containing or requiring one or more metal ions. In gen-
eral, an enzyme catalyzes only one reaction type (reac-
tion specificity) and operates on only a narrow range of
SUBSTRATES(substrate specificity). Substrate molecules
are attacked at the same site (regiospecificity), and only
one, or preferentially one of the ENANTIOMERs of chiral
substrate or of RACEMICmixtures, is attacked (enan-
tiospecificity).
See alsoCHIRALITY;COENZYME.
enzyme induction The process whereby an (inducible)
ENZYMEis synthesized in response to a specific inducer
molecule. The inducer molecule (often a substrate that
needs the catalytic activity of the inducible enzyme for
its METABOLISM) combines with a repressor and thereby
prevents the blocking of an operator by the repressor
leading to the translation of the gene for the enzyme. An
inducible enzyme is one whose synthesis does not occur
unless a specific chemical (inducer) is present, which is
often the substrate of that enzyme.
enzyme repression The mode by which the synthe-
sis of an ENZYMEis prevented by repressor molecules.
In many cases, the end product of a synthesis chain
(e.g., an amino acid) acts as a feedback corepressor by
combining with an intracellular aporepressor protein,
so that this complex is able to block the function of an
operator. As aresult, the whole operation is prevented
from being transcribed into mRNA, and the expression
of all enzymes necessary for the synthesis of the end-
product enzyme is abolished.
Eocene Part of the Tertiary period during the Ceno-
zoic era, lasting from about 54.8 to 33.7 million years
ago. Most of the orders of truly warm-blooded mam-
mals were present by the early Eocene.
See alsoGEOLOGICAL TIME.
eosinophil One of the five different types of white
blood cell (WBC) belonging to the subgroup of WBCs
called polymorphonuclear leukocytes. Characterized by
large red(i.e., eosinophilic) cytoplasmic granules.
Eosinophil function is incompletely understood.
They are prominent at sites of allergic reactions and
with parasitic larvae infections (helminths). Eosinophil
secretory products inactivate many of the chemical
mediators of inflammation and destroy cancer cells.
This phenomenon is most obvious with mast-cell-
derived mediators. Mast cells produce a chemotactic
factor for eosinophils.
Eosinophils are produced in the bone marrow, then
migrate to tissues throughout the body. When a foreign
substance enters the body, lymphocytes and neutrophils
release certain substances to attract eosinophils, which
release toxic substances to kill the invader.
See alsoEOSINOPHILIA.eosinophil chemotactic factor of anaphylaxis
(ECF-A) A substrate released from mast cells and
basophils during anaphylaxis, which attracts eosinophils.
A tetrapeptide mediator of immediate hypersensitivity.eosinophilia(blood eosinophilia) An abnormally
high number of eosinophils in the blood. Not a disease
in itself but usually a response to a disease. An elevated
number of eosinophils usually indicates a response to
abnormal cells, parasites, or allergens.
See alsoEOSINOPHIL.ephemeral Transitory, lasting for a brief time.epidermis Both plants and animals have epidermis,
the “skin.” Epidermis serves as a protective layer
against invasion of foreign substances both chemical
and animal (parasites).
In plants, it protects against desiccation; partici-
pates in gas exchange and secretion of metabolic com-
pounds; absorbs water; and is the site of receptors for
light and mechanical stimuli.
In mammals, the epidermis is a superficial layer of
the skin and is subdivided into five layers or strata—
the stratum corneum, the stratum basale, the stratum
spinosum, the stratum granulosum, and the stratum
lucidum—each with their own functions.epigenesis The complete and progressive develop-
ment and differentiation that starts from the beginning116 enzyme induction