CLINICAL CASES
67
3.
Be familiar with how immunoglobulins of Graves disease cause
hyperthyroidism.
4.
Be aware of the mechanism of action of PTU and methimazole.
5.
Understand the regulation of hormone levels.
D efinitions
Effector: The protein in a signal transduction pathway (e.g., a hormonal
response) that produces the cellular response.
G protein: A guanosine triphosphate (GTP)-binding protein that serves as
a transducer in a signal transduction pathway. On binding GTP and
releasing guanosine diphosphate (GDP), a G protein is able to activate
the effector enzyme (e.g., adenylate cyclase).
Graves disease: An autoimmune disease in which the B lymphocytes syn-
thesize an immunoglobulin (thyroid-stimulating immunoglobulin, TSIg)
that binds to and activates the TSH receptor in such a way that the thy-
roid hormones do not feedback inhibit the receptor-effector interaction,
leading to a hyperthyroid condition.
Hormone: A chemical signal that is produced in one set of cells and directs
the activity in another set of cells that can be endocrine, paracrine, or
autocrine.
Receptor: A protein that will perceive the signal of a hormone or other
chemical signal (e.g., a neurotransmitter) by binding it and transmitting
that signal further down the signal transduction pathway.
Second messenger: A molecule that is synthesized within a cell in
response to a receptor binding a hormone.
Transducer protein: A protein (such as the a-subunit of a G protein) that
transmits the signal from a hormone-bound receptor to the effector protein.
D ISC U SSIO N
Cellular biochemistry is a complex system of reactions and processes that
must be efficiently regulated and integrated with processes underway in other
cells and tissues. One method by which regulation is achieved is through the
interaction of hormones and their associated receptors located either within
or on the surface of the cell. A hormone is any substance in an organism that
carries a signal to change metabolic processes within a cell.
The hormonal-signaling process is summarized in Figure 7-1. Hormones
are released from secretory tissues in response to metabolic signals as well as
electrical or chemical signals from the nervous system. The released hormone
binds to a receptor, which can either be on the cell surface or, as in the case
of steroid and similar hormones, within the cell. The hormone-receptor
complex starts a series of events in which the signal is converted to other
chemical forms that bring about changes in the biochemical reactions within
the cell.
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