This oil-soluble vitamin-like substance is present in most eukaryotic cells, primarily in the mitochondria. It is a component of the electron transport chain and participates in aerobic cellular respiration, generating energy in the form of ATP. Ninety-five percent of the human bodys energy is generated this way.^[1][2] Therefore, those organs with the highest energy requirements—such as the heart and the liver—have the highest CoQ₁₀ concentrations.^[3][4][5]

History

Coenzyme Q was first discovered by professor Andrew L. Crane and colleagues at the University of Wisconsin–Madison Enzyme Institute in 1957.^[6][7] In 1958, its chemical structure was reported by Dr. Karl Folkers and coworkers at Merck; in 1968, Folkers became a Professor in the Chemistry Department at the University of Texas at Austin.^[7][8]

Chemical properties

The oxidized structure of CoQ10 is shown on the top right. The various kinds of Coenzyme Q can be distinguished by the number of isoprenoid side-chains they have. The most common CoQ in human mitochondria is Q₁₀. The 10 refers to the number of isoprene repeats. The image below has three isoprenoid units and would be called Q₃.

Biochemical role

CoQ is found in the membranes of many organelles. Since its primary function in cells is in generating energy, the highest concentration is found on the inner membrane of the mitochondrion. Some other organelles that contain CoQ₁₀ include endoplasmic reticulum, peroxisomes, lysosomes, and vesicles.

Supplementation

Because of its ability to transfer electrons and therefore act as an antioxidant, Coenzyme Q is used as a dietary supplement.

According to the Mayo Clinic^[9] "CoQ10 has been used, recommended, or studied for numerous conditions, but remains controversial as a treatment in many areas." Further clinical results are needed to determine whether the supplementation with Coenzyme Q10 is beneficial for healthy people.

Mitochondrial disorders

Supplementation of Coenzyme Q₁₀ is a treatment for some of the very rare and serious mitochondrial disorders and other metabolic disorders, where patients are not capable of producing enough coenzyme Q₁₀ because of their disorder. Coenzyme Q₁₀ is then prescribed by a physician.^[10]

Heart failure

There is some clinical evidence^[11] that supplementation with Coenzyme Q10 is beneficial treatment of patients with congestive heart failure. However, The American College of Cardiology published in 2005 an expert consensus document concluding that the value of coenzyme Q10 in cardiovascular disease has not been clearly established.^[12] The Mayo clinic says that there is not enough scientific evidence to recommend for or against the use of CoQ10 in patients with coronary heart disease.^[9]

Migraine headaches

Supplementation of Coenzyme Q₁₀ has been found to have a beneficial effect on the condition of some sufferers of migraine headaches. So far, three studies have been done, of which two were small, did not have a placebo group, were not randomized, and were open-label,^[13] and one was a double-blind, randomized, placebo-controlled trial, which found statistically significant results despite its small sample size of 42 patients.^[14] Dosages were 150 to 300 mg/day.

Cancer

It is also being investigated as a treatment for cancer, and as relief from cancer treatment side-effects.^[15]

Cardiac arrest

Another recent study shows a survival benefit after cardiac arrest if coenzyme Q₁₀ is administered in addition to commencing active cooling of the body to 90–93 degrees Fahrenheit (32–34 degrees Celsius).^[16]

Blood pressure

There are several reports concerning the effect of CoQ₁₀ on blood pressure in human studies.^[17] In a recent meta-analysis of the clinical trials of CoQ₁₀ for hypertension, a research group led by Professor Frank Rosenfeldt (Director, Cardiac Surgical Research Unit, Alfred Hospital, Melbourne, Australia) reviewed all published trials of Coenzyme Q₁₀ for hypertension, and assessed overall efficacy, consistency of therapeutic action, and side-effect incidence. Meta-analysis was performed in 12 clinical trials (362 patients) comprising three randomized controlled trials, one crossover study, and eight open-label studies. The research group concluded that coenzyme Q₁₀ has the potential in hypertensive patients to lower systolic blood pressure by up to 17 mm Hg and diastolic blood pressure by up to 10 mm Hg without significant side-effects.^[18]

Lifespan

One study demonstrated that low dosages of coenzyme Q₁₀ reduce oxidation and DNA double-strand breaks, and a combination of a diet rich in polyunsaturated fatty acids and coenzyme Q₁₀ supplementation leads to a longer lifespan in rats.^[19] Coles and Harris demonstrated an extension in the lifespan of rats when they were given coenzyme Q₁₀ supplementation.^[20] Another study demonstrated that coenzyme Q₁₀ extends the lifespan of c. elegans (nematode).^[21]

Biosynthesis

The benzoquinone portion of Coenzyme Q₁₀ is synthesized from tyrosine, whereas the isoprene sidechain is synthesized from acetyl-CoA through the mevalonate pathway. The mevalonate pathway is also used for the first steps of cholesterol biosynthesis.

Inhibition by statins and beta blockers

Coenzyme Q₁₀ shares a common biosynthetic pathway with cholesterol. The synthesis of an intermediary precursor of Coenzyme Q₁₀, mevalonate, is inhibited by some beta blockers, blood pressure-lowering medication,^[22] and statins, a class of cholesterol-lowering drugs.^[23] Statins can reduce serum levels of coenzyme Q₁₀ by up to 40%.^[24] Some research suggests the logical option of supplementation with coenzyme Q₁₀ as a routine adjunct to any treatment that may reduce endogenous production of coenzyme Q₁₀, based on a balance of likely benefit against very small risk.^[25][26]

Absorption and metabolism

CoQ₁₀ is a crystalline powder that is insoluble in water due to its low polarity. It has a relatively high molecular weight (863 g/mol) and its solubility in lipids is also limited so it is very poorly absorbed in the gastrointestinal tract.^[27],[28] Absorption follows the same process as that of lipids and the uptake mechanism appears to be similar to that of vitamin E, another lipid-soluble nutrient. Emulsification and micelle formation is required for the absorption of fats. For CoQ₁₀, this process is chiefly facilitated by secretions from the pancreas and bile salts in the small intestine.^[29] A general rule is that the higher the dose orally administered, the lower the percent of the dose absorbed.^[29]

Data on the metabolism of CoQ₁₀ in animals and humans are limited.^[27] A study with ¹⁴C-labeled CoQ₁₀ in rats showed most of the radioactivity in the liver 2 hours after oral administration when the peak plasma radioactivity was observed, but it should be noted that CoQ₉ is the predominant form of coenzyme Q in rats.^[30] It appears that CoQ₁₀ is metabolised in all tissues, while a major route for its elimination is biliary and fecal excretion. After the withdrawal of CoQ₁₀ supplementation, the levels return to their normal levels within a few days, irrespective of the type of formulation used.^[31]

Factors affecting ubiquinone levels

Use of statins reduce ubiquinone levels.
Aging, in individuals older than 20 years, reduces ubiquinone levels in internal organs.^[32][33]
UV exposure reduces ubiquinone levels in the skin.^[34]

Pharmacokinetics and bioavailability

Some reports have been published on the pharmacokinetics of CoQ10. The plasma peak can be observed 2–6 hours after oral administration, mainly depending on the design of the study. In some studies, a second plasma peak was also observed at about 24 hours after administration, probably due to both enterohepatic recycling and redistribution from the liver to circulation.^[35] Tomono et al. used deuterium-labelled crystalline CoQ10 to investigate pharmacokinetics in human and determined an elimination half-time of 33 hours.^[36]

Improving the bioavailability of CoQ₁₀

The importance of how drugs are formulated for bioavailability is well known. In order to find a principle to boost the bioavailability of CoQ₁₀ after oral administration, several new approaches have been taken and different formulations and forms have been developed and tested on animals or humans.^[27]

Reduction of particle size

The obvious strategy is reduction of the particle size to as low as the micro- and nano-scale. Nanoparticles have been explored as a delivery system for various drugs and an improvement of the oral bioavailability of drugs with poor absorption characteristics has been reported;^[37] the pathways of absorption and the efficiency were affected by reduction of particle size. This protocol has so far not proved to be very successful with CoQ10, although reports have differed widely.^[38],[39] The use of the aqueous suspension of finely powdered CoQ₁₀ in pure water has also only revealed a minor effect.^[31]

Soft-gel capsules with CoQ10 in oil suspension

A successful approach was to use the emulsion system to facilitate absorption from the gastrointestinal tract and to improve bioavailability. Emulsions of soybean oil (lipid microspheres) could be stabilised very effectively by lecithin and were utilised in the preparation of soft gelatine capsules. In one of the first such attempts, Ozawa et al. performed a pharmacokinetic study on beagle dogs in which the emulsion of CoQ₁₀ in soybean oil was investigated; about two times higher plasma CoQ₁₀ level than that of the control tablet preparation was determined during administration of a lipid microsphere.^[31] Although an almost negligible improvement of bioavailability was observed by Kommuru et al. with oil-based soft-gel capsules in a later study on dogs,^[40] the significantly increased bioavailability of CoQ10 was confirmed for several oil-based formulations in most other studies.^[28]

Novel forms of CoQ10 with increased water-solubility

Facilitating drug absorption by increasing its solubility in water is a common pharmaceutical strategy and has also been shown to be successful for Coenzyme Q10. Various approaches have been developed to achieve this goal, with many of them producing significantly better results over oil-based soft-gel capsules in spite of the many attempts to optimize their composition.^[27] Examples of such approaches are use of the aqueous dispersion of solid CoQ10 with tyloxapol polymer,^[41] formulations based on various solubilising agents, i.e. hydrogenated lecithin, ^[42] and complexation with cyclodextrins; among the latter, complex with β-cyclodextrin has been found to have highly increased bioavailability.^[43] and is also used in pharmaceutical and food industry for CoQ10-fortification.^[27] Also some other novel carrier systems like liposomes, nanoparticles, dendrimers etc can be used to increase the bioavailability of Coenzyme Q10.

Occurrence in nature

Fresh tissue samples from both mackerel and herring found the concentration of Coenzyme Q10 to be higher in the heart tissue (105-148 μg/g) compared to concentrations found in the body tissue. The red tissue of mackerel contained a higher concentration (67μg/g) of CoQ10 than the white tissue (15μg/g) whilst in herring tissue the concentration was found to range between 15–24 ug/g. A small seasonal variance in the concentrations of CoQ10 was observed in both fish ^[44].

References