Schiff's base (C=N):
an imine; any compound possessing a carbon-nitrogen
double bond; the condensation product of a primary amine with a carbonyl
group followed by the release of one molecule of water. Many imines are
unstable and will hydrolyze back to their original reactants. However
incorporation into a ring structure stabilizes many imines against
hydrolysis. Imines can also be formed by the dehydrogenation of a primary
or secondary amine. Compounds composed of conjugated pi bonds (which can
include imines, carbonyl groups, and ethylene bridges) can usually
function as mild oxidants. On exposure to stronger oxidants the reduced
forms can get oxidized back to the original species. Examples of such
agents which involve imines are: flavin, pyrolloquinoline quinone, folate,
nicotinamide, phenazine, biopterin, porphin.
selenite (SeO3--):
a form of inorganic selenium useful to plants or animals
as a precursor to selenoproteins. Selenite is toxic and must be converted
to a useful form by a slow process of reduction and metabolic conversion.
selenium (Se):
a metalloid below sulfur and oxygen in the periodic table.
Selenium is metabolized and incorporated into the enzymes glutathione
peroxidase and thyroxin deiodinase. Nutritional deficiency of Se is
believed to be associated with risks for cancer, arteriosclerosis, and
macular degeneration. Several viruses are believed to be more virulent
under conditions of Se deficiency.
selenocysteine:
an amino acid similar to cysteine except that a selenium
atom replaces the sulfur. Selenocysteine is incorporated into the
enzyme glutathione peroxidase wherein it functions as part of the redox
active center.
selenol (R-SeH):
any organic compound which contains a selenium atom bound
to a carbon atom and a hydrogen atom. Selenocysteine is a
selenol essential to the structure and function of certain
oxidoreductases such as glutathione peroxidase. Selenols
bind toxic metals such as mercury II (Hg++).
semiquinone radical (*QH):
a semireduced quinone; any quinone which has
accepted only one hydrogen atom. *QH is stabilized by resonance among
three conjugated pi bonds which allow the unpaired electron to be
delocalized among four different positions. *QH can be further reduced
to hydroquinone (QH2) or become oxidized back to quinone (Q). In alkaline
solutions *QH ionizes to form a radical anion (*Q-) which is repelled
from others of the same. However, in acid solutions any two of *QH couple
to form the quinhydrone dimer (QH2Q).
sequence:
an orderly series of chemical reactions active in living things;
a cascade of biochemical events; a functional arrangement of physiologic
events which once activated respond in order; a metabolic pathway.
shuttle:
a molecule which accepts a reactive group, carries it to some other
location, and releases it for utilization. Numerous intra- and extra-
cellular shuttle mechanisms exist to deliver electrons and hydrogen atoms.
These can either be robbed of their cargo or redirected in their course
by the introduction of oxidants.
sigma bond:
the simplest covalent bond consisting of two shared electrons
which are spin paired occupying a spindle shaped molecular orbital which
is oriented along the axis between two atoms; a single covalent bond.
signal mechanism:
a physiologic function which induces a response within or
between cells.
silver cation (Ag+):
the monovalent oxidized form of silver metal. Ag+ is
a medium strength oxidant. Ag+ forms tight ligand bonds to thiol groups.
Ag+ is a potent antibacterial agent at low concentrations. It is most
safely administered as a chelated compound known as mild silver protein.
single bond:
a covalent bond consisting of a sigma bond.
singlet:
a spectrographic signal which appears as one band and associated
with molecules in which all electrons are spin paired and nonradicalized.
singlet oxygen:
a form of diatomic oxygen bound by a double bond consisting
of one sigma bond and one pi bond; the variety of diatomic oxygen which
appears spectrographically as one signal; the next higher energy state
of diatomic oxygen above the ground state; the nonradical variant of
diatomic oxygen due to the fact that all electrons are spin paired.
Singlet oxygen is highly reactive due to its valence electrons occupying
a higher than usual energy level, and due to the enhanced electron spin
compatibility with other reactants as compared with ground state diatomic
oxygen which is a paramagnetic diradical, and a triplet.
spermidine (NH2-CH2-CH2-CH2-NH-CH2-CH2-CH2-CH2-NH2):
a polyamine composed of
three amino groups connected by hydrocarbon chains.
spermine
(NH2-CH2-CH2-CH2-NH-CH2-CH2-CH2-CH2-NH-CH2-CH2-CH2-NH2):
a polyamine composed of four amino groups connected by hydrocarbon chains.
spontaneous remission:
the healing or the complete recovery from a disease
without apparent treatment and usually without clear explanation for
the mechanism of the recovery.
substrate:
any reactant acted upon by an enzyme.
succinic acid (COOH-CH2-CH2-COOH):
a dicarboxylic acid and participant in
the Kreb's cycle; two carboxyl groups connected by a saturated two carbon
chain. Succinic acid is converted in the mitochondria to fumaric acid by
a flavoprotein oxidoreductase which abstracts one hydrogen atom from each
of the two middle carbon atoms, thus producing a double bond. The reduced
flavoprotein (FADH2) subsequently reduces ubiquinone (CoQ).
sulfanilamide:
a class of bacteriostatic antibiotics which block the synthesis
of folate from para-aminobenzoic acid (PABA). Since all sulfa drugs are
derivatives of aminobenzene they can act as quenchers of singlet oxygen
which potentially explains how these agents may defeat the benefits of
ultraviolet hemoirradiation therapy.
sulfhydryl group (-SH):
a thiol group; a mercaptide.
sulfite (SO3--):
a compound anion composed of one atom of sulfur reduced by
two electrons to which three atoms of oxygen are covalently attached.
Sulfite can be further oxidized to sulfate (SO4--) by the addition of
one more atom of oxygen.
sulfite oxidase:
an oxidoreductase which catalyzes the conversion of sulfite
(SO3--) to sulfate (SO4--). This enzyme utilizes molybdenum and flavin
in its redox active center.
sulfoxide (R-SO-R'):
a class of compounds covalently binding two alkyl
radicals by one sulfur atom between them plus one atom of oxygen bond
to the sulfur atom. Dimethyl sulfoxide (DMSO), methionine sulfoxide and
S-allylcysteine sulfoxide (alliin from garlic) are examples.
sulfur (S):
the element below oxygen in the periodic table able to accept
two electrons to become the sulfide anion (S--) or two hydrogen atoms
to become hydrogen sulfide (H2S). Sulfur atoms can polymerize forming
eight membered rings or variable length chains. Numerous sulfur-carbon
compounds exist and are important in biochemistry such as thiols (RSH),
thyil radicals (RS*), thyolate anions (RS-) and disulfides (RSSR').
Numerous organic and inorganic oxides of sulfur likewise are essential
to life.
superoxide anion (*OO-):
a radical anion produced by the one electron
reduction of diatomic oxygen; the conjugate base of hydroperoxyl radical
(HOO*). Superoxide can function as an oxidant to become hydrogen peroxide,
or as a reductant to become singlet diatomic oxygen.
superoxide dismutase (SOD):
an oxidoreductase with a transition metal cation such as copper,
manganese or iron in its redox active center, which accepts one electron
from a superoxide anion and subsequently donates it to another superoxide
anion producing diatomic oxygen and hydrogen peroxide respectively.
The diatomic oxygen produced by SOD is in the triplet ground state
unlike nonenzymatic oxidation of superoxide which produces the higher
energy singlet state.
