Calhelha, Ricardo C.Azevedo, CristinaBarroso, PatríciaFerreira, Isabel C.F.R.Estevinho, Leticia M.2011-02-212011-02-212005Calhelha, Ricardo; Azevedo, Cristina; Barroso, Patrícia; Ferreira, Isabel C.F.R.; Estevinho, Letícia (2005). Bioactive properties of pollen samples collected in the Northeast of Portugal . In 7º Encontro de Química dos Alimentos. Viseuhttp://hdl.handle.net/10198/3467Reactive oxygen species are formed during normal cellular metabolism, but when present in high concentration they become toxic. Mammalian ce lls possess intracellular defences such as superoxide dismutase, catalase or glutathione peroxidase in order to protect the cells against excessive levels of free radicals.engconference object