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.

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).

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.

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.

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.

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).

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.