imidazole:
a heterocyclic ring of five atoms (two nitrogens and three carbons)
having two double bonds. This ring is part of the side chain of histidine.
Certain prooxidant antibiotics are analogues of histidine.
imine (C=N):
any compound possessing a carbon to nitrogen double bond.
Imines can be formed by the nucleophilic addition of a primary amine to
a carbonyl group followed by the expulsion of water. They can also be
formed by the oxidation of a primary or secondary amine when one hydrogen
atom is abstracted from the amine nitrogen and another from an adjacent
carbon atom. Many imines are unstable in an aqueous environment and
readily hydrolyze. Imines which are part of a ring structure tend to be
resistent to hydrolysis. Conjugated imines are noted for their ability
to redox cycle.
induction:
a physiologic stimulus response function which serves to increase
the transcription of a specific locus of DNA to produce mRNA which codes
for the increased biosynthesis in the ribosomes of needed proteins.
Induction is mediated in many cases by specific transcription factors
which are released by some trigger or receptor mechanism. A released
transcription factor then migrates to and binds to some specific DNA
associated complex where transcription is initiated.
inhibition:
any means (usually chemical) of hindering, slowing, or stopping
the function of an enzyme. Some inhibitors permanently deactivate or
destroy an enzyme. In such cases the inhibition is considered permanent.
Others lessen enzyme activity only temporarily so that the inhibition is
considered temporary or reversible. Examples of reversible inhibition
include: oxidation of sensitive thiol groups to disulfides, binding of
the ligands of the enzyme to a toxic metal, blocking carbonyl groups by
imine adduct formation with a toxic amine, exposing a metalloenzyme to
another ligand or a chelating agent, blocking the substrate receptor site
with a false receptor, increasing product concentration, etc.
inhibitory protein kappa B (I-kappa-B):
a protein subunit of a larger protein complex occurring in the cytoplasm
to which nuclear factor kappa B (NF-kappa-B) is loosely bound.
Certain physiologic triggers including exposure to oxidants and
UV light cause the release of NF-kappa-B from this complex.
initiation:
the first reaction in a sequence of free radical reactions;
the event which starts a cascade of repetitive free radical reactions.
initiator:
any substance which can start a free radical chain reaction.
Initiators can be reductants or oxidants which tend to react by a single
electron transfer, or they can be unstable compounds which readily break
up to produce two seperate free radicals. Heat or photons can break
covalent bonds and thereby initiate a free radical chain reaction.
iron sulfur center (Fe2S2):
a compound of iron and sulfur which can redox cycle and serve as
the active center of certain oxidoreductases. These enzymes have
also been called nonheme iron proteins and ferredoxins. They serve
as reductants in many anaerobic bacteria. They reduce nitrogen
in nitrogen fixing bacteria. They shuttle electrons in photosynthesis
and in oxidative phosphorylation.
isocitric acid:
a tricarboxylic acid which is an important component of the Kreb's cycle.
It is oxidized/ dehydrogenated by isocitrate dehydrogenase.
isoprene (CH2=CHCH3-CH=CH2):
2-methylbutadiene; a precursor for numerous lipophilic organic compounds.
Isoprene units readily polymerize to produce rubbers.
isoprenoids:
organic compounds composed of or containing isoprene units.
Examples are: carotenoids, turpenes, rubbers, steroids, tocopherols,
ubiquinones, plastoquinone, and phylloquinone.
