Toxicon 48 (2006) 272–277 Protein phosphorylation profile and adduct formation in liver and kidney of microcystin-LR-treated mice R. Jayaraj, P.V. Lakshmana Rao à Division of Pharmacology and Toxicology, Defence Research and Development Establishment, Jhansi Road, Gwalior-474002, India Received 22 February 2006; received in revised form 13 May 2006; accepted 30 May 2006 Available online 14 June 2006 Abstract Microcystins are cyclic peptide toxins implicated in several livestock and human deaths. The toxicity of microcystins has been attributed to the highly specific inhibition of serine/theronine protein phosphatases-1 and 2A. Reversible protein phosphorylation is an essential regulatory mechanism in many cellular processes. We aimed to investigate the protein phosphatase inhibition, profile of phosphorylated proteins of serine and threonine residues and microcystin-protein phosphatase adduct in vivo after microcystin-LR exposure by intraperitoneal route in mice. At 1 LD 50 , there was significant inhibition of protein phosphatases 1 and 2A activity in liver after 30–120 min exposure but there was no effect in kidney. At 0.5 LD 50 there was no inhibition of protein phosphatase activity in both liver and kidney. Similarly, time- dependent phosphorylation of serine and threonine residues was observed at 1 LD 50 . Microcystin-LR—protein phosphatase adduct was time and dose dependent in liver. At 0.5 LD 50 the adduct could be detected at 1 and 3 days post- exposure. No adduct could be detected in kidney. r 2006 Elsevier Ltd. All rights reserved. Keywords: Microcystin-LR; Protein phosphatase; Microcystin adduct Microcystins are family of structurally related cyclic peptide toxins that are produced by a variety of cyanobacteria but most often the genus Micro- cystis. Acute illnesses and deaths in both humans and livestock following exposure of microcystin contaminated water sources have been reported worldwide (Chorus et al., 2000; Carmichael et al., 2001). Acute microcystin poisoning in mammals is characterized by disruption of hepatic architecture due to phosphorylation of cytoskeletal proteins leading to massive intrahepatic hemorrhage and death in few hours (Rao and Bhattacharya, 1996; Nidhi et al., 2003). Microcystins have been found to be potent inhibitors of protein phosphatase type 1 and 2A (Mackintosh et al., 1990; Craig et al., 1996). They are also suspected to be involved with promotion of primary liver cancer in humans exposed to long-term doses of these peptide toxins through drinking water (Yu, 1995). Over 60 microcystin variants have been reported and micro- cystin-LR (MC-LR) is the most toxic among them. The variable amino acids of the most common microcystin variants, MC-LR, RR and YR are leucine (L), arginine (R) and tyrosine (Y). Reversible protein phosphorylation is an essential regulatory mechanism in many cellular processes. ARTICLE IN PRESS www.elsevier.com/locate/toxicon 0041-0101/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.toxicon.2006.05.012 à Corresponding author. Tel.: +91 751 2233495; fax: +91 751 2341148. E-mail address: pvlrao@rediffmail.com (P.V. Lakshmana Rao).