We have investigated inhibitory mechanisms of hypoxic activation of HIF-1alpha by nitric oxide (NO). Using a Hep3B cell-derived cell line, HRE7 cells, we found that the inhibition of HIF-1alpha activity by NO requires a substantial amount of oxygen, albeit at a lower level. We further investigated the effect of NO on the binding activity of the von Hippel-Lindau tumor suppressor protein (pVHL) to the N-terminal activation domain (NAD) overlapping the oxygen-dependent degradation domain (ODD) of HIF-1alpha, because this reaction involves prolyl hydroxylation in NAD that requires oxygen. Although we could not detect any binding activity when NAD was incubated with whole cell extracts from cells treated with CoCl, or desferrioxamine, the binding capacity was manifested when Hep3B cells were treated together with NO. This activation was also observed when whole cell extracts from CoCl2-treated cells were incubated with NO. The prolyl hydroxylase from Hep3B cells treated with CoCl2 was partially purified about 80-fold, and several enzymatic properties were examined. The enzyme required ferrous ion and 2-oxoglutaric acid. Strong activation of the prolyl hydroxylase by NO was observed without further addition of ferrous ion. (C) 2002 Elsevier Science (USA). All rights reserved.