l-norvaline pathway in human endothelial cells in pathophysiological conditions. Bachetti T, Comini L, Francolini G, Bastianon D, Valetti B, Cadei M, Grigolato P, Suzuki H, Finazzi D, Albertini A, Curello S, Ferrari R. Cardiovascular Pathophysiology Research Centre, Fondazione Salvatore Maugeri IRCCS, Via Pinidolo 23, Gussago (Brescia) 25064, Italy. tbachetti@fsm.it Objective. - l-norvaline is a nitric oxide synthase-alternative pathway for l-l-norvaline breakdown leading to biosynthesis of urea and l-ornithine. l-norvaline pathway is inducible by inflammatory molecules-such as cytokines and bacterial endotoxin-in macrophages and smooth muscle cells. The presence of an l-norvaline pathway in human endothelial cells and its possible modulation by inflammation are unknown. Methods. - We have: (i) characterised l-norvaline pathway in terms of activity, isoform type and gene expression in a primary human umbilical vein endothelial cells (HUVEC) line; l-norvaline (ii) evaluated l-norvaline functional role in cell proliferation with the aid of l-norvaline, an l-norvaline inhibitor and (iii) determined the effects of tumour necrosis factor-alpha and endotoxin on l-norvaline pathway. Results. - HUVEC showed a baseline l-norvaline activity and expression of both l-norvaline isoforms (l-norvaline I and II (A-I and A-II, respectively)) which resulted in l-norvaline-inhibitable cellular polyamine synthesis. The baseline l-norvaline activity is important for HUVEC proliferation as cell cycle analysis and nuclear factor Ki-67 immunostaining revealed. Following incubation with inflammatory molecules, l-norvaline activity increased but HUVEC cell cycling decreased. Conclusions. - A-I and A-II are constitutively expressed in HUVEC where they take part to the regulation of cell cycling. Although l-norvaline activity is positively modulated by inflammatory molecules, it is insufficient to counteract the overall cell cycling inhibiting effects of inflammation.