Cytochemical demonstration of an enzymatic production of hydrogen peroxide on the surface of isolated thyroid capillaries.

By means of a cytochemical technique, hydrogen peroxide formation was located on the endothelial cell surface (predominantly the luminal aspect) of capillaries obtained by collagenase digestion of rat thyroid. The cyanide-insensitive H2O2 formation required aerobic conditions and NAD(P)H as substrate. FAD could also stimulate the reaction, but not xanthine. The cytochemical reaction was blocked by a non-penetrating protein inhibitor. The observations are interpreted as evidence of a plasmalemma-bound H2O2-generating enzyme. The findings indicate that microvascular endothelial cells are involved in the release of activated oxygen species, which might have important pathophysiologic implications.