Crucial Enzymes in the Oxygen Order of Human Health – Oxyenzymes
Majid Ali, M.D.
In this tutorial , I present fundamrentals of enzymes of central importance in the free radical pathology and immunology.
It is of some interest to note that the essential role of eukocyte myeloperoxidase in preserving the function integrity of the phagocytes was first demonstrated in disseminated candidiasis in 1969 by Lehrer and Cline. We may add parenthetically that altered gut ecology, it appears to many of us, is one of the most pervasive and debilitating health problem of our times. And clearly the Candida overgrowth and infection represent important entities within the broad spectrum of disorders which result from impaired gut colony. The true importance of this syndrome, seen in a patient with inherited myeloperoxidase deficiency, however, was the discovery of the existence of another oxygen-dependent microbicidal system existed came from the fact that the patient with this deficiency did not suffer from recurrent infections as do patients with CGD.
The next important insight into our understanding of the biological defense mechanism came from the work of Babior and his colleague in 1973. They demonstrated the production of superoxides by phagocytes during respiratory bursts. In studies using reduction of cytochrome C, the workers demonstrated that the superoxide (O2-) is the primary molecule produced in respiratory bursts. Further, the reaction of superoxides with hydrogen peroxide results in production of secondary molecules which are much more potent microbicidal agents.
The Superoxide Forming Enzyme
What is the initial event in the respiratory burst leading to the production of superoxides and secondary oxidizing radicals? This question remains unanswered to date. Considerable evidence points to NADPH oxidase as the primary enzyme in the plasma membrane of the phagocytes has been demonstrated in many studies. Teleologically, this would seem to be an ideal location for this enzyme, since its the plasma membrane which invaginates to engulf the bacteria and other particulate matter during phagocytoses resulting in the formation of the phagosome. The enzyme, thus, finds itself in an ideal position to deliver oxidants for maximal microbicidal efficiency. NADPH oxidase is one of the flavoenzymes, and appears to be highly specific for oxygen. It is unable to transfer electrons from NADPH to several other electron acceptors which function well with other flavoenzyme.
It seems probable that the superoxides generating oxidase represent an electron chain of which the flavoenzyme is but one component. A neutrophil quinone appears to be another likely member of this hypothetical electron transport chain.
Antioxidant Systems of Biological Defense Mechanisms
Nature seemed to have designed the free radicals as primary defense mechanisms for individual cells against foreign material. The free radicals, indeed, are potent microbicidal agents. The peroxide-halide-myeloperoxidase system is an effective cellular weapon system for decarboxylating foreign proteins. OCl– is a potent poison for a host of cellular enzymes including heme enzymes, enzymes with iron-sulfur centers, enzymes with sulfhydryl compounds, and other enzyme sub groups.
In the natural scheme of things, free radicals may be regarded as cellular daggers are fully capable of damaging host cells and tissues, including the phagocytes themselves. Thus, free radicals unless reined in by other molecules capable of neutralizing them, serve as weapons of self destruction.
If free radicals are considered as cellular daggers, the antioxidant may be regarded as the cellular sheath for these daggers to prevent self-inflicted injury.
Superoxide dismutases is a subgroup of enzymes which catalyze the conversion of superoxide to hydrogen peroxide and oxygen:
2O2– + 2H+ —> O2 + H2O2
The above reaction does occur spontaneously, even at physiologic pH values. This reaction is considered bimolecular, indicating the possibility of production of superoxides at high levels before spontaneous dismutation occurs.
Two distinct types of super dismutase occur in human tissues: one containing copper and zinc subunits with a molecular weight of 33,000, and a second containing manganese subunit with a molecular weight of 35,000 daltons. The liver cells contain twenty times as much dismutases as the neutrophils. Even with dismutases, the activated neutrophils produce free radicals far in excess of the capacity of leukocyte dismutases. Activated leukocytes, thus, produce too many free floating cellular daggers and not enough dagger sheaths. Activated leukocytes are suicidal cells.
Catalase is a 263,000 daltons tetrameric heme enzyme which is ubiquitous among human tissues. It catalyzes the conversion of hydrogen peroxide to oxygen and water:
2H2O2 —> O2 + H2O
maintenance of the physical chemical and energy status within the physiologic limits. A cell assures its health and the integrity of its plasma membrane by a host of life processes. Notable among them are:
1. Phosholipid and lipoprotein composition of the cell membrane.
2. Enzymatic activity in the cell membrane, especially enzymatic pathways involved with generation of oxidant radical and antioxidant mechanisms.
3. Mineral composition of the cell membrane.
4. Membrane receptors for both complimentary ligand systems and a host of antigens.
Cell membrane lipid peroxidation may be simply defined as the oxidative damage to the structural components and functional characteristics of the cell membrane, and the damage caused to the internal cellular milieu by secondary chain reactions initiated by the early events at the cell membrane. This definition goes beyond the conventional definition of lipid peroxidation as oxidation of the polyunsaturated lipids in the cell membrane. The main weakness of such limited definition of cell membrane peroxidation, in our viewpoint, is that it relegates this essential process to a secondary role in the preservation of cell health and disease.
Catalase serves as a second line antioxidant defense system. Catalase-mediated destruction of hydrogen peroxide is generally inefficient at the peroxide levels found under physiologic conditions.
Glutathione-dependent Antioxidant System
Under physiologic conditions, glutathione-dependent destruction of H2O2 represents the primary cellular defense system. This system includes two well defined enzymes. Glutathione peroxidase is tetramer with a molecular weight of 84,000. It is one of the few enzymes which contain selenium as selenocysteine. In addition to H2O2, glutathione peroxidase also catalyzes the reduction of alkyl hydroperoxides and hydroperoxides formed during the anti-oxidation of polyunsaturated fatty acids in cell membranes:
ROOH + 2GSH —> ROH + H2O + GSSG
The second member of this enzyme system is glutathione reductase, a flavoenzyme which requires FAD as the physiologic factor.
The glutathione-dependent antioxidant system catalyzes the reduction of hydrogen peroxide to water at the expense of NADPH: these reactions are as follows:
2GSH + H2O2 —> GSSG + 2H2O
catalyzed by glutathione peroxidase, and
GSSG + NADPH + H+ —> 2GSH + NADP+
Neutrophils and other human tissues contain several compounds which play an antioxidant role under physiologic conditions and which are not dependent on any system. Three principle antioxidant in this group are:
1. Ascorbic acid
Taurine reacts with OCl– to form a stable nonreactive N-chloramine. OCl– is a highly reactive free radical, hence the value of taurine in clinical nutritional protocols.
Ascorbic acid is a nonspecific free radical scavenger. It has a high affinity for potent oxidants such as OH–. There is some evidence that ascorbic acid provides a radical scavenger system based on the cyclic oxidation and re-reduction of ascorbic acid by an enzyme contained in neutrophils which can convert dehydroascorbate to ascorbate.