LIPIDS PART II
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Transcript of LIPIDS PART II
ECDA
August 2009
STEROIDS
AND HORMONES
STEROIDS AND HORMONES Steroids and hormones are alicyclic
compounds, all of which are derivatives of a fused tetracyclic ring system, the cyclopentanoperhydrophenanthrene ring.
These tetracyclic hydrocarbons are much more common in eukaryotes than in prokaryotes.
STEROIDS AND HORMONES The most important and most prominent
compound under this classification is cholesterol.
Cholesterol is an important component of many eukaryotic cell membranes.
Cholesterol is also the precursor of two major classes of steroids: the steroid hormones and the bile acids.
Cholesterol is a 27-carbon sterol
STEROIDS AND HORMONES
STEROIDS AND HORMONES Bile acids are the primary degradation
products of cholesterol. They are made in the liver, stored in the
gallbladder, and secreted into the small intestine
Beginning in the duodenum, they aid in the solubilization of dietary lipids, thereby facilitating their digestion by intestinal lipases.
Dietary Lipid Digestion
STEROIDS AND HORMONES Steroid hormones play a key role in the
regulation of metabolism In general, steroids exert their action and effect
changes in the cell by mediating DNA/RNA synthesis.
○ When the steroid hormone binds at receptor sites in cell membrane, both the hormone and receptor enter the cell.
○ The mobile receptor breaks up and returns to the membrane as soon as the hormone binds to cytoplasmic receptors which allow it to enter the nucleus and affect synthesis of some proteins.
STEROIDS AND HORMONES Examples of steroid hormones derived
from cholesterol are important sex hormones such as testosterone, estrogen, and progesterone
Some steroid hormones serve as regulatory substances of metabolism. These are aldosterone (mineralocorticoid) and cortisol (glucocorticoid).
SEX HORMONES
PROGESTERONE
Precursor of other steroids
Prepares uterus for implantation of an egg
Prevents ovulation during pregnancy
SEX HORMONES
ESTROGEN
Responsible for sexual development in the female
Promotes and maintains female sex characteristics
SEX HORMONES
TESTOSTERONE
Promotes male sexual development
Promotes and maintains male sex qualities
METABOLIC HORMONES
ALDOSTERONE
Synthesized in the adrenal glands
Increases retention of Na ions by the renal tubules
METABOLIC HORMONES
CORTISOL
Synthesized in the adrenal glands
Promotes gluconeogenesis
Suppresses inflammatory reactions
ANABOLIC STEROIDS Also called anabolic-
androgenic steroids (AAS)
Are a class of steroid hormones related to testosterone
They increase protein synthesis within cells which results in the buildup of cellular tissues, especially muscles
ANABOLIC STEROIDS Anabolic-androgenic steroids have two
different but overlapping, types of effects:ANABOLIC
○ Promotes anabolism or cell growthIncreases appetite, stimulation of bone marrow, increase
production of RBCs, increased protein synthesis, etc.
ANDROGENIC○ Affect the development and maintenance of
masculine characteristicsIncreased libido, growth of penis and clitoris, impaired
sperm production, sebaceous gland production, etc.
ANABOLIC STEROIDS AAS have many adverse
effects: Raises blood pressure especially
in those that are pre-hypertensives
Increases risk for cardiovascular disease
Increase in acne occurrence Can cause massive liver damage Can cause testicular atrophy Increase in body hairs Increased frequency and
duration of erections
ANABOLIC STEROIDS
EICOSANOIDS
EICOSANOIDS
Eicosanoids are a diverse group of hormones, most of which are derivatives of arachidonic acid (20:4Δ5,8,11,14)
Most prominent among the group is a series of cyclopentanoic acids known as prostaglandins (PG)
Closely related to PG are thromboxanes (TXA2), also are eicosanoids
EICOSANOIDS Prostaglandins were initially isolated from the
seminal fluid and were therefore thought to be produced by the prostate glands. However, more studies have shown that these substances are present in various tissues in the body
Different prostaglandins exist in the body: PGE2, PGF2, PGG2, PGH2, PGI2, etc.
○ PG stands for “prostaglandin”○ The 3rd letter refers to a particular structure○ The subscript 2 notes the number of double bonds
EICOSANOIDS
Initial steps for the synthesis of PGs and TXA2
are the oxidation and cyclization of arachidonic acid to yield the PGG2 and PGH2
PROSTAGLANDINS
PGE2
In human adipose cells causes 15-fold increase in the concentration of cyclic AMP or cAMP, an energy containing molecule used in membrane transport reactions
It is also observed to cause dilatation of blood vessels
PROSTAGLANDINS
PGF2
PGF2 has a physiological function opposite PGE2, that is it constricts blood vessels
PROSTAGLANDINS
PGH2
The precursor of various prostaglandins and tromboxanes
Formation of this PG from PGG2 is catalyzed by the peroxidase component of the enzyme
PROSTAGLANDINS
PGI2
Derived from PGH2 with enzyme PGI2 synthase
Synthesized in endothelial cells that line blood vessels (arterial walls)
Inhibits platelet aggregation by binding to a receptor on the plasma membrane of platelets
EICOSANOIDS
TXA2
Derived from PGH2 with enzyme TXA2 synthase
Has opposite effects of PGI2: it causes platelet aggregation leading to formation of blood clots (major cause of coronary artery disease)
Aspirin (Aspilet) can inhibit platelet thromboxane formation
Aspirin activates cyclooxygenase by acetylation of a serine at or near the active site of the enzyme.
Possible actions of PGI2 and TXA2 in blood clotting and thrombosis.
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