David S Newman. Cambridge World History of Food. Editor: Kenneth F Kiple & Kriemhild Conee Ornelas. Volume 1, Cambridge University Press, 2000.
Potassium (K) is found in virtually all aerobic cells and is essential to life. It is the third most abundant element in the human body (after calcium and phosphorus) and the eighth most abundant element in the earth’s crust, with a mass percent of 1.8, which means that every 100 grams (g) of the earth’s crust contains 1.8 g of potassium. Potassium is a very reactive alkali metal with an atomic number of 19 and an atomic weight of 39.098 atomic mass units (amu). Its outer “4s” electron is not bound very tightly to the atom, which is therefore easily ionized to K+ (Dean 1985), and potassium reacts readily with chlorine to form the salt potassium chloride. Potassium chloride is a white crystalline solid at room temperature with alternating potassium ions and chloride ions on the lattice sites. Potassium is found primarily in seawater and in natural brines in the form of chloride salt. The minerals mica and feldspar also contain significant quantities of potassium (Dean 1985).
The Discovery of Elemental Potassium
Potassium was first isolated in 1807 by Humphry Davy (1778-1829), who electrolyzed “potash” with a newly invented battery designed to contain a series of voltaic cells, with electrodes made out of zinc and copper plates dipped in a solution of nitrous acid and alum. In Davy’s time, the term “potash” referred to any number of different compounds, including “vitriol of potash” (potassium sulfate), “caustic potash” (potassium hydroxide), and “muriate of potash” (potassium chloride as well as potassium carbonate), the last of which was formed by leaching ashes from a wood fire and evaporating the solution to near dryness in an iron pot. Today, potash is usually potassium carbonate, although potassium chloride is still called potash by fertilizer manufacturers (Kent 1983: 262). The potash Davy used was potassium hydroxide that he had dried and melted. He wrote of his experiment:
A small piece of pure potash, which had been exposed for a few seconds to the atmosphere, so as to give conducting power to the surface, was placed upon an insulated disc of platina, connected with the negative side of a battery of the power 250 of 6 and 4, in a state of intense activity; and a platina wire communicating with the positive side, was brought in contact with the upper surface of the alkali. The whole apparatus was in the open atmosphere. Under these circumstances a vivid action was soon observed to take place. The potash began to fuse at both its points of electrization. There was a violent effervescence at the upper surface; at the lower, or negative surface, there was no liberation of elastic fluid; but small globules having a high metallic lustre, and being precisely similar in visible characters to quicksilver formed, and others remained and were merely tarnished, and finally covered with a white film which formed on their surfaces. These globules, numerous experiments soon shewed to be the substance I was in search of, and a peculiar inflammable principle the basis of potash. (Davy 1839-40, 5: 60)
The next day, Davy isolated sodium metal by electrolyzing soda ash (sodium hydroxide) in much the same way. In the history of chemistry, isolating potassium and sodium was no mean accomplishment. It had been suspected by several people, but especially by Antoine Lavoisier (1743-94), that potash was a compound and that the “basis of potash” was, indeed, a metal (Partington 1962, 3: 485). Davy’s experiments confirmed this suspicion. Several years earlier, in 1801, Carl Friedrich Kielmeyer, also suspecting that potash was an oxide of some metal, had attempted to electrolyze potash using a voltaic pile but was unsuccessful (Partington 1964, 4: 45). Thus, the credit for discovering the two most important alkali metals clearly goes to Humphry Davy.
Between 1808 and 1809, the French chemists Louis Thenard (1777-1857) and Joseph Gay-Lussac (1778-1850) found that only small quantities of potassium and sodium could be derived by the electrolysis of fused alkali hydroxides and went on to develop a much improved method for producing larger quantities of both (Partington 1964, 4: 94). Thenard and GayLussac reacted the fused alkali with red-hot iron turnings in an iron gun barrel lined with clay and sand and collected the condensed metal vapor in a receiver attached to the gun barrel. An explosion using this dangerous device nearly blinded Gay-Lussac.
A variation that further improved the method for producing potassium used potassium carbonate as the source of potassium and carbon instead of iron as the reducing agent to produce elemental potassium and carbon dioxide as the reaction products. In 1827, Frederich Wohler (1800-82) first employed potassium produced by this technique to isolate metallic aluminum in more or less pure form. He reacted anhydrous aluminum chloride with potassium metal as the reducing agent and obtained enough aluminum metal to measure its properties (Ihde 1964: 467). Today, metallic potassium is usually produced by reacting molten sodium with molten potassium chloride and condensing the gaseous potassium formed by this reaction. There are very few industrial uses for elemental potassium, although many of its compounds are widely utilized throughout industry and agriculture. For example, potassium nitrate is commonly employed as a fertilizer in the tobacco industry where chloride-containing fertilizers are undesirable.
Potassium in Living Organisms
Despite all of potassium’s various functions, its principal function in living organisms is in the transportation of ions across cell membranes. In most animal cells, the internal concentration of potassium ions is between 20 and 30 times higher than the external concentration of potassium ions found in the extra-cellular fluids. Most cells also have considerably different internal concentrations of sodium ions than are found externally. Neither the reason for the existence of these ionic gradients across the cell membrane nor, for that matter, potassium’s principal role in cell metabolism was understood until comparatively recently, and many of potassium’s functions in living organisms are still being investigated. It can be said with certainty, however, that moving sodium ions and potassium ions across membranes is an important activity in most organisms, and that if this activity stops, the organism dies.
The Sodium-Potassium-ATPase Pump
The significant step toward understanding potassium’s role in animal cells was taken in 1957. In that year, the Danish biochemist Jens Skou, who later won the 1997 Nobel Prize in chemistry, found an enzyme in crab nerve cells that hydrolyzed adenosine triphosphate (ATP) into adenosine diphosphate (ADP) and any one of several phosphorus-containing anions, such as dihydrogen phosphate, in the presence of magnesium ions only if both sodium ions (Na+) and potassium ions (K+) were present (Skou 1965: 6). Magnesium ions are always required for enzymes (called ATPases) to catalyze the hydrolysis of ATP.
The unusual property of this enzyme was that neither K+ nor Na+ alone had any significant effect on its activity. A short time later, Skou proposed that this Na+-K+-ATPase complex was part of a transmembrane pump that pumped Na+ and K+ into and out of cells, and that the energy needed for this process was supplied by the hydrolysis of ATP. Many different kinds of animal tissues were soon found to exhibit similar Na+-K+-ATPase activity, and it was shown that this enzyme was, indeed, a protein that resided in a cell’s membrane, with its sodium ion receptor facing the interior of the cell (or the cytoplasm), and its potassium ion receptor facing the external environment. The Na+-K+-ATPase pumped sodium ions out of the cell and potassium ions into the cell, accompanied by the hydrolysis of ATP. Brain, nerve, and muscle cells—and the electric organ of the electric eel—were discovered to be particularly rich in K+-Na+-ATPase activity (Lehninger 1970: 617).
“Membrane transport”—the pumping of ions across cell membranes—is such an important part of a cell’s total activity that it is estimated that more than one-third of the ATP consumed by a resting animal is expended transporting ions across membranes (Stryer 1988: 950). The Na+-K+ pump, in particular, maintains proper electrochemical potentials across cell membranes and maintains the proper concentrations of Na+ and K+, both internally and externally. Because these concentrations are usually far from the equilibrium concentrations, the ions must be pumped against their respective concentration gradients. The sodium-ion and potassium-ion concentration gradients in most animal cells control cell volume, drive the transport of sugars and amino acids across cell membranes, and control the electrical excitability of both nerve and muscle cells.
In the most ubiquitous transport systems in animal cells, the Na+-K+ pump removes 3 sodium ions from the interior of the cell and pumps in 2 potassium ions from the cell’s surroundings, using the transmembrane Na+-K+-ATPase to facilitate the transport of the ions. One molecule of ATP is hydrolyzed to supply the necessary free energy.This process is shown schematically in Figure IV.B.6.1. Typically, the internal concentration of potassium ions is approximately 20 times greater than the external concentration of potassium ions.This concentration gradient is maintained also by the free energy supplied by the hydrolysis of ATP and the transmembrane enzyme pump. The ATP supplies energy to the enzyme by phosphorylating it in the presence of sodium ions and magnesium ions. The phosphorylated enzyme is then dephosphorylated in the presence of potassium ions to regenerate the original enzyme and form a phosphorus-containing ion, such as hydrogen phosphate. The energy stored in the ATP is transferred to the enzyme via the phosphate group bonding to it. The enzyme uses this energy to move ions against their gradients and then gets rid of the phosphate group via hydrolysis.
Potassium in the Nerves and the Nervous System
Potassium ions and sodium ions are found in the nerve cells of virtually all animals and play a vital role in the transmission of nerve impulses. These ions regulate the nerve’s transmembrane potential, and it is the transmembrane potential difference between the interior and exterior of a nerve cell that causes the transmission of nerve impulses along the nerve (Atkins 1994: 334). Many K+-Na+ pumps are distributed throughout the nervous system. When a nerve is resting, there is a high internal potassium ion concentration and a high external sodium ion concentration. When the cell experiences a pulse, the nerve cell membrane’s structure alters, becoming permeable to sodium ions. Because the membrane is now more permeable to sodium ions, the ions rapidly flow into the interior of the cell and reduce the size of the sodium ion concentration gradient so that it becomes smaller than it was when the nerve was resting. In other words, the Na+ ions and K+ ions exchange places. Because the membrane’s potential arises primarily from the difference in concentration on either side of the membrane, the voltage across the membrane drops. This change in voltage triggers the adjacent part of the cell wall to alter its structure. The pulses of collapsing potential pass along the nerve. Behind each pulse, the Na+-K+ pump restores the proper internal sodium ion concentration by pumping out sodium ions and pumping in potassium ions.
Potassium in Muscle Tissue
Generally speaking, muscle tissue can be divided into two classes, smooth and striated. Striated muscle is under voluntary control and has a striated or striped appearance under a light microscope. Striated muscle found in vertebrates contains 2 protein filaments that interact with each other. One of the filaments contains myosin and the other contains 3 proteins—actin, tropomyosin, and troponin. Striated muscle contraction is regulated primarily by calcium ions (Ca++), and the calcium ion concentration is itself regulated by a Ca++-ATPase pump, very similar in kind and function to the Na+-K+-ATPase pumps found in other kinds of cells. In mammalian striated muscle, however, fatigue is associated with a loss of intercellular K+ and a gain in intercellular Cl–, Na+, and H2O, and it may indeed be the case that, in humans, fatigue is related to these changes in the potassium ion concentration gradient across muscle cells. Lowering the internal potassium ion concentration and raising the external potassium ion concentration depolarizes the cell membrane. The rapid recovery of muscle function following brief rest periods is caused by a reestablishment of the proper potassium ion concentrations and a restoration of the resting potential of the muscle cell’s membrane.
Smooth muscle, such as that found in the heart or veins, is not subject to voluntary control. The contraction of smooth muscle is controlled by the degree of phosphorylation of its light chains. Phosphorylation leads to contraction and dephosphorylation leads to relaxation. In smooth muscle, Na+-K+ pumps and active transport are directly involved with contraction and relaxation. For example, in vascular smooth muscle, the function of the Na+-K+-ATPase pump is to transport 2 potassium ions into the cell for every 3 sodium ions it takes out. The energy for this transport is supplied by phosphorylation. As in striated muscles, extracellular calcium ions also play an important role in cell function, but in smooth muscles this role is primarily to regulate the K+-Na+ equilibria extant in the cell (O’Donnell and Owen 1994).
The importance of the Na+-K+ pump in smooth muscle function has some intriguing consequences. It is entirely possible—and evidence is mounting that it is more than just possible—that a principal underlying cause of hypertension in humans is an inhibitor of Na+-K+-ATPase activity or an impaired Na+-K+ pump (Blaustein et al. 1986; O’Donnell and Owen 1994: 687). It is also well known that a certain class of compounds, called cardiotonic steroids, specifically inhibit the Na+-K+ pump. These steroids prevent the dephosphorylation of Na+-K+-ATPase. As a consequence, these compounds are very important in the treatment of heart disease.
For example, digitalis (an extract from purple fox-glove leaves that contains a mixture of cardiac steroids), which is perhaps the best-known remedy for congestive heart failure and has been widely used for centuries, increases the force of heart muscle contraction by inhibiting the Na+-K+-ATPase pump (Voet and Voet 1990: 496).This increases the sodium ion concentration inside the cell, thereby causing a reduction in the concentration gradient across the membrane, because the internal sodium ion concentration becomes closer to the external sodium ion concentration. Reducing the sodium ion concentration gradient reduces the extrusion of calcium ions via the Na+-Ca++ exchanger, which increases the internal calcium ion concentration. A high intracellular calcium ion concentration causes the heart muscle to contract (Stryer 1988: 955). Ion exchangers, such as the Na+-Ca++ exchanger, are conduits—or ports—that allow ions to diffuse in and out of a cell via concentration gradients rather than through active transport, and they are not fueled by ATP.
The medicinal effects of digitalis were known long before the compounds themselves were identified and long before the Na+-K+-ATPase pump was discovered. In the 1770s, a woman in Shropshire, England, using extracts from some 20 different herbs, prepared a cocktail that had a remarkable effect on curing congestive heart failure or “dropsy,” as it was called. Many people suspected that this woman was a witch because of the curative power of her concoctions. A physician by the name of William Withering heard about the woman’s remarkable medicine and, after considerable effort, found that foxglove was the significant herb in the cocktail. He published his findings in 1785 in his classic work, An Account of the Fox-glove and Some of its Medical Properties. A line from this paper is worth noting: “It [the foxglove] has a power over the motion of the heart to a degree yet unobserved in any other medicine, and this power may be converted to salutary ends” (Estes and White 1965: 110).
Potassium in Protein Synthesis
Protein synthesis on ribosomes requires a high potassium concentration for maximum efficiency. Ribosomes are cellular bodies that serve as the sites for protein synthesis and can be thought of as the cell’s protein factories because protein assembly from amino acids, controlled by RNA, takes place on the ribosome’s surface (Lehninger 1970: 616).
Potassium in Glycolysis
Potassium is necessary for glycolysis, which is a form of fermentation that ultimately converts glucose (C6H12O6) into pyruvate (C3COCOO–) with the associated production of ATP. Glycolysis can be thought of as a fundamental aspect of the generation of metabolic energy that occurs in virtually all living organisms, including humans. Glycolysis requires potassium ions for maximum activity of one of the enzyme catalysts, pyruvate kinase, which is involved in the process (Stryer 1988: 350). If there is sufficient oxygen present, the pyruvate enters the mitochondria where it is completely oxidized to CO2 and H2O. If there is insufficient oxygen present, as is often the case in muscle contraction, pyruvate is converted to lactic acid. In yeast, which is an anaerobic organism, pyruvate is converted to ethanol and carbon dioxide.
In 1897 and 1898, the German chemist Eduard Buchner discovered that when yeast cells were crushed with sand and diatomaceous earth (kieselguhr), the cell-free liquid extract was able to ferment sucrose into alcohol and carbon dioxide. Buchner’s results showed, for the first time, that fermentation could occur outside a living cell, and his fermentation experiments contributed greatly to the overthrow of the “vital force” theory then prevalent in the biological sciences,and started the field of modern biochemistry. Even Louis Pasteur thought that fermentation could occur only in living cells. Buchner was awarded the Nobel Prize in chemistry for this work in 1907 (Partington 1964,4:309).
Renal Control of Potassium
In humans, the kidneys are responsible for most potassium excretion, although fecal matter does contain about 10 percent of the potassium ingested (Stanton and Giesbisch 1990). However, renal malfunction as a result of disease or trauma often will prevent the proper elimination of potassium from the body. For example, because maintenance of the proper potassium ion balance is vital, the secretion of potassium ions by the distal tubule is one of the kidney’s most important functions, and it is the distal tubule that, more or less, regulates the quantity of potassium ions eliminated in the urine. More than 75 percent of the filtered potassium is reabsorbed in the proximal tubule, and this percentage remains nearly constant no matter how much is filtered. About 50 percent of the urinary potassium is secreted into the urine by the distal tubules in normally functioning kidneys. If this secretion is prevented—as, for example, in polycystic kidneys—dangerous levels of potassium ions can accumulate and cause heart failure. Na+-K+-ATPase pumps, H+-K+-ATPase pumps, and K+– Cl–Na+ cotransport have all been found to control potassium in tissues within the kidneys.
Difficulties with potassium elimination are also encountered if the production of a hormone in the adrenal gland, aldosterone, is inhibited. Aldosterone catalyzes the elimination of potassium ions so that lowering the concentration of this enzyme reduces the rate of potassium ion elimination and can increase potassium ion concentrations in the blood to dangerous levels.
Potassium and Health
Potassium is essential for human life. A normal adult male, weighing 70 kilograms (kg), contains approximately 135 g of the mineral in his body. About 98 percent of this potassium is found in his cells and the other 2 percent in extracellular fluids (Macrae, Robinson, and Sadler 1993, 6: 3668). Potassium deficiency is called hypokalemia and—because potassium is present in most foods—when it occurs, it is often in areas where people exist on subsistence or starvation diets. Severe diarrhea, diabetes, and prolonged use of cortisone can also cause hypokalemia.
For living cells to function properly it is essential they maintain a correct balance between internal potassium ion concentrations and external potassium ion concentrations. There are many factors that influence this balance, in addition to the various pumps and transport systems already described. For example, there must be a normal water balance in the organism, because if the amount of K+ inside a cell remains fixed and the amount of water increases, the potassium ion concentration will decrease proportionately, or if the amount of water decreases, the potassium ion concentration will increase. A potassium deficiency resulting from urine loss often occurs during the treatment of heart disease because the medication used prevents sodium and water retention. To reduce this deficiency, foods high in potassium are often prescribed.
Recently, a possible connection between potassium and hypertension has been discovered (Brancati 1996). It was found that African-Americans, a particularly high-risk group for hypertension and acute myocardial infarction, benefited greatly from a diet rich in potassium. Indeed, in a double-blind test, all of the subjects who received potassium supplements reduced their blood pressure—regardless of age, gender, body weight, or alcohol consumption. It was not clear why potassium was especially beneficial for African-Americans. It may be that their diets are particularly low in potassium, but it is more likely the case that, for some reason, African-Americans are especially sensitive to potassium. This finding is certainly also consistent with the mechanism of the Na+-K+-ATPase pump discussed earlier.
Potassium and Diet
Potassium is one of the most important elements in the human diet. According to the National Academy of Sciences, a healthy adult should consume between 1,875 milligrams (mg) and 5,625 mg of this mineral daily (National Academy of Sciences 1980: 173). It is probable that the diets of our hunter-gatherer ancestors contained some 2 mg of potassium for every calorie consumed. It is also known that people who consume more than 4 g of potassium each day have a much lower incidence of disease.
Unfortunately, a modern diet typically contains only 0.5 mg of potassium for every calorie consumed, although it is the case that practically all successful dietary weight-loss programs, dietary cholesterol-lowering programs, and dietary blood-pressure-lowering programs contain foods that are high in potassium. Some of these are oranges, tomatoes, peas, spinach, bananas, cantaloupe, and fish (Pennington 1985). Foods that are very high in potassium content, with the exception of potatoes, are relatively low in energy content, which makes them especially healthful for normal adults.
Potassium is a relatively abundant alkali metal that comprises a significant fraction of the earth’s crust. Its salts and oxides are widely used in industry and agriculture. Potassium’s role in membrane transport and other metabolic processes make it vital to virtually every living organism. Recently, antibiotics using potassium ions have been discovered, and it is beginning to appear that eating foods rich in potassium or taking potassium supplements can reduce hyper-tension in humans. Most foods contain some potassium, and the maintenance of good health in normal adults seems to require a diet that contains 4 g of potassium per day.