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Active transport is the carriage of a solute across a biological membrane from low to high concentration that requires the expenditure of (metabolic) energy.

Adsorption is the biological process through which drugs reach the blood stream, when administered other than intravenously

ADME Abbreviation for Absorption, Distribution, Metabolism, Excretion. (See also Pharmacokinetics; Drug disposition).

Affinity is the tendency of a molecule to associate with another. The affinity of a drug is its ability to bind to its biological target (receptor, enzyme, transport system, etc.). For pharmacological receptors it can be thought of as the frequency with which the drug, when brought into the proximity of a receptor by diffusion, will reside at a position of minimum free energy within the force field of that receptor. For an agonist (or for an antagonist) the numerical representation of affinity is the reciprocal of the equilibrium dissociation constant of the ligand-receptor complex denoted K _A, calculated as the rate constant for offset (k _-1) divided by the rate constant for onset (k ₁).

Agonist is an endogenous substance or a drug that can interact with a receptor and initiate a physiological or a pharmacological response characteristic of that receptor (contraction, relaxation, secretion, enzyme activation, etc.).

Alkaloids are a varied family of alkaline, nitrogen-containing substances, usually plant-derived, reacting with acids to form salts. Normally intensely bitter, alkaloids form a body of substances widely used in drug and herbal therapy. They are usually biologically active and have a toxic potential. The term is more pharmaceutical and medical than chemical since alkaloids come from a variety of otherwise unrelated organic compounds. (Examples a variety of otherwise unrelated organic compounds. (caffeine, morphine, berberine).

Allosteric binding sites are contained in many enzymes and receptors. As a consequence of the binding to Allosteric binding sites, the interaction with the normal ligand may be either enhanced or reduced.

Allosteric enzyme is an enzyme that contains a region to which small, regulatory molecules (.effectors.) may bind in addition to and separate from the substrate binding site and thereby affect the catalytic activity. On binding the effector, the catalytic activity of the enzyme towards the substrate may be enhanced, in which case the effector is an activator, or reduced, in which case it is a de-activator or inhibitor.

Allosteric regulation is the regulation of the activity of allosteric enzymes. (See also Allosteric binding sites; Allosteric enzymes).

Ames test is based on the assumption that any substance that is mutagenic (for the bacteria used in his test) may also turn out to be a carcinogen. Although, in fact, some substances that cause cancer in laboratory animals (dioxin, for example) do not give a positive Ames test (and vice-versa), the ease and low cost of the test make it invaluable for screening substances in our environment for possible carcinogenicity. The bacterium used in the test is a strain of Salmonella typhimurium that caries a defective (mutant) gene making it unable to synthesize the amino acid histidine from the ingredients in its culture medium. However, some types of mutations (including this one) can be reversed, a back mutation, with the gene regaining its function. These revertants are able to grow on a medium lacking histidine. The mutagenic effect of the chemical causes many bacteria to regain the ability to grow without histidine, forming the colonies seen around the disk. Ames test includes a mixture of liver enzymes to ensure that the metabolic component is included.

Amino Acid refers to compounds containing carboxylic and free amine functions. In biomedical literature, it refers toany of twenty nitrogen-containing acids that are the building blocks for proteins and required for human growth.

Anabolic Steroids are synthetic steroids used to increase muscle mass and weight. Anabolic steroids are versions of the natural hormone testosterone but have fewer masculinizing effects. Anabolic steroids have been used to reverse AIDS-related wasting syndrome on an individual basis, but trial data are lacking.

Analgesics are compounds that reduce pain

Analog is a drug whose structure is related to that of another drug but whose chemical and biological properties may be quite different. (See also Congener).

Anemia is the incapacity of blood to carry enough oxygen to the body.s tissues. Anemia may be caused by an abnormally low number of red blood cells or low levels of hemoglobin, the protein that carries oxygen in red blood cells. It is a condition that is often caused by AZT as well as by other drugs and illnesses.

Anorexia refers to the lack or loss of appetite that leads to significant decline in weight.

Angiogenesis is the process of new blood vessel growth. Tumors and Kaposi's sarcoma lesions stimulate angiogenesis to supply themselves with blood.

Antagonist is a drug or a compound that opposes the physiological effects of another. At the receptor level, it is a chemical entity that opposes the receptor-associated responses normally induced by another bioactive agent.

Antibiotics are substances that kill (cytotoxic) or inhibit (cytostatic) the growth of microorganisms. An antibiotic is used to combat disease and infection.

Antibody: a protein in the blood created by the immune system, also known as immunoglobulin. Antibodies coat, mark for immune destruction or render harmless foreign particles like bacteria, viruses or harmful toxins. Antibodies also tag infected cells, making them vulnerable to attack by the immune system. Each antibody attaches itself to a single specific chemical sequence on an antigen.

Antigen is a foreign substance, usually a protein, that stimulates an immune response. An antigen contains several subunits called epitopes (see) that are targets of specific antibodies and cytotoxic T-lymphocytes (see).

Antigen Presenting Cell (APC) is a cell, such as a macrophage or dendritic cell that digests foreign bodies and exhibits the resulting pieces of the protein (antigen) on its surface in an effort to find and activate the CD4 T-helper cells responsive to that antigen.

Antimetaboliteis a structural analog of an intermediate (substrate or coenzyme) in a physiologically occurring metabolic pathway that acts by replacing the natural substrate thus blocking or diverting the biosynthesis of physiologically important substances.

Antiretrovirals are substances that stops or suppresses the activity of a retrovirus such as HIV. AZT, ddC, ddI and d4T are examples of antiretroviral drugs.

Antisense molecule is an oligonucleotide or analog thereof that is complementary to a segment of RNA (ribonucleic acid) or DNA (deoxyribonucleic acid) and that binds to it and inhibits its normal function.

Aphasia: loss of ability to speak or understand speech.

Apoptosis or programmed cell death is a set of ordered events that enables the selective removal of cells from tissue.

Aspergillus is a fungus that infects the lungs, causing a disease known as aspergillosis. The infection can spread through the blood to other organs and cause lesions of the skin, ear, nasal sinuses or lungs, as well as occasionally the bones, meninges, heart, kidneys or spleen.

Ataxia refers to lack of muscular coordination

Autacoid is a biological substance secreted by various cells whose physiological activity is restricted to the vicinity of its release; it is often referred to as local hormone.

Autocrine factors are those produced by the same cells they bind to act upon.

Autoradiography is a sensitive and simple method of recording spatial distribution of radioisotope-labeled substances within a specimen material. Radioisotopic emissions release energy to the sensitive silver halide grains in the emulsion layer of a photographic film, forming a latent image. When treated with a developing agent, the autoradiograms yield visible density images, which can be observed by the eye or measured by a densitometer for precise analysis

Autoreceptor, present at a nerve ending, is a receptor that regulates, via positive or negative feedback processes, the synthesis and/or release of its own physiological ligand. (See also Heteroreceptor ).