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From the August 1st, 1996 issue of Smart Drug News [v5n2]. Copyright (c) 1996. All rights reserved.

Research Update:

Mitochondrial Nutrition,
Aging and Cognition

by Ward Dean, M.D., and Steven Wm. Fowkes

Every cell of the body contains many tiny organelles called mitochondria (see inset, Figure 1 below). These mitochondria produce most of the energy used by the body. Cells with a high metabolic rate (heart muscle cells) may contain many thousands of mitochondria. Some cells may contain only dozens.

Mitochondrial energy production is a foundation for health and wellbeing. It is necessary for physical strength, stamina and consciousness. Even subtle deficits in mitochondrial function can cause weakness, fatigue and cognitive difficulties. Chemicals which strongly interfere with mitochondrial function are known to be potent poisons. During aging, mitochondrial function may become compromised.

Mitochondrial energy production is accomplished by two closely linked metabolic processes. First, the citric acid cycle converts biological fuel (carbohydrates and fatty acids) into ATP (adenosine triphosphate) and hydrogen (in the form of NADH and FADH2) (see sidebar) for further explanation of NADH and FADH2). Second, the electron transport chain combines hydrogen with oxygen to generate abundant ATP in a highly efficient and tightly controlled manner (see Figure 1 above). Mitochondrial efficiency has been reported to be close to 70%, which compares quite favorably with internal combustion engines (about 10% efficient) or hydrogen-oxygen fuel cells used in spacecraft (approximately 40% efficient). The process of generating ATP with oxygen is called oxidative phosphorylation. This process generates approximately ten times more ATP than the citric acid cycle alone, and generates more ATP than any other energy-producing pathway (e.g., glycolysis). Oxidative phosphorylation is the primary energy process for all aerobic organisms.

The utilization of oxygen by mitochondria is accomplished by a highly specialized group of five protein complexes embedded in the inner membrane of the mitochondria. Complex I accepts fuel from the citric acid cycle in the form of NADH, which donates electrons to the chain (see Figure 1 above). Part of the energy of this electron is used to pump a proton (i.e., acidity) across the inner membrane, after which the electron is passed to Complex III via coenzyme Q. Complex II accepts electrons from FADH2 and also passes them to Complex III via coenzyme Q. Complex III uses another part of the energy of the electron to pump another proton across the inner mitochondrial membrane. The electron is then passed to Complex IV via cytochrome C where it uses most of the remaining energy to pump the third proton across the membrane. The deenergized electron is then transferred to oxygen to generate water.

The relative excess of protons in the intermembrane space creates a pH and redox gradient across the inner mitochondrial membrane. The energy of this gradient is used by Complex V to convert adenosine diphosphate (ADP) to adenosine triphosphate (ATP), the chemical energy “currency” of the cell. ATP can then be transported to where work needs to be done.

Mitochondrial Aging

One interesting property of mitochondria is that they have their own DNA (deoxyribonucleic acid), the stuff of which genes and chromosomes are made. Mitochondrial DNA (mtDNA) is quite different from nuclear DNA in several respects. First, it exists as a simple plasmid (a DNA loop), and in this respect, it is more akin to bacterial DNA than the chromosomal DNA of higher organisms. Second, mtDNA is not associated with histones. Histones are positively charged “storage” proteins around which nuclear DNA is wound for safekeeping (like thread on a spool). Third, most of the complex DNA repair mechanisms that correct damage to nuclear DNA are missing from mitochondria. All of these features have prompted some scientists to speculate that mitochondria are ancient remnants of primitive symbiotic bacteria. Whether this view is correct or not, the relatively unprotected and unrepaired mtDNA suffers more than ten times the damage that nuclear DNA does [Miguel, 1991, 1992; Shigenaga et al., 1994]. This leads to mitochondrial dysfunction, disruption of cellular energy production, and accelerated cellular aging [Miguel, 1980].

Mitochondrial electron transport is not perfect. Even under ideal conditions, some electrons “leak” from the electron transport chain. These leaking electrons interact with oxygen to produce superoxide radicals. With mitochondrial dysfunction, leakage of electrons can increase significantly. The close proximity of mtDNA to the flux of superoxide radicals (or hydroxyl radicals), and it’s lack of protection and repair mechanisms, leads to free radical-mediated mutations and deletions. Mitochondrial aging has been proposed as an underlying cause of 1) free-radical stress, 2) degenerative disease and 3) aging [Miguel, 1980, 1991, 1992, Shigenaga et al., 1994].

Evidence is accumulating that mitochondrial dysfunction underlies many common pathologies. Mitochondrial defects have been identified in Parkinson’s disease, Alzheimer’s disease [Hutchin and Cortopassi, 1995], heart disease, fatigue syndromes, numerous genetic conditions, and nucleoside therapy for AIDS. Also, many common nutritional deficiencies can impair mitochondrial efficiency.

Parkinson’s Disease

The mitochondria of patients with Parkinson’s disease exhibit several deficiencies. One of the most well characterized is diminished Complex I activity, an effect which has been measured in brain, muscle and platelets. Although some researchers have been unable to measure this effect in muscle and platelet, some of this difficulty may be related to older, less-sensitive measurement techniques [Shults, 1995].

Complex I deficits have been observed to directly follow the administration of dopa or dopamine to rats. This effect is dose dependent and may be the direct result of chronic dopa therapy in humans [Przedborski et al., 1995]. This deficit may be prevented by both vitamin C and deprenyl [Przedborski et al., 1995].

Inhibition of Complex I enzymes can diminish ATP production, increase leakage of electrons, and increase superoxide production. This may be a primary cause of the increased oxidative stress associated with Parkinson’s disease.

In a similar manner, hydroxyl radical stress in rats increases in a dose-dependent manner with dopa administration [Smith et al., 1994]. Unlike Complex I inhibition, however, hydroxyl radical production is not blocked by deprenyl. One of the major pathways producing hydroxyl radical stress is the breakdown of hydrogen peroxide, a byproduct of superoxide detoxification. This is entirely consistent with the observation that hydroxyl-radical production is increased with Complex I inhibition [Smith et al., 1994]. Since reduced iron (Fe2+) catalyzes the catabolism of hydrogen peroxide into hydroxyl radicals, the association between tissue iron burdens and Parkinson’s disease (incidence and progression) should receive more attention [Nufert, 1996]. We’ll look into this more closely in a future article.

Due to these factors, serious questions have been raised about the wisdom of using L-dopa for the treatment of Parkinson’s disease. L-Dopa is prescribed for it’s ameliorative effect on symptoms, not for it’s effect on underlying disease pathology. Now that evidence is emerging that dopa may actually aggravate some of the causes of Parkinson’s disease, it may be time to reconsider the costs and benefits of dopa therapy.

The primary benefits of dopa are symptomatic. The possible costs are increased oxidative stress to dopaminergic neurons and earlier disability and death. Although definitive human clinical studies of this effect are lacking, dopa causes increased oxidative stress in cell cultures of “normal” dopaminergic neurons, and tissue cultures of neurons from Parkinson’s patients exhibit marked neurotoxicity from exposure to dopa [Mena, 1992].

One study recently attempted to measure the net progressive disability associated with different Parkinson’s therapies. In a year-long, randomized, double-blind, placebo-controlled study of early Parkinson’s patients, researchers reported significant clinical benefit from deprenyl in combination therapy with both dopa and bromocriptine [Olanow et al., 1995]. The researchers rated the severity of symptoms of 100 subjects and then put them on one year of therapy. After one year, they discontinued the therapy and remeasured the severity of symptoms two months after discontinuing deprenyl or seven days after discontinuing dopa (Sinemet) or bromocriptine. Placebo-treated subjects (dopa-plus-placebo and bromocriptine-plus-placebo groups) deteriorated by 5.8 points (Unified Parkinson’s Disease Rating Score), while deprenyl-treated subjects (dopa-plus-deprenyl and bromocriptine-plus-deprenyl groups) deteriorated by only 0.4 points, indicating a strong disease-stabilizing effect from deprenyl.

Inhibition of Complex I activity is also observed in MPP+ toxicity, an acute condition which closely mimics Parkinson’s disease in the spectrum of symptoms and the localization of brain damage to the dopaminergic neurons of the brain. Deprenyl provides significant protection against MPP+ toxicity, as well as increased antioxidant defenses against hydrogen peroxide, superoxide ions and hydroxyl radicals [Wu et al. 1993]. The similarity between MPP+ (and other neurotoxic agents) and NAD+ may be significant (see Figure 3 below). Perhaps these toxins bind to or interfere with the NADH/NAD+ receptor site on Complex I.

The mitochondria of Parkinson’s patients also exhibit milder inhibition of Complex III activity, and a relative deficit of alpha-ketoglutarate dehydrogenase complex (KGDHC). KGDHC plays a key role in the citric acid cycle and directly produces the Complex I substrate NADH.

Although KGDHC is too unstable to measure enzymatically in post-mortem studies, it has been measured by immunohistological methods and found to be significantly depleted in the lateral part of the substantia nigra regions of Parkinson’s patients (see Figure 4 at left) [Mizuno et al., 1995].

Similar reductions in KGDHC levels in the cortex of Alzheimer’s patients have been noted [Mastrogiacomo et al., 1993].

Cardiolipin and ALC

One of the mitochondrial components which may play a critical role in the aging of mitochondria is cardiolipin (diphosphatidylglycerol), a special phospholipid that is unique to the inner mitochondrial membrane and which provides important structural support to several of the enzymes in the electron transport chain [Hoch, 1992]. Carnitine is necessary to transport long-chain fatty acids into the mitochondrion for use as fuel and for the manufacture of cardiolipin. Medium- and short-chain fatty acids less than 8 carbons in length do not require carnitine transport.

Cardiolipin levels decrease with age, as does mitochondrial efficiency. Acetyl-L-carnitine (ALC) restores falling cardiolipin levels in aged rat mitochondria to youthful levels [Okayasu, 1985].

ALC treatment also restores ADP carrier activity [Paradies et al., 1992] and cytochrome oxidase activity (Complex IV) [Paradies et al., 1996].

Since the amounts of cytochrome protein and ADP carrier protein in aged mitochondria is close to that in young mitochondria, it is the enzyme efficiencies which are being adversely affected by aging influences — and restored by ALC administration. ALC has no effect on cytochrome oxidase activity in young rat mitochondria.

By restoration of cardiolipin levels, cytochrome oxidase activity and ADP carrier transport, ALC also restores overall respiratory activity (oxygen energy conversion) of aged rat mitochondria to normal levels [Paradies et al., 1996]. We believe it may help normalize human mitochondrial function as well.

Coenzyme Q

Coenzyme Q (ubiquinone) is a critical electron transfer molecule that transports electrons from Complexes I and II to Complex III. It is present in much higher amounts than the complex proteins and is probably mobile within the membrane. It can exist in reduced (quinol) and oxidized (quinone) forms, as well as an intermediate radical form (a semiquinone radical). Deficiencies of coenzyme Q are associated with numerous pathologies, the most common of which is probably cardiomyopathy (heart muscle disease). The heart muscle is especially rich in mitochondria due to it’s extremely high energy requirements. It is no accident that cardiolipin was first extracted from heart muscle mitochondria.

Coenzyme 1

NADH (also called coenzyme 1) is a key electron transfer molecule between the citric acid cycle and Complex I. NAD (short for nicotinamide adenine dinucleotide) exists in both oxidized (NAD+) and reduced (NADH) forms. Both forms participate in countless reactions throughout the body, where NAD+ serves as an electron acceptor and NADH as an electron donor. The electron transport chain starts with NADH on Complex I and ends with oxygen on Complex IV (see Figure 1).

Although the mechanism of inhibition of Complex I in Parkinson’s disease is not known, NADH supplementation has demonstrated clinical value in treating Parkinson’s disease [Birkmayer, 1996].

Under average circumstances, about one-third of NAD is produced from vitamin B3 (niacin or niacinamide) and about two-thirds from the catabolism of tryptophan [Mayes, 1993].

Lipoic Acid

Lipoic acid (lipoate) is an essential component of the alpha-ketoglutarate dehydrogenase complex (KGDHC), the closely associated collection of enzymes that generates NADH from the decarboxylation of alpha-ketoglutarate within the citric acid cycle (see Figure 1). Also called thioctic acid, lipoate exists in both oxidized (disulfide) and reduced (dithiol) forms. Thiamine diphosphate (a vitamin B1 derivative) and FAD (a riboflavin derivative) are also cofactors of KGDHC.

Lipoate, thiamine diphosphate and FAD also serve as a cofactors in the pyruvate dehydrogenase complex, an enzyme complex quite similar to KGDHC in both structure and function. Like KGDHC, the pyruvate dehydrogenase complex generates NADH. While KGDHC generates succinyl-CoA within the citric acid cycle, pyruvate dehydrogenase complex generates acetyl-CoA that feeds the citric acid cycle. Specifically, acetyl-CoA is a substrate for citrate synthase (see Figure 1) to generate citric acid (citrate) at the start of the citric acid cycle.

Of all of the enzymes of the citric acid cycle, only the KGDHC and citrate synthase catalyze directional reactions (illustrated by single-headed arrows in Figure 1). All of the other reactions are reversible (they can run forwards and backwards, illustrated by double-headed arrows). Both of these directional or “driving” reactions force the citric acid cycle to flow in the correct direction, the direction that generates NADH, FADH2 and ATP. Both of these directional reactions require the involvement of dehydrogenase complexes which are dependent on lipoic acid and vitamins B1 and B2 for their activity.

In addition to it’s cofactor role, lipoic acid is a powerful antioxidant that is effective at scavenging both water- and lipid-soluble free radicals [Packer, 1992]. It picks up some of the free radicals that vitamin C and E miss [Packer, 1996]. Lipoate decreases the excitotoxicity of glutamate and is used to treat diabetic polyneuropathy [Suzuki et al., 1992].

Supplementation

There is plenty of evidence that documents the potential effectiveness of diet and dietary supplements on the mitochondrial pathologies underlying Parkinson’s disease and aging. Not only do numerous nutrients play indispensable roles in mitochondrial energy function, nutrients also serve vital antioxidant functions that ameliorate the free-radical byproducts of oxidative phosphorylation.

In aged rats and mice, antioxidant supplements of vitamins C and E, and the amino acid cysteine, are effective in 1) lowering the amount of oxidized glutathione and 2) reducing DNA damage [Vina, et al., 1996]. Untreated, old rodents have several times more oxidized glutathione in their livers and up to six times more oxidized glutathione in their brains. Such changes reflect the general increase in oxidative stress that occurs with age and a gradual decrease in the competence of the antioxidant defense system. One obvious mitochondrial component of this defense is the production of reduced NADH, FADH2 and NADPH which can directly reduce (recycle) oxidized substrates into their reduced forms.

Although antioxidant therapy is an obvious approach to deal with increased oxidative stress and decreased antioxidant levels, scientists and doctors have been slow to apply this technology. Researchers have started investigating the effect of vitamin E towards this end, but antioxidants are much more effective in combinations than they are singly. More importantly, vitamin E is lipid soluble and provides minimal antioxidant protection to the aqueous (watery) metabolic compartments of the brain that are stressed in Parkinson’s disease. It makes much better sense to employ a broad-spectrum antioxidant intervention which emphasizes water-soluble antioxidants like vitamin C, glutathione, N-acetylcysteine, polyphenols, proanthocyanidins, lipoate, NADH, DMSO, etc. This general approach has been pioneered by Annetta Freeman with outstanding results.

Cognitive Enhancement

At this time, the degree of mitochondrial involvement in age-related mental decline (ARMD) and age-associated memory impairment (AAMI) is not known. A significant amount of the mitochondrial DNA (mtDNA) damage seen in Parkinson’s disease is also observed in age-matched controls. Such observations suggest that reductions in mitochondrial efficiency and ATP output may underlie many age-associated phenomena. The successful use of mitochondrial support nutrients to ameliorate serious mitochondrial diseases may prove to be generalizable to the subclinical complaints of normal, healthy, aging humans.


References

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