lecithin include eggs, red meats, spinach,

and nuts. See also LIPOPROTEIN, LIPIDS, CONJU-

GATED PROTEIN, HIGH-DENSITY LIPOPROTEINS

(HDLP), LOW-DENSITY LIPOPROTEINS (LDLP),

SOYBEAN PLANT, SOYBEAN OIL, CHOLINE, SIGNAL

TRANSDUCTION, LINOLEIC ACID, ACETYLCHOLINE,

LECITHIN (crude, mixture), FATS, MICELLE, DIGESTION (WITHIN ORGANISMS).

Lectins A class of proteins that have the capability to rapidly (and reversibly) combine with specific sugar molecules (e.g., those sugar molecules or glycoproteins on the surface of adjacent cells, within an organism). Lectins are a common component of the surface (membranes) of plant and animal cells, and are so specific (regarding sugar molecules that they will or won’t combine with/attach to) that they discriminate between different monosaccharides and different oligosaccharides (i.e., on the surfaces of adjacent cells within an organism). This capability to reversibly combine with sugar (i.e., carbohydrate) molecules (on the surface of adjacent cells) is utilized by:

•Bacteria and other microorganisms, to adhere to (sugar molecules on surface of ) host cells, as the first step in the process of infecting those host cells

•White blood cells (e.g., lymphocytes), to adhere to the walls of blood vessels (endothelium), as the first step to leaving the bloodstream to go fight infection (pathogens, trauma) in tissue adjacent to that blood vessel. The lectin (glycoprotein) that adheres to the (endothelial sugar molecule on) blood vessel wall is called L-selectin, or the homing receptor. The two sugar molecules (glycoproteins) on the blood vessel wall (endothelium) are called P-selectin and E-selectin (also known as ELAM-1)

•Cancerous tumor cells, to adhere to the walls of blood vessels (endothelium) as part of the tumor-proliferation process known as metastasis (i.e., new tumors are “seeded” throughout the body via this process).

Separate and apart from the above impacts, some plant lectins (e.g., in the seeds of certain

plants) are toxic to some of the animals that consume those seeds. See also PROTEIN, SUGAR

MOLECULES, GLYCOPROTEIN, LEUKOCYTES,

SELECTINS, LYMPHOCYTES, MONOCYTES, NEUTRO-

PHILS, ENDOTHELIAL CELLS, ENDOTHELIUM, CAN-

CER, METASTASIS, SIGNAL TRANSDUCTION.

Leptin A protein hormone that is produced by fat cells (adipose tissue) in the body. When leptin is produced and travels to cells whose surface bears leptin receptors (e.g., in the brain), those (brain) cells receive signal (transduction) indicating fullness/satiety. Leptin has been found to be present in the bloodstream of obese humans at a concentration of approximately four times the concentration found in bloodstreams of lean humans. High levels of leptin present in the bloodstream disrupt some of the activities of insulin (hormone which regulates blood sugar levels), and may possibly lead to dia-

betes. See also HORMONE, PROTEIN, BIOLOGICAL ACTIVITY, INSULIN, ADIPOSE.

Leptin Receptors Cellular receptors which L are specific to leptin. In 1996, H. Ralph Snodgrass discovered that leptin receptors

are involved in the “sorting” of immature blood cells (from bone marrow) to create subpopulations. See also LEPTIN, RECEPTORS.

Lethal Mutation Mutation of a gene to yield no, or a totally defective, gene product (protein), thereby making it unable to function, and hence unable to sustain the life of the organism.

Leucine (leu) A monocarboxylic essential amino acid. See also AMINO ACID, ESSENTIAL

AMINO ACIDS, ALS GENE.

Leukocytes (white blood cells) A diverse family of nucleated cells that has many immunological functions. See also

EOSINOPHILS, BASOPHILS, LYMPHOCYTE, B LYM-

PHOCYTES, MONOCYTES, GRANULOCYTES.

Leukotrienes Lipid mediator molecules (synthesized from arachidonic acid) released by certain cells (T cells), which “signal” leukocytes (white blood cells) during the initial stages of an infection or an allergic reaction. When thus activated, the leukocytes migrate to the site of infection to combat the pathogens (or allergens), and mediate the inflam-

mation. See also LIPIDS, LEUKOCYTES, MAST CELLS, SIGNALING, SIGNAL TRANSDUCTION,

T CELLS , PATHOGEN, ARACHIDONIC ACID, ALLER-

GIES, SIGNALING MOLECULE.

Levorotary (L) Isomer An isomer of an optically active compound; rotates (when illuminated) the plane of plane-polarized light to the left. See also STEREOISOMERS, DEX-

TROROTARY (D) ISOMER.

LH See LUTEINIZING HORMONE.

Library A set of cloned DNA fragments together representing the entire genome. See

also DEOXYRIBONUCLEIC ACID (DNA), GENOME.

Ligand (in biochemistry) In general, a molecule or ion that can bind to (interact with) a protein molecule. For example, a pharmaceutical that binds to a receptor protein molecule on the surface of a cell may be called

a ligand. See also PROTEIN, RECEPTORS, T CELL RECEPTORS, ENDOCYTOSIS, CD4 PROTEIN, INVA-

SIN, LIGAND (IN CHROMATOGRAPHY), CHELATION.

Ligand (in chromatography) A term used to describe a substance (the ligand) that has the capacity for specific and noncovalent

L(reversible) binding to some protein. A ligand may be a coenzyme for a specific enzyme. The ligand can be covalently attached (immobilized) by means of the appropriate chemical reaction to the surface of certain porous column material. When a mixture of proteins containing the enzyme to be isolated is passed through the column, the enzyme, which is capable of tightly binding to the ligand, does so, and is in this manner held to the column. The other proteins present, which have no specific affinity for the ligand, pass on through the column. The protein/ligand complex is then dissociated and the enzyme eluted from the column, which may be accomplished by passing more free (unbound) coenzymes through the column. The ligand may be hormones (i.e., used to isolate receptor molecules) or any other type of molecule that is capable of binding specifically and reversibly to the desired protein or protein complex. See also

AFFINITY CHROMATOGRAPHY, SUBSTRATE (IN

CHROMATOGRAPHY), CHROMATOGRAPHY, PROTEIN,

PEPTIDE, ANTIBODY, MONOCLONAL ANTIBODIES

(MAb).

Ligase An enzyme used to catalyze the joining of single-stranded DNA segments. See also

DEOXYRIBONUCLEIC ACID (DNA).

© 2002 by CRC Press LLC

Ligation The formation of a phosphodiester bond to link two adjacent bases separated by a nick in one strand of a double helix of DNA (deoxyribonucleic acid). The term can also be applied to blunt-end ligation and to the joining of RNA (ribonucleic acid) strands. See

also DEOXYRIBONUCLEIC ACID (DNA), LIGASE.

Light-Chain Variable (VL) Domains The regions (domains) of the antibody (molecule’s) “light chain” that vary in their amino acid sequence. The “chains” (of atoms) comprising the antibody (immunoglobulin) molecule consist of a region of variable (V) amino acid sequence and a region in which the amino acid sequence remains constant

(C). An antibody molecule possesses two antigen binding sites, and it is the variable domains of the light (VL) and heavy (VH) chains which contribute to this (antigen binding ability). See also ANTIBODY, IMMUNO-

GLOBULIN, PROTEIN, SEQUENCE (OF A PROTEIN MOLECULE), ANTIGEN, AMINO ACID, COMBINING SITE, DOMAIN (OF A PROTEIN), HEAVY-CHAIN

VARIABLE (VH) DOMAINS.

Lignans A category of phytochemicals that play defensive roles (e.g., against infections by bacteria, fungi, etc.) within land plants (e.g., those grown by man for crops). Lignans are also sometimes referred to by some people as “phytoestrogens,” and are typically beneficial to the health of humans that consume them. Lignans are found in virtually all fruits, vegetables, and cereals (grains); generally within the seed coats, stems, leaves, or flowers. One of the beneficial lignans commonly consumed by humans is sesamin, found in seeds of the sesame plant (Sesamum indicum); which acts as an antioxidant. See also PHYTOCHEM-

ICALS, PHYTOESTROGENS, ISOFLAVONES, ANTI-

OXIDANTS, OXIDATIVE STRESS.

Lignins A category of phenolic (“ring-shaped” molecules) polymeric (i.e., composed of more than one molecular unit) compounds produced by land plants within the cell walls (i.e., exterior of cell’s plasma membrane) of those plants, to reinforce/strengthen those cell walls. See also CELL, POLYMER, PLASMA

MEMBRANE.

Lignocellulose A complex biopolymer comprising the bulk of woody plants. It consists

of polysaccharides and polymer phenols.

See also POLYSACCHARIDES, LIGNINS. Limonene See PHYTOCHEMICALS.

Linkage A phenomenon discovered by Thomas Hunt Morgan in the early 1900s via his experiments with fruit flies. This term describes the tendency of genes to be inherited together as a result of their locations being physically close to each other on the same chromosome; measured by percent recombination between loci. Because the locus (location of gene on the chromosome) determines the likelihood that two genes will go together into offspring, “marker genes” that are linked to a gene (e.g., for a given trait or disease) of interest can be utilized to predict the presence of that (trait or diseasecausing) gene. See also GENE, LOCUS, CHRO-

MOSOMES, LINKAGE GROUP, MARKER (GENETIC

MARKER), MAP DISTANCE, LINKAGE MAP.

Linkage Group Includes all loci (in DNA molecule) that can be connected (directly or indirectly) by linkage relationships; equivalent to a chromosome. See also LOCUS, CHRO-

MOSOMES, LINKAGE, CHROMATIDS, CHROMATIN,

LINKAGE MAP, DEOXYRIBONUCLEIC ACID (DNA).

Linkage Map A depiction of gene loci (on chromosomes) based on the frequency of recombination (of linked genes) in the offspring’s genome. See also LINKAGE, LINKAGE

G R O U P , G E N E , L O C U S , M A R K E R (G E N E T I C

MARKER).

Linker A short synthetic duplex oligonucleotide containing the target site for some restriction enzyme. It may be added to the ends of a DNA (deoxyribonucleic acid) fragment prepared by cleavage with some other enzyme reconstructions of recombinant DNA.

Linking The process of “attaching” a drug or a toxin to a monoclonal antibody, or another homing molecule of the immune system. Because this attachment must be reversible, so that the homing molecule can release the drug or toxin after delivering that drug or toxin to the desired site in the body (e.g., delivery of a toxin to a tumor, to kill the tumor), linking is a difficult process to reliably achieve. See also IMMUNOTOXIN, CONJU-

GATE, MONOCLONAL ANTIBODIES (MAb), TOXIN.

© 2002 by CRC Press LLC

Linoleic Acid One of the so-called “omega-6” (n-6) polyunsaturated fatty acids (PUFA), it has historically comprised approximately 53% of the total fatty acid content of soybean oil. It is an essential fatty acid for humans. When consumed by humans, linoleic acid causes LDLP cholesterol levels in the blood to decrease, which reduces risk of coronary heart disease (CHD). The human body converts linoleic acid to the n-6 highly unsaturated fatty acid (HUFA) arachidonic acid. See

lipid molecules which possess both a hydrophilic (“water loving”) and a hydrophobic (“water hating”) moiety. These (membrane) lipids thus spontaneously form closed bimolecular sheets in aqueous (water-containing) media, in which the hydrophobic ends of each lipid molecule are in the center of the bimolecular membrane and the hydrophilic ends of the lipid molecules are on the outside (i.e., touching the water molecules). See also

LIPIDS, PLASMA MEMBRANE, MOIETY.

Lipid Vesicles See LIPOSOMES.

Lipids From the Greek word lipos, fat, lipids are water-insoluble (fat) biomolecules that are highly soluble in organic solvents such as chloroform. Lipids serve as fuel molecules, highly concentrated energy stores, “signaling” molecules, and components of cell membranes. Membrane lipids are relatively small molecules that have both a hydrophilic (“water loving”) and a hydrophobic (“water hating”) moiety. These (membrane) lipids

Lspontaneously form closed bimolecular sheets in aqueous media (water) which are barriers to the free movement (flow) of polar

molecules. See also FATS, MOIETY, LIPOPROTEIN,

CHOLESTEROL, SIGNALING, SIGNALING MOLECULE,

SIGNAL TRANSDUCTION, PLASMA MEMBRANE,

ANTIOXIDANTS, OXIDATIVE STRESS, LIPID BILAYER,

LEUKOTRIENES, OLEOSOMES.

Lipolytic Enzymes See LIPASE.

Lipophilic A “fat loving” molecule, or portion of a molecule. Relating to, or having strong affinity for, fats or other lipids. See also LIPIDS,

FATS.

Lipopolysaccharide (LPS) See ENDOTOXIN.

Lipoprotein A conjugated protein containing a lipid or a group of lipids. For example, low-density lipoproteins (also known as “bad” cholesterol) are a “package” of cholesterol (lipid) surrounded by a hydrophilic protein. Low-density lipoproteins (LDLPs) and very low-density lipoproteins (VLDLs) are the specific lipoproteins that are most likely to deposit cholesterol (plaque) on artery walls, which increases risk of coronary heart disease (CHD). See also PROTEIN,

LOW-DENSITY LIPOPROTEINS (LDLP), VERY LOW-

DENSITY LIPOPROTEINS (VLDL), CONJUGATED

PROTEIN, HYDROPHILIC, LIPIDS, CHOLESTEROL,

APOLIPOPROTEINS.

© 2002 by CRC Press LLC

Lipoprotein-Associated Coagulation (Clot) Inhibitor (LACI) A protein that prevents formation of blood clots. This occurs because LACI inhibits the controlled series of zymogen activations (enzymatic cascade) which causes the formation of fibrinogen (precursor to fibrin), leading subsequently to clot formation. See also FIBRIN, FIBRONECTIN,

ZYMOGENS.

Liposomes Also called lipid vesicles or vesicle. Aqueous (watery) compartments enclosed by a lipid bilayer. They can be formed by suspending a suitable lipid, such as phosphatidyl choline, in an aqueous medium. This mixture is then sonicated (i.e., agitated by highfrequency sound waves) to give a dispersion of closed vesicles (i.e., compartments) that are quite uniform in size. Alternatively, liposomes can be prepared by rapidly mixing a solution of lipid in ethanol with water, which yields vesicles that are nearly spherical in shape and have a diameter of 500 Å (Angstroms). Larger vesicles (10,000 Å or 1 mm, or 0.00003937 inch in diameter) can be prepared by slowly evaporating the organic solvent from a suspension of phospholipid in a mixed solvent system.

Liposomes can be made to contain certain drugs for protective, controlled release delivery to targeted tissues. For example, pharmaceuticals which tend to be rapidly degraded in the bloodstream could be enclosed within liposomes so that more of the nondegraded pharmaceutical would remain by the time it reached the targeted tissue. The controlled release property enables larger doses (of drugs possessing toxic side effects) to be prescribed, knowing that the drug will be released in the body over an extended period of time. See also

LIPIDS, MICRON, ANGSTROM (Å).

Lipoxidase See LIPOXYGENASE (LOX).

Lipoxygenase (LOX) A “family” of enzymes that is naturally produced within its seeds (soybeans) by the soybean plant (Glycine max (L.) Merrill). In the presence of moisture and certain other conditions, lipoxygenase enzymes catalyze a chemical reaction in which objectionable “beany” flavor can be produced from certain components of the soybean. That “beany” flavor decreases the