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.halogen:
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.
hapten:
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.
hematoxylin:
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.
histamine:
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.
HMG CoA:
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.
hydration:
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.
hydrogenation:
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.
hydrolase:
an enzyme which catalyses the spliting of a molecule accompanied
by the addition of water. Most digestive enzymes are hydrolases.
hydrolysis:
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.
hydrophilic:
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.
hydropyridine:
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.
hypericin:
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.
