A free radical is a molecule,
atom or molecular fragment which contains an unpaid electron in
its outer orbital shell (R ). Each atomic nucleus is surrounded
by one or more orbitals containing a maximum of 2 electrons All
compounds contain 2 electrons in each of its orbitals which spin
in opposite directions. The covalent bonds of organic compounds
share two electrons spinning in opposite directions one supplied
by each atom or molecule pan in the bond. Any molecule can
become a free radical by gaining or losing an electron in the
outer shell. O2 + e Organic compounds also form free radicals
RH2 -e- - RH. O2
Organic molecules are held
together by covalent bonds. Normally, when covalent bonds split
one fragment becomes a positively charged ion by losing an
electron and the other becomes a negatively charged ion by
holding both of the shared pair of electrons (heterolytic
splitting). Free radicals are formed from organic compounds when
a covalent bond is split symmetrically and each retains an
unpaired electron in its outer shell (homolytic splitting).
Homolytic splitting produces free radicals which result in lipid
Free radicals are also formed
as a product of normal cellular chemical reactions. Oxidative
metabolism is a major contributor of free radicals. As O2 is
reduced to water, over SO% is univalently reduced with the
production of the superoxide radical O2.
O2 is combined spontaneously
or dismutated with H+ to produce H2O2 which readily breaks down
in the presence of the metals Fe++ &Cu+ or O2 to form OH.
radicals which are highly injurious to adjacent structures:
lipid membranes, proteins, DNA and precellular matrix 2 O2. + 2
H202 + Fez+
O2 + H2O2
-SOD-> H2O2 + O2
- OH + OH + Fe3+
-, 0H + OH- + O2
(SOD = Superoxide dismutase)
The autoxidation of transition
metals can produce superoxide radicals:
O2 + Fe++ > Fe+++ + O2
O2 + Cu+ > Cu++ + O2
Free radical damage is also accelerated
by various deficiency states and inborn errors or genetic
defects of metabolism
Defects in antioxidant or free radical
scavenge systems result in oxidative stress and cellular damage.
Parkinson's Alzheimer's and Huntington's disease, multiple
sclerosis, progressive myoclonic epilepsy, familial ALS,
post–traumatic epilepsy, ant arteriosclerosis, arc diseases
which are now thought to be at least partly due to oxidative
stress and free radical damage.
All living cells have developed mechanisms
for protection against oxidative stress.Mammalian cells have
very specific and elaborate antioxidant mechanisms which prevent
free radical damage. Superoxide dismutase (SOD), a ubiquitous
superoxide scavenging enzyme. is present in both intra– and
extracellular fluid (ECF) compartments. Copper and Zinc SOD's
are present in ECF and cytosol and manganese SOD is present in
mitochondria. These SOD's dismutate 0~ and H+ to H~O~. H.O. is a
potential cellular toxin because of its reactivity with 0 and
transition metals, however, in the normal situation it is
readily converted to 0. and H.0 by catalase (CAT) and
glutathione peroxidase (GPX). CAT is present in mitochondna and
cyrosolic peroxisomes of most tissues, however, it is found in
very low concentrations in brain. GPX is the major antioxidant
enzyme in the brain, being present in mitochondria and in the
cytosol. GPX requires selenium as a necessary cofactor and is
decreased in selenium deficient states. Glutathione transferase
(GST) conjugates glutathione (GSH) to reactive organic compounds
which become pharmacologically inactive. GPX requires reduced
GSH which is oxidized (GSSH) during the conversion of H2O2 to 0.
and H,O. GSSH is reconverted to reduced GSH by glutathione
reductase which requires vitamin B, (riboflavin) as a cofactor.
Vitamin E is an important membrane
antioxidant which prevents membrane peroxidation by scavenging
OH'. Oxidized vitamin E is reconverted to active E by vitamin C
which is another important antioxidant free radical scavenger.
Beta carotene is an antioxidant which works most effectively in
low oxygen tensions and is an important retinal antioxidant.
From the above, one can appreciate that the
balance between cellular oxidation and free radical production
is most important in maintaining homeostasis and preventing
cellular damage and death. Elaborate systems have developed to
accomplish this. However, a number of disease processes and
defects in antioxidant mechanisms can lead tO both progression
and initiation of tissue injury and cell death.
We (B.J Wilder. M. D. and Russell Hurd. M.S.)
over a number of years have been working on drug injury and
tissue protection with a number of antioxidants. Concurrent with
our work and that of others, techniques for measuring free
radical scavenging enzymes assays ( FRESA) have been developed.
In 1992 we measured FRESA levels in patients
with progressive myoclonic epilepsy and familial progressive
cerebellar degeneration and found abnormalities in SOD. GPX, GSH
and lipid peroxidation (LP). We initiated treatment with
selenium. antioxidant vitamins and N–acetylcysteine, a potent
free radical and H,O. scavenger and supplier of GSH. After
observing favorable clinical effects we have expanded our
studies to include other progressive degenerative neurological
diseases. We have been joined by other interested investigators,
Drs. Basim Uthman Wendell Helveston and Jean Cebula..
We are now studying the effects of
N–Acetylcysteine and antioxidant vitamins and trace metals in
patients with: Friedreich's ataxia, spinocerebellar ataxia,
ataxia telangiectasia, olivopontocerebellar degeneration.
amyotrophic lateral sclerosis, multiple sclerosis, Huntington's
disease, and others.
We will soon initiate studies
in other degenerative diseases and in the prevention of post
traumatic epilepsy and in the amelioration of post ischemic