Regulation of Metabolism 66919.2 REGULATION OF ENERGY
METABOLISM
The plasma contains circulating glucose, fatty acids, amino
acids, and other molecules that can be used by the body tis-
sues. The synthesis of energy reserves of glycogen and fat
following a meal and the utilization of these reserves between
meals are regulated by the action of hormones.and degenerative diseases associated with aging, promotes the
malignant growth of cancers, and contributes to all inflammatory
diseases (such as glomerulonephritis, rheumatoid arthritis, and
lupus erythematosus). It has been implicated in ischemic heart
disease, stroke, hypertension, and a variety of neurological dis-
eases, including multiple sclerosis, Alzheimer’s disease, Parkin-
son’s disease, and others. The wide range of diseases associated
with oxidative stress stems from the widespread production of
superoxide radicals in the mitochondria of all cells that undergo
aerobic respiration.
The body protects itself against oxidative stress through vari-
ous means, both enzymatic and nonenzymatic. The enzymes that
help to prevent an excessive buildup of oxidants include superox-
ide dismutase (SOD), catalase, and glutathione peroxidase. The
SOD enzyme catalyzes a reaction where two superoxide radicals
form hydrogen peroxide ( H 2 O 2 ) and O 2. Hydrogen peroxide,
though not a free radical, is considered a reactive oxygen spe-
cies because it can generate the highly reactive hydroxyl radi-
cal. Elimination of hydrogen peroxide is accomplished by the
enzyme catalase. In this reaction, two hydrogen peroxide mol-
ecules (H 2 O 2 ) react to form H 2 O and O 2. Also, the enzyme glu-
tathione peroxidase catalyzes a reaction where H 2 O 2 reacts with
NADPH 1 H^1 to form NADP and H 2 O.
The body also protects itself from oxidative stress through
nonenzymatic means ( fig. 19.1 ). One of the most impor-
tant protective mechanisms is the action of a tripeptide called
glutathione. When it is in its reduced state, glutathione can react
with certain free radicals and render them harmless. Thus, glu-
tathione is said to be the major cellular antioxidant. Ascorbic
acid (vitamin C) in the aqueous phase of cells, and a - tocopherol
(the major form of vitamin E) in the lipid phase, help in this
antioxidant function by picking up unpaired electrons from free
radicals. This is said to “quench” the free radicals, although in
the reaction vitamins C and E themselves gain an unpaired elec-
tron and thus become free radicals. Because of their chemical
structures, however, they are weaker free radicals than those
they quench. Many other molecules present in foods (primarily
fruits and vegetables) have been shown to have antioxidant prop-
erties, and research on the actions and potential health benefits
of antioxidants is ongoing.
| CHECKPOINT
- Explain how the metabolic rate is influenced by
exercise, ambient temperature, and the assimilation
of food.
2a. Distinguish between the caloric and anabolic
requirements of the diet.
2b. List the water-soluble and fat-soluble vitamins and
describe some of their functions. - Identify the free radicals and reactive oxygen
species, and describe their significance.
LEARNING OUTCOMESAfter studying this section, you should be able to:- Identify the energy reserves and circulating energy
substrates of the body. - Describe the functions of adipose tissue and the
dangers of obesity. - Explain how different neurotransmitters and
hormones regulate hunger. - Describe the different types of caloric expenditures
and identify the hormones that regulate metabolic
balance.
The molecules that can be oxidized for energy by cell respiration
may be derived from the energy reserves of glycogen, fat, or pro-
tein. Glycogen and fat function primarily as energy reserves; for
proteins, by contrast, this represents a secondary, emergency func-
tion. Although body protein can provide amino acids for energy, it
can do so only through the breakdown of proteins needed for mus-
cle contraction, structural strength, enzymatic activity, and other
functions. Alternatively, the molecules used for cell respiration
can be derived from the products of digestion that are absorbed
through the small intestine. Since these molecules—glucose, fatty
acids, amino acids, and others—are carried by the blood to the
cells for use in cell respiration, they can be called circulating
energy substrates ( fig. 19.2 ).
Because of differences in cellular enzyme content, differ-
ent organs have different preferred energy sources. This concept
was introduced in chapter 5, section 5.3. The brain has an almost
absolute requirement for blood glucose as its energy source, for
example. A fall in the plasma glucose concentration to below
about 50 mg per 100 ml can thus “starve” the brain and have
disastrous consequences. Resting skeletal muscles, by contrast,
use fatty acids as their preferred energy source. Similarly, ketone
bodies (derived from fatty acids), lactic acid, and amino acids
can be used to different degrees as energy sources by various
organs. The plasma normally contains adequate concentrations
of all of these circulating energy substrates to meet the energy
needs of the body.
Terms relating to the formation of energy reserves of glyco-
gen and fat ( glycogenesis and lipogenesis ), and the production
of different circulating energy substrates ( glycogenolysis, lipoly-
sis, gluconeogenesis, and ketogenesis ), are useful when describ-
ing metabolism. These terms and processes were introduced in