half reaction:
conceptually part of a redox reaction, one half reaction being the donation or release of electrons, the other half being the acceptance or uptake of electrons. The direction (release versus uptake) and the tendency of a half reaction to occur is predictable by the Nernst equation and measurable by a volt meter.
a catagory of element which characteristically accepts only one electron in redox reactions to obtain a complete orbital shell. A reduced halogen is called a halide anion. Examples of halogens are fluorine, chlorine, bromine, iodine and astatine. Halide anions can carry one to four atoms of oxygen each if in an alkaline solution. The number of oxygen atoms so bound determines the name of the anion as follows: 1) hypohalite, 2) halite, 3) halate, and 4) perhalate. These oxygens can be released to add to various molecules or to abstract hydrogen or electrons from them.

an allergen produced by chemical combination of a molecule of small molecular weight bound to a larger molecule such as a protein. Electrophiles, oxyradicals, and formaldehyde activated substances can readily form haptens with biologic macromolecules.

a polyphenol extractable from logwood using ether. Upon oxidation hematoxylin converts to the brightly colored orthoquinone hematein used in cytology as a stain. When coadministered intravenously in an aqueous solution with DMSO, hematoxylin has demonstrated antineoplastic effects.

heterocyclic compound:
a compound having a ring structure wherein the ring is composed of more than one type of element. Examples are rings containing carbon with nitrogen, oxygen, or sulfur.

hexose monophosphate shunt:
phosphogluconate pathway; pentose phosphate pathway; a complex series of molecular rearrangements involving phosphate esters of various sugar molecules. This functions to produce twelve of NADPH and six of CO2 for every one of glucose-6-phosphate metabolized. This pathway is the main source of NADPH for numerous reductases.

4-imidazolethylamine; a physiologic signaling agent which promotes inflammation, bronchial constriction, and gastric secretion; the product of decarboxylation of the amino acid histidine. Once produced it must subsequently be inactivated, either by methylation or by oxidation.

beta-hydroxy-beta-methylglutaryl-coenzyme A; a precursor to mevalonic acid which is a precursor to isoprenoid synthesis. HMG CoA is first reduced and then hydrolyzed to release mevalonic acid.

HMG CoA reductase:
a NADPH dependent oxido-reductase which in successive steps reduces the terminal carboxyl group of HMG CoA to a primary alcohol. This process requires two of NADPH (four reducing electrons).

homogentisic acid:
2,5-dihydroxy-phenylacetic acid; alkapton; an intermediate in tyrosine catabolism; a hydroquinone carboxylic acid. Homogentisic acid occurs abundantly in the urine in the genetic disease alkaptonuria. Being a hydroquinone it oxidizes on exposure to air or on exposure to ferric iron.

1) the attachment of water to other molecules binding loosely and reversibly; 2) covalent addition of water to a molecule.

hydrazine (NH2-NH2):
a diamino compound consisting of two nitrogen atoms covalently bound by only one sigma bond plus four hydrogens, bound two to each nitrogen atom. Hydrazine can be readily oxidized releasing its four hydrogen atoms and producing common nitrogen gas (N2). As such it has been used as a rocket fuel. In vivo it undergoes successive one and two electron oxidations with free radical intermediates. Some of these, especially those formed from alkylhydrazines (RNH-NH2), are mutagenic. Hydrazine is an inhibitor of gluconeogenesis and has been used to treat and prolong the life of terminal cancer victims, thus illustrating the dependency of tumors on glucose. It is given orally as the ammonium sulfate salt known as hydrazine sulfate.

hydride anion (H-):
a hydrogen atom which has accepted one electron from a stronger donor; the fully reduced form of hydrogen; the equivalent of two electrons and one proton. Strongly reductive metals react with hydrogen gas to produce hydrides. In living things the equivalent of the hydride species is carried by NAD+ when it is reduced to NADH.

the addition of hydrogen to a molecule; reduction by donation of hydrogen atoms rather than just electrons; saturation of olefins by catalytic addition of hydrogen.

hydrogen (H2 or [H]):
the lightest element having atomic weight of one due to its nucleus containing only one proton; an explosively flammable gas; a species of reducing equivalent due to the electron which is relocated with any transfer of the hydrogen atom.

hydrogen bond:
1) the weak attraction of a covalently bond hydrogen atom (as in a polar sigma bond) to the electronegative pole of another group; 2) a means by which polar molecules are held together weakly by a bridge of one proton between electronegative ligands.

hydrogen peroxide (H2O2 or HOOH):
a strong oxidizing agent composed of two atoms of oxygen covalently linked by a sigma bond plus two hydrogen atoms covalently bound one to each oxygen. H2O2 is commonly used topically as an antiseptic and therapeutic agent. It is produced physiologically in low levels. It can act as the final electron acceptor for certain peroxidases. It has demonstrated immune stimulating and antiinfection effects when given in dilute form intravenously.

hydrogen sulfide (H2S or HSH):
a molecule composed of sulfur covalently bound to two hydrogen atoms. H2S is an extremely toxic gas having a foul odor. It can be produced by the reduction of elemental sulfur, by acidification of metalic sulfides, and by the putrification of sulfur containing organic matter. It binds as a ligand to numerous metallic cations and is an inhibitor of certain metalloenzymes. H2S is highly toxic due to its ability to bind to cytochrome A. This disables the utilization of diatomic oxygen in the mitochondria, halts oxidative phosphorylation, and allows reductants to accumulate. H2S can be detoxified by methylation to become dimethylsulfide, by oxidation to sulfate, and by exchange with disulfides.

an enzyme which catalyses the spliting of a molecule accompanied by the addition of water. Most digestive enzymes are hydrolases.

the spliting of a molecule upon addition of water. Examples are the separation of esters, amides, or imines by the addition of water to produce acid plus alcohol, acid plus amine, or aldehyde plus amine respectively.

hydronium cation (H3O+):
a positively charged species composed of one molecule of water to which a solubilized proton is attached. Hydronium is the main proton carrier in water solutions.

hydroperoxide group (-OOH):
a reactive portion of a molecule composed of one hydrogen atom covalently bound to one oxygen atom which is in turn bound to another oxygen which is in turn bound to the remainder of the molecule. Hydroperoxides are readily formed from a chain reaction involving diatomic oxygen which is added to a radicalized organic molecule to form an alkyl peroxyl radical (ROO*). ROO* subsequently abstracts hydrogen from another molecule to become ROOH. Singlet (energized) oxygen can add to olefins to produce hydroperoxides without radicalized intermediaries.

hydroperoxyl radical (HOO*):
an oxyradical composed of two atoms of oxygen covalently bound to each other, with one of the oxygen atoms covalently bound to a hydrogen atom, leaving the other holding an unpaired electron. HOO* can be produced by protonation of superoxide radical (-OO*). In aqueous solution at neutral pH about 1% to 2% of -OO* is believed to be present as its conjugate acid HOO*. Due to its neutral charge, HOO* can more readily penetrate lipid compartments than -OO* and more readily approach and abstract electrons or hydrogen atoms from potential donors. HOO* is a potent oxidant and will readily convert to HOOH either by spontaneuos dismutation or by abstraction of hydrogen from numerous donors.

the characteristic of a tendency to prefer solution in aqueous media. Most hydrophilic species are also lipophobic. They tend to be ionic or highly polar, much like the solvents they prefer.

the fully reduced phase of a nicotinamide; NADH or NADPH; NAD+ or NADP+ plus a hydride equivalent.

hydroquinone (QH2):
a diphenolic compound composed of an aromatic ring and two hydroxyl groups (-OH). To be considered a hydroquinone the -OH groups must be situated at the 1,2 (ortho-) or the 1,4 (para-) carbon positions. This structure allows the hydroxyl groups to conjugate in the sense that abstraction of the two hydroxyl hydrogen atoms permits an immediate electronic rearrangement to the quinone structure. Hydroquinones are useful in many situations requiring a mild organic reductant. As such they are effective donor type antioxidants. Thousands of types of hydroquinones are produced naturally in the plant kingdom.

hydroxyl radical (HO*):
an oxyradical composed of one atom of oxygen covalently bound to one atom of hydrogen, thus leaving one unpaired electron. HO* is a powerful and rapidly acting oxidant towards potential donors. HO* penetrates readily into aqueous and lipid media due to its neutral charge and small size. It also forms adducts with many if not most compounds possessing a pi bond thus producing a bound hydroxyl group and radicalization at the opposite side of the former pi bond. HO* is the most rapidly acting and probably the most destructive oxyradical.

an extended anthroquinone; a dimer of two emodin analogues; the medicinally active agent extractable from St. John's wort. Hypericin can serve as an inhibitor of monoamine oxidase and has been found to offer broad spectrum antiviral and antibacterial benefits.

hypochlorite anion (ClO-):
a molecular species composed of one atom of oxygen and one atom of chlorine and one electron; a monooxygenated chloride anion; the conjugate base of hypochorous acid (HClO). ClO- is readily produced by reaction of chlorine gas (Cl2) with water under alkaline conditions (OH-). Acidification of hypochlorite solutions (ClO-) produces hypochlorous acid (HClO) which can oxidize chloride anions (Cl-) producing chlorine gas (Cl2). ClO- or HClO can oxidize hydrogen peroxide (H2O2) producing singlet oxygen (O=O). Activated white blood cells utilize the enzyme myeloperoxidase which uses hydrogen peroxide (H2O2) to produce hypochlorite by oxidizing ambient chloride (Cl-).

hypoxia inducible factor (HIF-1):
a transcription factor which is activated under conditions of hypoxia which induces biosynthesis of the enzymes of glycolysis.