several pathological situations. Oxidation induced by ROS can result in cell membrane disintegration, membrane protein damage and DNA (Deoxyribose Nucleic Acid) mutation, which can further initiate or propagate the development of diseases, such as cancer, liver injury and cardiovascular disease. Human bodies are exposed to free radicals from external sources such as radiation from the sun or X-rays, ozone and nitrous oxide, heavy metals like Mercury, Lead, smoke, alcohol, saturated fats, air pollution from cigarette smoke and other chemicals and pollutants. Our bodies also produce free radicals during essential activities, such as energy production and immunity. Stress is also another factor which produces adrenaline-related products, which not only restricts blood flow to our skin, but also generates potent, destructive free radicals. Therefore, antioxidants with free radical scavenging activities may have great relevance in the prevention and treatment of diseases associated with oxidants (Paolo et al., 1991, Feskanich et al., 2000, Gordon, 1996, Halliwell, 1996, Lim et al., 2006). P. guajava is a common plant because the fruit is edible and different part of the plant has enjoyed many medicinal applications as antibacterial, anti- inflammatory, anti-malaria, haemostatic, antispasmodic, as a tonic, anti-diarrhoeal, antidiabetic amongst others (Abdelrahim et al., 2002, Dutta et al., 2000, Rabe and Staden, 1997, Muruganandan et al., 2001, Olajide et al., 1999, Sen et al, 1995, Ticzon, 1997, Conway, 2001). Also much biological work has been carried out on P. guajava and many compounds of medicinal importance have been isolated (Arima and Danno, 2002, Begum et al., 2002, Michael et al., 2002, Morton, 1987, Gnan and Demello, 1999, Pranee et al., 1999, Hernandez, 1980, Ali et al., 1996, Lutete et al., 1994). Guava contains lycopene which according to some researchers may play a role in the prevention of different forms of cancer,