Caution Solutions should be sterile-filtered, not autoclaved, if sterile solution is necessary. Spermidine deaminates with time; solutions should be stored frozen. Prepare new solutions frequently. Biochem/physiol Actions Inhibits neuronal nitric oxide synthase (nNOS). Binds and precipitates DNA; may be used for purification of DNA binding proteins. Stimulates T4 polynucleotide kinase activity.