Ascorbic acid reduction of compound I of mammalian catalases proceeds via specific binding to the NADPH binding pocket

Monofunctional Heme-Catalases

The review focuses on four issues that are critical for the understanding of monofunctional catalases. How hydrogen peroxide (H2O2) reaches the active site and outcompetes water molecules to be able to function at a very high rate is one of the issues examined. Part of the answer is a gate valve system that is instrumental to drive out solvent molecules from the final section of the main channel. A second issue relates to how the enzyme deals with an unproductive reactive compound I (Cpd I) intermediate. Peroxidatic two and one electron donors and the transfer of electrons to the active site from NADPH and other compounds are reviewed. The new ascribed catalase reactions are revised, indicating possible measurement pitfalls. A third issue concerns the heme b to heme d oxidation, why this reaction occurs only in some large-size subunit catalases (LSCs), and the possible role of singlet oxygen in this and other modifications. The formation of a covalent bond between the proximal tyrosine with the vicinal residue is analyzed. The last issue refers to the origin and function of the additional C-terminal domain (TD) of LSCs. The TD has a molecular chaperone activity that is traced to a gene fusion between a Hsp31-type chaperone and a small-size subunit catalase (SSC).

The Result of Vitamin C Treatment of Patients with Cancer: Conditions Influencing the Effectiveness

Vitamin C (ascorbic acid, AA) is a weak sugar acid structurally related to glucose. All known physiological and biochemical functions of AA are due to its action as an electron donor. Ascorbate readily undergoes pH-dependent autoxidation creating hydrogen peroxide (H₂O₂). In vitro evidence suggests that vitamin C functions at low concentrations as an antioxidant while high concentration is pro-oxidant. Thus, both characters of AA might be translated into clinical benefits. In vitro obtained results and murine experiments consequently prove the cytotoxic effect of AA on cancer cells, but current clinical evidence for high-dose intravenous (i.v.) vitamin C's therapeutic effect is ambiguous. The difference might be caused by the missing knowledge of AA's actions. In the literature, there are many publications regarding vitamin C and cancer. Review papers of systematic analysis of human interventional and observational studies assessing i.v. AA for cancer patients' use helps the overview of the extensive literature. Based on the results of four review articles and the Cancer Information Summary of the National Cancer Institute's results, we analyzed 20 publications related to high-dose intravenous vitamin C therapy (HAAT). The analyzed results indicate that HAAT might be a useful cancer-treating tool in certain circumstances. The AA's cytotoxic effect is hypoxia-induced factor dependent. It impacts only the anoxic cells, using the Warburg metabolism. It prevents tumor growth. Accordingly, discontinuation of treatment leads to repeated expansion of the tumor. We believe that the clinical use of HAAT in cancer treatment should be reassessed. The accumulation of more study results on HAAT is desperately needed.

Flavonoid-induced conversion of catalase to its inactive form--Compound II

Flavonoids (FlaOHs), plant polyphenols, are ubiquitous components of human diet and are known as antioxidants. However, their prooxidant activity has also been reported. We have recently found that FlaOHs inhibit catalase, the heme enzyme which catalyzes the decomposition of hydrogen peroxide (H2O2) into water and molecular oxygen. The catalytic cycle proceeds with the formation of the intermediate, Compound I (Cpd I), an oxoferryl porphyrin π-cation radical, the two-electron oxidation product of a heme group. Under conditions of low H2O2 fluxes and in the presence of an appropriate substrate, Cpd I can undergo one-electron reduction to inactive Compound II (Cpd II), oxoferryl derivative without radical site. Here we show that in vitro, under low fluxes of H2O2, FlaOHs reduce Cpd I to inactive Cpd II. Measurable amounts of Cpd II can be formed even in the presence of reduced nicotinamide adenine dinucleotide phosphate (NADPH) at concentration comparable with the investigated FlaOHs. Possible mechanisms of electron transfer from FlaOH molecule to the heme are discussed.