H2O2 production, protection, and signaling actions. Activation of various cell surface receptors activates Nox situated either in the plasma membrane or in the membrane of organelles such as endosomes to produce H2O2. To function as an intracellular signaling molecule, H2O2 must be imported into the cytosol. Cytosolic H2O2 enhances protein tyrosine phosphorylation by inactivating protein tyrosine phosphatases while activating protein tyrosine kinases. Transient protection of the H2O2 signal from abundant cytosolic peroxiredoxin appears to result from the reversible inactivation of these enzymes through either hyperoxidation or phosphorylation.