NAD+ Oxidized form of nicotinamide adenine dinu-
cleotide. Note that despite the plus sign in the symbol,
the COENZYMEis anionic under normal physiological
conditions. NAD+is a coenzyme derived from the B vita-
min niacin. It is transformed into NADH when it accepts
a pair of high-energy electrons for transport in cells and
is associated with catabolic and energy-yielding reactions.
NADH Reduced form of nicotinamide adenine dinu-
cleotide (NAD). Called coenzyme I, it is an electron
donor essential for a variety of oxidation-reduction
reactions.
NADP+ Oxidized formof nicotinamide adenine di-
nucleotide phosphate. Note that despite the plus sign in
the symbol, the COENZYME is anionic under normal
physiological conditions. An enzyme commonly associ-
ated with biosynthetic reactions. NADP is a hydrogen
carrier in a wide range of redox reactions.
NADPH Reduced form of nicotinamide adenine di-
nucleotide phosphate. An energy-rich compound pro-
duced by the light-reaction of photosynthesis. It is used
to synthesize carbohydrates in the dark-reaction.
naiad(larva, nymph) Used to describe the interme-
diate stage, between egg and adult, of a dragonfly’s
development.
natural killer cell(NK cell; NK lymphocyte) A
large granular lymphocyte killer cell that attacks and
kills tumorous, cancerous, and virus-infected cells
through phagocytosis and by using chemicals that
destroy the abnormal cells. They are not specifically
targeted to any antigen the way other lymphocytes are,
and therefore they do not need additional stimulation
to attack and kill. Part of the innate immune response.
See alsoLYMPHOCYTE.natural selection Survival and reproduction of a
genotype that leaves more progenies who possess
attributes of fitness, survivability, and adaptation to
their environment over that of another genotype. Those
better adapted to their environment are likely to
increase in frequency over a number of generations
over those that are less adapted, and this difference is
not due to chance. It is the environment’s strong influ-
ence on the reproductive success of individuals in a
population; one of the major forces of evolution. First
promoted by Charles Darwin in the 19th century.
See alsoEVOLUTION.NCE SeeNEW CHEMICAL ENTITY.NDA(new drug application) The process of submit-
ting a new drug for approval. After a new drug appli-
cation (NDA) is received by the federal agency in241