We have studied a series of 27 disease-causing mutations located in different domains of the CBS molecule representing~70% of patient-derived mutant alleles. Expression in prokaryotic and eukaryotic cells showed a propensity of mutants to misfold/misassemble, which was accompanied by their impaired catalytic activity. Homocystinuria is an inborn error of metabolism biochemically characterized by cystathionine beta-synthase deficiency, leading to homocysteine (Hcy) tissue accumulation. Clinically, patients present mental retardation, seizures, and vascular complications. Folic acid therapy has been used, since it is a methyl donor in Hcy metabolism, as well as its own antioxidant properties. In the present study we evaluated the effect of Hcy on the activities of creatine kinase and respiratory chain enzymes (complex II, succinate dehydrogenase and cytochrome c oxidase) in parietal cortex of rats. We also evaluated the neuroprotector effect of folic acid on biochemical alterations elicited by Hcy. Acute treatment: 22 day-old rats received folic acid daily administrations for one week, 12 hours later the animals were subjected to a single Hcy administration. Chronic treatment: 6 day-old rats received daily folic acid and/or Hcy injections up to 28th day-of-life. Results showed that Hcy acute administration reduced cytochrome c oxidase activity, while chronic Hcy administration increased it. Hcy did not alter other enzymes evaluated. Folic acid prevented Hcy effects, probably by acting as an antioxidant. In conclusion, folic acid prevented the effects of Hcy administration on respiratory chain activity, suggesting a relevant role of this vitamin preventing neurotoxic effects of hyperhomocysteinemia. In the present study we evaluate the effect of chronic homocysteine administration on some parameters of inflammation, such as cytokines (TNF-α, IL-1β and IL-6), chemokine CCL2 (MCP-1), nitrite levels, prostaglandin E2, as well as acetylcholinesterase activity in rat hippocampus. Wistar rats received daily subcutaneous injections of homocysteine (0.3-0.6 μmol/g body weight) or saline (control) from the 6th to the 28th day-ofage. Rats were sacrificed 1 or 12 h after the last injection. Results showed that chronic hyperhomocysteinemia significantly increased proinflammatory cytokines (TNF-α, IL-1β and IL-6), chemokine CCL2 (MCP-1), prostaglandin E2 and nitrite levels in hippocampus.of rats. Acetylcholinesterase activity also was increased after homocysteine administration. Our findings show that chronic hyperhomocysteinemia increases inflammatory parameters, suggesting that this process might be associated, at least in part, to the brain dysfunctions characteristic of some homocystinuric patients. Supported by CNPq and FAPERGS. The purpose of this study was to develop a chronic chemically-induced model of mild hyperhomocysteinemia in adult rats. We produced levels of Hcy in the blood (30 μM), comparable to those considered a risk factor for the development of neurological and cardiovascular diseases, by injecting subcutaneously homocysteine (0.03 μmol/g of body weight) twice a day from the 30th to the 60th postpartum day. Controls received saline in the same volumes. Using this model, we evaluate the effect of chronic administration of homocysteine on redox status in blood and cerebral cortex of adult rats. Reactive oxygen species and thiobarbituric acid reactive substances were significantly increased in plasma and cerebral cortex, while nitrite levels were reduced in cerebral cortex, but not in plasma of rats subjected to chronic hyperhomocysteinemia. We also observed that homocysteine disrupted enzymatic and non-enzymatic antioxidant defenses in the blood and cerebral cortex of rats. Considering that experimental animal models are useful to understand the pathophysiology of human diseases, the present model of mild hyperhomocysteinemia may be useful in the investigation of additional mechanisms involved in tissue alterations caused by homocysteine. Supported by CNPq and FAPERGS. Homocystinuria is an inborn error of metabolism caused by cystathionineβ-synthase deficiency, leading to tissue accumulation of homocysteine (Hcy). Affected patients may present with seizures and mental retardation. It has been demonstrated that homocysteine might promote glutamatergic excitotoxicity due to overstimulation of NMDA receptors. Studies show that guanosine (Guo) prevents seizures induced by changes in glutamatergic system in rats. In the present study we evaluated the influence of Guo on the effects elicited by severe hyperhomocysteinemia on Na+, K± ATPase activity and glutamate uptake. Wistar rats received daily twice subcutaneous injection of D,L-Hcy (0.3-0.6 μmol/g body weight), and/or Guo (7.5 mg/kg) once a day from 6th to their 21th day of life. Twelve hours after the last injection the rats were sacrificed and hippocampus was dissected. Results showed that hyperhomocysteinemia reduced activity of Na+, K±ATPase and glutamate uptake in rat hippocampus. Guo prevents only the decrease Na+, K±ATPase activity. Our findings suggest that Hcy might alter neuronal excitability and increase levels of glutamate in the synaptic cleft which could lead to excitotoxicity. The mechanisms of prevention of Guo on the activity of Na+, K±ATPase is still unknown and need further studies to be elucidated. Supported by CNPq, FAPERGS. An increased reactive oxygen species (ROS) production and apoptosis rate have been associated with several disorders of cobalamin metabolism, particularly with the cblC type of methylmalonic aciduria combined with homocystinuria . In order to analyze the role of homocysteine in stress response, we have evaluated several parameters related to oxidative stress and apoptosis in fibroblasts from patients with defects of MTHFR, MTRR and MTR genes causing homocystinuria. All cell lines showed a significant increase in ROS content and in MnSOD expression level, and also a higher rate of apoptosis with similar levels to the ones found in cblC fibroblasts. The amount of phosphorylated forms of p38 and JNK stresskinases was also increased. ROS content and apoptosis rate were increased in control fibroblasts and glioblastome cell line by shRNA-mediated silencing of MTRR gene expression, indicating the possible function of homocysteine in these processes. The toxic built-up of homocysteine in patients with homocystinuria might be partially responsible for the deleterious effects in stress response, and support the potential of using antioxidants as a novel therapeutic strategy to improve the severe neurological outcome of these rare diseases. The key regulatory point of transsulfuration pathway is catalysed by cystathionine beta-synthase (CBS), a homotetrameric enzyme activated by S-adenosyl methionine (SAM), which binds to CBS C-terminal domain. However, the activation mechanism remains unclear.