acid (as'id) [L. acidus, sour]
1. Any substance that liberates hydrogen ions (protons) in solution; a hydrogen ion donor. An acid reacts with a metal to form a salt, neutralizes bases, and turns litmus paper red.
3. A sour substance.
4. Slang term for lysergic acid diethylamide (LSD).
, the acid that gives the sour taste to vinegar. It is also used as a reagent. Glacial (highly purified) acetic acid contains at least 99.5% acetic acid by weight.
, a ketone body formed when fats are incompletely oxidized. It was formerly called acetylacetic acid. Synonym: diacetic acid
; diacetic acid
acetylacetic acid See: acetoacetic acid
acetylsalicylic acid Abbreviation: ASA
, a colorless corrosive acid used in making acrylic polymers and resins.
adenylic acidAdenosine monophosphate.
alpha-hydroxy acid Abbreviation: AHA
Any of a class of water-soluble acids derived from fruit or milk, having a hydroxyl moiety in the first position in the molecule. AHAs are used in chemical peels and other skin care products to remove the outer layer of the epidermis. This chemical exfoliation is promoted for its cosmetic effects on wrinkled or sun-damaged skin.
, an omega-3 fatty acid derived from plants, esp. seeds (canola oil, flaxseed, walnuts and pumpkins) and from some fish (salmon and mackerel).
, a natural coenzyme and antioxidant, used for short-term treatment of peripheral neuropathies.
amino acid See: amino acid
aminobenzoic acidPara-aminobenzoic acid.
, a hemostatic drug. It is a specific antidote for an overdose of a fibrinolytic agent.
aminoglutaric acidGlutamic acid.
aminosalicylic acidPara-aminosalicylic acid.
aminosuccinic acidAspartic acid.
, an omega-6 fatty acid formed by the action of enzymes on phospholipids in cell membranes. The acid is found in many foods. It is metabolized primarily by the cyclo-oxygenase or 5-lipoxygenase pathways to produce prostaglandins and leukotrienes, which are important mediators of inflammation. Corticosteroids inhibit formation of arachidonic acid from phospholipids when cell membranes are damaged. Nonsteroidal anti-inflammatory agents such as salicylates, indomethacin, and ibuprofen inhibit the synthesis of prostaglandins and leukotrienes.
, a compound intermediate in the synthesis of arginine, formed from citrulline and aspartic acid.
, an acid derived from Aristolochia
, a genus of flowering plants, and used as an herbal remedy. It is promoted as an aphrodisiac, a weight loss agent, and an anticonvulsant.
The acid is a known carcinogen, and its use has been associated with and may cause end-stage renal disease and cancers of the urinary tract that may occur many years after usage has stopped.
ascorbic acidVitamin C.
, a nonessential amino acid that is a product of pancreatic digestion. Synonym: aminosuccinic acid
, a crystalline acid from which phenobarbital and other barbiturates are derived.
, a white crystalline acid having a slight odor. It is used in keratolytic ointments and in food preservation. Saccharin is a derivative of this acid.
Any of the complex acids that occur as salts in bile, e.g., cholic, glycocholic, and taurocholic acids. They give bile its foamy character, are important in the digestion of fats in the intestine, and are reabsorbed from the intestine to be used again by the liver. See: enterohepatic circulation
An acid containing hydrogen and one other element.
, a white crystalline acid that in water forms a very weak acid solution poisonous to plants and animals. It is soluble in water, alcohol, and glycerin. See: boric acid poisoning
Boric acid is toxic and should be used only rarely. It is particularly dangerous because it can be accidentally swallowed by children or used in food because of its resemblance to sugar.
, a viscous fatty acid with a rancid odor, derived from butter but rare in most fats. It is used in disinfectants, emulsifying agents, and pharmaceuticals.
carbolic acidPhenol (1).
, an acid formed when carbon dioxide is dissolved in water.
Any acid containing the carboxyl group –COOH. The simplest examples are formic and acetic acids.
cell-free fetal nucleic acidFree fetal nucleic acid.
, a bile acid formed in the liver by hydrolysis of other bile acids. It is formed from the breakdown of cholesterol and helps digest consumed fats.
, an insoluble white powder derived from cinnamon. It is used as a flavoring agent in cooking and in the preparation of perfumes and medicines.
, an acid found naturally in citrus fruits or prepared synthetically. It acts as a sequestrant, helping to preserve food quality.
conjugated linoleic acid Abbreviation: CLA
Any of the isomers of linoleic acid effective against cancer, obesity, diabetes, and atheromata in laboratory rodents. CLAs have not been shown to have similar beneficial effects in humans.
S, an acid produced by the oxidation of cysteine. Further oxidation produces taurine.
, a crystalline acid found in bile.
deoxyribonucleic acid, desoxyribonucleic acid See: DNA
diacetic acidAcetoacetic acid.
p-dichlorosulfamoyl benzoicacid Halazone.
4,8-dihydroxyquinaldic acidXanthurenic acid.
docosahexaenoic acid, docosahexanoic Abbreviation: DHA
, an omega-3 fatty acid found in the oils of cold-water fish and in algae. DHA plays a role in the development of nerve cell membranes and is required for the normal growth and development of the infant brain. Lack of DHA has been linked to growing numbers of people suffering from depression.
, a toxin that resembles glutamate, the main excitatory amino acid of the brain. When ingested, it may cause continuous seizures.
eicosapentaenoic acid Abbreviation: EPA
, an omega-3 fatty acid found in fish oils, containing 20 carbons and five double bonds.
endogenous uric acid
Uric acid derived from purines undergoing metabolism from the nucleic acid of body tissues.
essential fatty acid Abbreviation: EFA
A fatty acid (alpha-linoleic and linoleic) that is essential for health and must be present in the diet because it cannot be synthesized in the body. See: digestion
ethylenediaminetetraacetic acid Abbreviation: EDTA
, a chelating agent that, in its calcium or sodium salts, is used to remove metallic ions such as lead and cadmium from the body and as a food preservative. See: chelation
exogenous uric acid
Uric acid derived from purines from food made up of free purines and nucleic acids. See: urate; uraturia
Any of numerous monobasic acids with the general formula Cn
-COOH (an alkyl radical attached to a carboxyl group).
Fatty acids are insoluble in water. This insolubility would prevent their being absorbed from the intestines, but the action of bile salts on the fatty acids enable thems to be absorbed. Fatty acids include acetic, butyric, capric, caproic, caprylic, formic, lauric, myristic, palmitic, and stearic acids. Unsaturated fatty acids have one or more double or triple bonds in the carbon chain. They include those of the oleic series (oleic, tiglic, hypogeic, and palmitoleic) and the linoleic or linolic series (linoleic, linolenic, clupanodonic, arachidonic, hydrocarpic, and chaulmoogric). See: fat
, a water-soluble B complex vitamin needed for DNA synthesis and amino acid metabolism. It is present in green leafy vegetables, beans, and yeast. It is used to treat megaloblastic and macrocytic anemias and to prevent neural tube defects (NTDs) and cardiovascular disease in adults. The U.S. Public Health Service recommends that all women of childbearing age who may become or are pregnant should consume 0.8 mg of folic acid daily to reduce their risk of having a child affected with spina bifida or other NTDs. See: neural tube defect
; vitamin B9
Folic acid should not be used to treat pernicious anemia (a vitamin B12
deficiency) because it does not protect patients against the development of changes in the central nervous system that accompany this type of anemia.
, the active form of folic acid. It is used to counteract the effects of folic acid antagonists and to treat folic acid deficiency anemia.
HCOOH, the first and strongest member of the monobasic fatty acid series. It occurs naturally in certain animal secretions, e.g., the sting of insects such as bees and ants, and in muscle, but it is also prepared synthetically.
, an intermediate product in the metabolism of histidine.
free fatty acid Abbreviation: FFA
The form in which a fatty acid leaves the cell to be transported for use in another part of the body. FFAs are not esterified and may be unbound (not bound to protein). In the plasma, the nonesterified fatty acids released immediately combine with albumin to form bound free fatty acids.
free fetal nucleic acid Abbreviation: ffNA
Fetal RNA or DNA in blood or body fluids. It is used to determine the sex of the fetus (as in pregnancies in which X-linked inherited diseases are a concern) or to identify other genetically transmitted illnesses, e.g., trisomies. Synonym: cell-free fetal nucleic acid
, one of the organic acids in the Krebs cycle. It is used as a substitute for tartaric acid in beverages and baking powders.
fuming nitric acid
Concentrated nitric acid (more than 86% nitric acid) that emits toxic fumes that cause choking if inhaled. See: fumes
gadolinium-diethylenetriamine pentaacetic acid Abbreviation: Gd-DTPA
A radiographic contrast agent, used in magnetic resonance imaging to enhance the appearance of blood vessels.
Contrast agents containing gadolinium should not be given to patients with diminished renal function.
COOH, a colorless crystalline acid. It occurs naturally as an excrescence on the twigs of trees, esp. oaks, as a reaction to the deposition of gall wasp eggs. It is used as a skin astringent and in the manufacture of writing inks and dyes.
gamma-aminobutyric acid Abbreviation: GABA
, the principal inhibitory neurotransmitter of the brain.
gamma-linolenic acid Abbreviation: GLA
, an essential fatty acid promoted by alternative medicine practitioners as a treatment for skin and inflammatory disorders, cystic breast disease, and hyperlipidemia.
COOH, an oxidation product of glucose that is present in the urine. Toxic products (salicylic acid, menthol, phenol) that have entered the body through the intestinal tract are detoxified in the liver by conjugation with glucuronic acid.
)·COOH, an amino acid formed in protein hydrolysis and an excitatory neurotransmitter in the central nervous system. Synonym: aminoglutaric acid
OH·CHOH·COOH, an intermediate product of the oxidation of fats.
, a bile acid that hydrolizes to glycine and cholic acid.
, an alpha-hydroxy acid derivative used to remove the outer layer of skin to rejuvenate its appearance.
, an acid produced by the action of glycine oxidase on glycine or sarcosine.
COOH, an acid formed and excreted by the kidneys. It is formed from the combination of benzoic acid and glycine. The synthesis takes place in the liver and, to a limited extent, in the kidneys.
An acid formed in the liver, kidney, and other tissues. It is metabolized to form creatine.
, an intermediate product of tyrosine catabolism. It is found in the urine in alkaptonuria. Synonym: alkapton
hyaluronic acid Abbreviation: HA
, an acid mucopolysaccharide found in the extracellular matrix of connective tissue that acts as a binding and protective agent. It is found in synovial fluid and in the vitreous and aqueous humors of the eye. Patients with osteoarthritis have elevated serum levels of HA. Synonym: hyaluronan
HI, an acid used in solution in various forms of chemical analyses. Synonym: hydrogen iodide
HCl, an inorganic acid normally present in gastric juice. It destroys fermenting bacteria that might cause intestinal tract disturbances.
HCN, a colorless, extremely poisonous, highly volatile acid that occurs naturally in plants but is also produced synthetically. It acts by preventing cellular respiration. Hydrocyanic acid is used in electroplating, fumigation, and in producing dyes, pigments, synthetic fibers, and plastic. Exposure of humans to 200 to 500 parts of hydrocyanic acid per 1,000,000 parts of air for 30 min is fatal. Synonym: hydrogen cyanide
HF, a corrosive solution of hydrogen fluoride in water. It can be used in dentistry to etch composites and porcelain surfaces and is used industrially to etch glass. See: hydrogen fluoride
Exposure to the skin and aerodigestive tract causes severe burns with local necrosis and systemic manifestations resulting from disordered calcium and potassium metabolism. Treatments with calcium gluconate can be beneficial.
hydrosulfuric acidHydrogen sulfide.
Any of the acids containing one or more hydroxyl (–OH) groups in addition to the carboxyl (–COOH) group, e.g., lactic acid, CH3COHCOOH).
hydroxy-iminodiacetic acid Abbreviation: HIDA
A chemical that, when bound to radioactive technetium, is used to demonstrate the formation and flow of bile. See: HIDA scan
C4H8O3, any of the acids present in the urine, esp. in diabetic ketoacidosis, when the conversion of fatty acids to ketones increases.
, an herbal extract promoted for the treatment of weight loss. Placebo-controlled studies have not found any benefit to the treatment.
HClO, an acid used as a disinfectant, deodorant, and bleaching agent. It is usually used in the form of one of its salts.
An acid formed as a result of oxidation of amino acids in the body.
An acid containing no carbon atoms. Synonym: mineral acid
, a radiopaque agent formerly used in cholecystography.
, a radiopaque contrast medium used in radiographic studies of the gallbladder.
Any organic acid containing the ketone CO (carbonyl radical).
, an organic acid formed in muscles during anaerobic cell respiration in strenuous exercise. It is also formed during anaerobic muscle activity when glucose cannot be changed to pyruvic acid in glycolysis. It contributes to muscle aches and fatigue. Synonym: lactacid
COOH, an acid formed when certain simple sugars are acted on by dilute hydrochloric acid.
, a saturated, naturally occurring fatty acid present in certain foods, including peanuts. It is also found in wood tar, various cerebrosides, and in small amounts in most natural fats. The acid is also a by-product of lignin production.
, an omega-6 fatty acid found in vegetables, nuts, grains, seeds, fruits and their oils. Oils rich in linoleic acid include (in descending order) safflower, sunflower, corn, soybean and cottonseed.
, an omega-6 fatty acid, thought to be cardioprotective. It reduces the production of cytokines and down-regulates serum cell adhesion molecules thought to be intermediates in atherosclerosis.
An obsolete term for uric acid.
, a crystalline acid derived from ergot. Its derivative, lysergic acid diethylamide (LSD), is a potent hallucinogen. See: LSD
lysophosphatidic acid Abbreviation: LPA
P, an acid purified from the ascitic fluid of patients with ovarian cancer. LPA stimulates the growth of ovarian cancer and may be a useful screening test for the disease.
, an acid found in sour fruits such as apples and apricots and active in the aerobic metabolism of carbohydrates.
, a dibasic acid formed by the oxidation of malic acid and active in the Krebs cycle in carbohydrate metabolism. Malonic acid is found in beets. Its inhibition of succinic dehydrogenase is the classic example of competitive inhibition.
, a colorless hydroxy acid. Its salt is used to treat urinary tract infections. Synonym: phenylglycolic acid
mineral acidInorganic acid.
, a colorless acid used to make methyl methacrylate.
monounsaturated fatty acid
A fatty acid containing one double bond between carbon atoms. It is found in olive oil and is the predominant fat in the Mediterranean diet. It is thought to reduce low-density lipoprotein levels without affecting high-density lipoprotein levels. See: Mediterranean diet
A nontechnical term for hydrochloric acid.
n-3 fatty acidOmega-3 fatty acid.
n-6 fatty acidOmega-6 fatty acid.
, a colorless, poisonous, fuming corrosive acid, widely used in industry and in chemical laboratories.
, a weak acid chemical reagent used in biological laboratories.
An acid, such as lactic acid or sulfuric acid, that accumulates in the body as a result of digestion, disease, or metabolism. It cannot be excreted from the body by ventilation but must be excreted by organs other than the lungs, e.g., by acidification of the urine.
NUCLEIC ACID: DNA and RNA
Any of the high-molecular-weight molecules that carry the genetic information crucial to the replication of cells and the manufacturing of cellular proteins. They have a complex structure formed of sugars (pentoses), phosphoric acid, and nitrogen bases (purines and pyrimidines). Most important are ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). See: illustration
octadecanoic acidStearic acid.
, a toxic acid found in shellfish. The toxin is produced by oceanic phytoplankton consumed by filter-feeding marine animals such as clams, crabs, and mussels and is the cause of diarrheal shellfish poisoning. Ingestion of these shellfish by humans results in profuse watery diarrhea.
, a monounsaturated fatty acid found in most organic fats and oils.
omega-3 fatty acid, ?-3 fatty acid
Any of the long-chain polyunsaturated fatty acids found in the oils of some saltwater fish, and in canola, flaxseed, walnuts, and some vegetables. These acids include eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Alpha-linolenic acid (found in flaxseed and chia) can be metabolically converted to omega-3 fatty acids in the body. People whose diets are rich in omega-3 fatty acids have a reduced incidence of cardiovascular disease. Synonym: n-3 fatty acid
omega-6 fatty acid, ?-6 fatty acid
Any of the long-chain polyunsaturated fatty acids, such as linoleic acid and arachidonic acid, thought to influence cardiovascular and growth function when balanced with omega-3 fatty acids in eicosanoid production. Linoleic acids are derived from vegetable oils; arachidonic acids, from animal fats. Synonym: n-6 fatty acid
omega-9 fatty acid, ?-9 fatty acid
Any of the nonessential unsaturated fatty acids that have a double carbon bond in the ninth position from the end of their fatty acid tail. They include oleic acid (present in olive oil), stearic acid, and erucic acid.
An acid containing the carboxyl radical, –COOH. Organic acids include acetic acid, formic acid, lactic acid, and all fatty acids.
, a crystalline acid occurring in milk. It is a precursor in the formation of pyrimidine nucleotides.
osmic acidOsmium tetroxide.
, the simplest dibasic organic acid. Its potassium or calcium salts occur naturally in rhubarb, wood sorrel, and other plants. It is the strongest organic acid and is poisonous. When properly diluted, it removes ink or rust stains from cloth. It is used also as a reagent.
oxaloacetic acid, oxalacetic acid
, a product of carbohydrate metabolism resulting from oxidation of malic acid during the Krebs cycle. It may be derived from other sources.
, a saturated fatty acid occurring as esters in most natural fats and oils.
, an acid of the vitamin B complex, occurring naturally in yeast, liver, heart, salmon, eggs, and various grains. It is part of coenzyme A, which is necessary for the Krebs cycle and for conversion of amino acids and lipids to carbohydrates. Synonym: vitamin B5
para-aminobenzoic acid Abbreviation: PABA
COOH, an acid of the vitamin B complex, used as a dietary supplement, an antirickettsial drug, a reagent, and a sunscreen agent. Synonym: aminobenzoic acid
para-aminohippuric acid Abbreviation: PAH, PAHA
, a derivative of aminobenzoic acid. The salt, para-aminohippurate, is used to test the excretory capacity of the renal tubules.
para-aminosalicylic acid Abbreviation: PAS, PASA
, a white or nearly white, practically odorless powder that darkens when exposed to air or light. It is a second-line drug used to treat multidrug-resistant tuberculosis. Synonym: aminosalicylic acid
C17H24O16, an acid derived from pectin by hydrolyzing its methyl ester group.
pentanoic acidValeric acid.
peptide nucleic acid Abbreviation: PNA
A synthetic nucleic acid analog in which natural nucleotide bases are linked to a peptide-like backbone instead of the sugar-phosphate backbone found in DNA and RNA. PNA has numerous uses in gene regulation, splicing, and therapy; in hybridization; and as a molecular diagnostic assay.
, a colorless unstable liquid compound. It is the highest oxygen-containing acid of chlorine, strong and dangerously corrosive.
phenylglycolic acidMandelic acid.
An acid formed by oxidation of phosphorus. The phosphoric acids are orthophosphoric acid, H3
; pyrophosphoric acid, H4
; metaphosphoric acid, HPO3
; and hypophosphoric acid, H4
. The salts of these acids are phosphates. Orthophosphoric acid, a tribasic acid, is used as a 30% to 50% solution to etch enamel of teeth in preparation for bonding of resin dental restorations.
, a crystalline acid formed when phosphorus is oxidized in moist air.
, a pale, water-soluble acid that is found in cereal grains and, if ingested, may interfere with the absorption of calcium and magnesium.
OH, a yellow crystalline powder that precipitates proteins and explodes when heated or charged. It is used as a dye and a reagent. Its salts are used in the Jaffé reaction (used to measure serum creatinine). Synonym: trinitrophenol
poly DL lactic acidPolyglactin.
, a polymer of glycolic acid anhydride units. It is used to manufacture surgical sutures, clips, and mesh.
, a carboxylic acid present in sweat.
C8H9NO4, a crystalline acid that is the principal end product of pyridoxine metabolism, excreted in human urine.
, an organic acid that plays an important role in the Krebs cycle. It is an intermediate product in the metabolism of carbohydrates, fats, and amino acids. Its quantity in the blood and tissues increases in thiamine deficiency because thiamine is essential for its oxidation.
, a crystalline acid present in some plants, including cinchona bark, and berries.
, a metabolite of vitamin A used in the treatment of cystic acne.
ribonucleic acid See: RNA
, an unsaturated hydroxy acid making up about 80% of fatty acids in the glycerides of castor oil. It has a strong laxative action.
, a white crystalline acid derived from phenol used to make aspirin, as a preservative and flavoring agent, and in the topical treatment of some skin conditions such as warts and wrinkles. See: chemical peeling
C9H9NO4, an acid found in the urine after ingestion of salicylic acid or its derivatives.
saturated fatty acid
A fatty acid in which the carbon atoms are linked to other carbon atoms by single bonds. See: fatty acid; unsaturated fatty acid
Any of a family of acids containing silica, such as H2
(metasilicic acid), H2
(orthosilicic acid), or H2
(pyrosilicic acid). When silicic acid is precipitated, silica gel is obtained.
, a monobasic fatty acid occurring naturally in plants and animals. It is used in the manufacture of soap and pharmaceutical products such as glycerin suppositories. Synonym: octadecanoic acid
, an intermediate in carbohydrate metabolism.
Any of the organic compounds having the general formula SO2OH, derived from sulfuric acid by replacement of a hydrogen atom.
, a crystalline acid soluble in water or alcohol. It is used as a reagent for precipitating proteins, as in testing for albumin in urine.
, a colorless, corrosive, oily, viscous acid prepared from sulfur dioxide and used in many industrial processes and in clinical laboratories. Industrial accidents involving sulfuric acid through contact with skin or inhalation of aerosols are common.
, an inorganic acid and a powerful chemical reducing agent used commercially, esp. for as a bleach.
, a mixture of digallic acid esters of d
(+) glucose prepared from oak galls and sumac. It yields gallic acid and glucose on hydrolysis.
, an acid obtained from by-products of wine fermentation. It is widely used in industry in the manufacture of carbonated drinks, flavored gelatins, dyes, and metals. It is also used as a reagent. It is thought to be an allergen.
S, a bile acid that hydrolyzes to cholic acid and taurine.
Any of the polymers found in the cell walls of some gram-positive bacteria, such as the staphylococci.
, an antifibrinolytic drug that has approx. 10 times the potency of and more sustained activity than aminocaproic acid. It is used to decrease bleeding time during surgical procedures. Loss of blood is decreased when this drug is used.
The solid fat produced by heating liquid vegetable oils in the presence of hydrogen and certain metal catalysts. Partial hydrogenation changes some of the unsaturated bonds to saturated ones. The more trans-fatty acids in the diet, the higher the serum cholesterol and low-density lipoprotein cholesterol.
trichloroacetic acid Abbreviation: TCA
A drug used as a caustic to destroy certain types of warts, condylomata, keratoses, and hyperplastic tissue.
unsaturated fatty acid
An organic acid in which some of the carbon atoms are linked to other carbon atoms by double bonds, thus containing less than the maximum possible number of hydrogen atoms, e.g., unsaturated oleic and linoleic acids as compared with the saturated stearic acid. Polyunsaturated fatty acids include linoleic acid and alpha-linoleic acid. See: fatty acid; saturated fatty acid
CLUMP OF URIC ACID CRYSTALS (×400)
, a crystalline acid occurring as an end product of purine metabolism. It is formed from purine bases derived from nucleic acids (DNA and RNA). It is a common constituent of urinary stones and gouty tophi. See: illustration
Uric acid must be excreted because it cannot be metabolized. Uric acid output should be between 0.8 and 1g/day if the patient is on an ordinary diet.
Increased elimination is observed after ingestion of proteins and nitrogenous foods, after exercise, after administration of cytotoxic agents, and in gout and leukemia. Decreased elimination is observed in kidney failure, lead poisoning, and in those who eat a protein-free diet.
, an oily fatty acid having a distinctly disagreeable odor, existing in four isomeric. Synonym: pentanoic acid
valproic acid Abbreviation: VPA
, an acid used to treat seizure disorders.
vanillylmandelic acid, vanilmandelic acid Abbreviation: VMA
, a principal metabolic product of catecholamines. VMA makes up approx. 90% of the metabolites of the catecholamines epinephrine and norepinephrine and is secreted in the urine. People with pheochromocytoma produce excess amounts of catecholamines; therefore there are increased amounts of VMA in their urine.
An acid produced from carbon dioxide (CO2). It can be excreted by the body by ventilation (colloquially, “blowing off CO2”).
, an acid excreted in the urine of pyridoxine-deficient animals after they have been fed tryptophan. Synonym: 4,8-dihydroxyquinaldic acid
gamma-aminobutyric acid Abbreviation: GABA
, the principal inhibitory neurotransmitter of the brain.