Returning to Sweden in 1935, he worked at the
Karolinska Institute, and in 1936 he was appointed
head of the newly established biochemical department
of the Nobel Medical Institute that was opened in 1937.
He carried out research on various oxidation
enzymes, contributing to the knowledge of cytochrome-
c, peroxidases, catalases, flavoproteins, and pyridin-
proteins, particularly the alcohol dehydrogenases. For
his work on the nature and effects of oxidation
enzymes, he was awarded the 1955 Nobel Prize in
physiology or medicine.
He was a member of numerous scientific organiza-
tions and in 1954 was chief editor of the journal
Nordisk Medicin.He died on August 15, 1982.
therapeutic index For a substance used to alleviate
disease, pain, or injury, the therapeutic index is the
ratio between toxic and therapeutic doses (the higher
the ratio, the greater the safety of the therapeutic dose).
thermolysin Acalcium- and zinc-containing neutral
protease isolated from certain bacteria.
thermolysis An uncatalyzed bond cleavage resulting
from exposure of a compound to a raised temperature.
thermoregulation The process of regulating body
temperature. There are various types of thermoregula-
tion. Ectothermic regulation is temperature based on an
organism’s behavior in response to the temperature of
the environment, e.g., amphibians and reptiles. Het-
erothermic regulation is maintaining a constant body
temperature but having the ability to fluctuate at other
times. The body temperature can rise without hurting
the animal, or it can drop significantly when the animal
enters a state of torpor, with body temperature drop-
ping almost to the level of the surroundings and with a
concurrent reduction in metabolic rate, heart rate, respi-
ration, and other functions. This can be in the formof
hibernation during winter or estivation during summer.
thick filament Amyosin filament and part of the
myofibril in muscles. Contains myosin molecules.
thigmomorphogenesis The process in which a plant
reacts to a mechanical disturbance, such as forming
thicker stems in response to strong winds.thigmotropism The sense of touch response, such as
bending or turning of a plant when coming into con-
tact with a solid surface.thorium-lead dating Amethod to measure the age
of rocks and other materials that contain thorium and
lead through the use of the natural radioactive decay of(^232) Th (half-life about 4.7 billion years) as it decays to
(^208) Pb. Because of the very long half-life, this dating
technique is usually restricted to ages greater than 10
million years.
threatened species The classification provided to an
animal or plant likely to become endangered within the
foreseeable future throughout all or a significant por-
tion of its range.
See alsoENDANGERED SPECIES.
three-dimensional quantitative structure-activity
relationship(3D-QSAR) The analysis of the quanti-
tative relationship between the biological activity of a
set of compounds and their spatial properties using sta-
tistical methods.
threshold potential The potential at which a neuron
will fire and then reset to its reset potential; when a
neuron receives inputs that increase its voltage from its
resting voltage and reaches its threshold voltage, it
sends out an action potential.
thylakoids Enclosed membrane structures inside
CHLOROPLASTs and photosynthetic bacteria.
See alsoPHOTOSYNTHESIS.
thymus A major component of the lymphatic system.
In the thymus, lymphoid cells undergo a process of mat-
uration and education prior to release into circulation.
thymus 325