Evidence for Sodium Azide as an Artifact Mediating the Modulation of Inducible Nitric Oxide Synthase by C-Reactive Protein

Abstract

C-reactive protein (CRP) is an acute-phase protein identified as a cardiovascular risk marker. In recent years, an increasing number of studies have investigated the possible direct effects of CRP on the vasculature, using mainly commercial CRP. In the present work, a potential role for CRP as a modulator of inducible nitric oxide synthase (iNOS) induction was explored. Cultured human aortic vascular smooth muscle cells (HASMC) were stimulated for 18 hours with 10 ng/mL interleukin-1beta (IL-1b), resulting in a marked increase of iNOS levels and NO production, as determined by Western blotting and nitrite measurement, respectively. Commercial CRP (1 to 100 mg/mL) concentration-dependently inhibited the effects elicited by IL-1b. Unexpectedly, similar results were observed when the commercial CRP solution was replaced by the corresponding vehicle medium containing growing concentrations of sodium azide. The inhibitory effects of commercial CRP or vehicle medium were lost on sodium azide removal by dialysis. In conclusion, sodium azide from the commercial CRP solution, but not CRP itself, mainly accounts for the inhibitory effect on IL-1b-evoked iNOS induction and NO release. Care should be taken before attributing any biologic role to commercial CRP containing sodium azide.