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2016-11-08
Steroids: cholesterol,
Bile acids and steroid hormones
Vitamin D
1. Structure and stereochemistry of steroids
2. Cholesterol as a precursor of bile acids, steroid hormones and
vitamin D3
3. Structure of the primary and secondary bile acids, conjugation with
glycine and taurine
4. Steroid hormones - progestins, glucocorticoids, mineralocorticoids,
androgens, estrogens
5. Vitamin D and its active forms
Structure and stereochemistry of
steroids
Steroids
are
derivatives
of
perhydrocyclopentanophenanthrene
system.
the
ring
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STEROID NUMBERING SYSTEM
18
11
19
1
2
A
3
10
17
C 13
D
14
9
B8
16
15
7
5
4
12
6
Steroid ring
• The steroid ring is not
planar, but flattened in a
way that it extends as much
as possible.
• All the rings are in as much
of a chair conformation as
possible.
• There is no rotation possible
around bonds in the rings,
which makes sterols pretty
rigid, except for the chain at
C17.
Conformation
• There are four rings in
a steroid skeleton and
hence there are three
fusion points. A/B, B/C
and C/D rings share two
carbons each (fusion).
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An A-B cis steroid
The methyl groups are attached at points
of ring junction and are called angular
methyl groups.
Other groups on the same side of the
steroid plane as the angular methyl
groups are said to be β substitutents
Groups below the plane of the steroid
ring system are said to be α substituents
The main sterol found in the membranes of eukaryotic cells is cholesterol.
The polar head in this lipids is the hydroxyl in carbon C3.
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Cholesterol
One of the most widely occuring steroids, was first isolated in 1770.
Contains 8 chiral C atoms, this means that 28 or 256 stereoisomers are
possible, but only one of them is cholesterol.
Contains 27 carbons
Cholesterol sources, biosynthesis and
degradation
• Slightly less than half of the cholesterol in the body derives
from biosynthesis de novo.
• Biosynthesis in the liver accounts for approximately 10%, and
in the intestine approximately 15%, of the amount produced
each day.
• Cholesterol synthesis occurs in the cytoplasm and
microsomes from the two-carbon acetate group of
acetyl-CoA.
Biosynthesis of Cholesterol
The process has five major steps:
• 1. Acetyl-CoAs are converted to 3-hydroxy-3-methylglutarylCoA (HMG-CoA)
• 2. HMG-CoA is converted to mevalonate
• 3. Mevalonate is converted to the isoprene based
molecule, isopentenyl pyrophosphate (IPP), with
the concomitant loss of CO2
• 4. IPP is converted to squalene
• 5. Squalene is converted to cholesterol.
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Acetyl CoA is the source of all carbon atoms in cholesterol
Acetyl CoA
CoA
Acetoacetyl CoA
Acetyl CoA
CoA
β -hydroxy- β- methylglutaryl CoA
HMG-CoA
reductase
Mevalonate
Squalene
Farmesyl pyrophosphate
Cyclization
HMG-CoA
Reductase
HMG-CoA reductase
1.
integral membrane protein in the ER
2.
carries out an irreversible reaction
3.
is an important regulatory enzyme in cholesterol synthesis
Fate of Cholesterol
• Incorporated into
hepatocyte membranes
• Exported
– Bile acids
– Cholesterol esters
– Free cholesterol
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Cholesterol Esters
• Acyl-CoA:cholesterol
acyl transferase (ACAT)
is an ER membrane
protein
• ACAT transfers fatty acid
of CoA to C3 hydroxyl
group of cholesterol
• Excess cholesterol is
stored as cholesterol
esters in cytosolic lipid
droplets
Fig. 8
ROLES OF CHOLESTEROL
AND BILE ACIDS (SALTS)
The physiological roles of cholesterol include:
a)
b)
c)
an important lipid component of biological membranes,
precursor of steroid hormones and
source of bile acids.
Bile acids are polar derivatives of cholesterol and aid in:
a)
b)
c)
lipid digestion
lipid absorption
cholesterol excretion
Bile Acids Synthesis and Utilization
• The end products of cholesterol utilization are the bile
acids, synthesized in the liver.
• Synthesis of bile acids is one of the predominant
mechanisms for the excretion of excess cholesterol.
• However, the excretion of cholesterol in the form of
bile acids is insufficient to compensate for an excess
dietary intake of cholesterol.
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Bile
•
•
•
Bile is formed in the liver and
secreted down the bile duct into
the gallbladder, from where it
passes into the intestine
The gallbladder stores bile
produced by the liver between
meals.
During digestion, the gallbladder
contracts and supplies bile
rapidly to the duodenum by way
of the common bile duct
Bile
•
Bile is a complex solution of salts
and protein, containing micelles
composed of cholesterol,
phospholipids and bile salts.
• The composition of these
micelles is critical, as imbalance
may lead to crystalization of
cholesterol in the gallbladder,
leading to the formation of
gallstones.
GALLSTONES
• Cholesterol gallstones can form as a result of excessive
secretion of cholesterol or of insufficient amounts of bile acids
and lecithin relative to cholesterol in bile
• If the amount of cholesterol in bile acids get above 15%, then
gallstones may result.
• If cholesterol gets too high, then they may crystallize out of
solution in the gall bladder, where bile is concentrated tenfold
before secretion.
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Functions of Bile Salts
Important for cholesterol excretion:
1. As metabolic products of cholesterol
2. Solubilizer of cholesterol in bile
Emulsifying factors for dietary lipids,
a prerequisite step for efficient lipid digestion
Cofactor for pancreatic lipase and PLA2
Facilitate intestinal lipid
formation of mixed micelle
absorption
by
Bile salts
•
•
•
detergent character of bile salts is due to the hydrophobic-hydrophilic
nature of the molecules
the presence of hydroxyl (or sulfate) and the terminal carboxyl group on the
tail gives the molecule its hydrophilic face
the steroid ring with its puckered plane provides the hydrophobic face
Emulsification of Dietary Lipids
in Duodenum: Role of Bile Salts
• Emulsification increases the surface area of lipid droplets,
therefore the digestive enzymes can effectively act.
• Mechanisms:
1. Mechanical mixing by peristalsis
2. Detergent effect of bile salts:
Bile salts interact with lipid particles and aqueous
duodenal contents, stabilizing the particles as they
become smaller, and preventing them from coalescing.
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Absorption of Lipids by Intestinal Mucosal
Cells: Role of Bile salts
Mixed micelles:
Disc-shaped clusters of amphipathic lipids.
Arranged with their hydrophobic groups on the inside and
their hydrophilic groups on the outside.
Micelle includes end products of lipid digestion, bile salts
and fat-soluble vitamins
Note: Short- and medium-chain fatty acids do not require
mixed micelle for absorption by intestinal cells
The bile acids are amphipathic, with detergent properties.
They emulsify fat globules into smaller micelles, increasing the surface area
accessible to lipid-hydrolyzing enzymes.
Synthesis of Bile Salts
Fig. 9
•
•
•
Fig. 10
Rate-limiting step performed by the 7α-hydroxylase (CYP7A1) and is
regulated by bile salt concentration
End product: Cholic acid series & Chenocholic acid series
Bile salts can be conjugated & become better detergents
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Synthesis of bile salts
Primary bile acids
Bile Salts
•
•
•
•
Bile acids & salts are effective detergents
Synthesized in the liver
Stored & concentrated in the gallbladder
Discharged into gut and aides in absorption of
intraluminal lipids, cholesterol, & fat soluble vitamines
• Bile acid refers to the protonated form while bile salts
refers to the ionized form
– The pH of the intestine is 7 and the pKa of bile salts is 6,
which means that 50% are protonated
• These terms are sometimes used interchangeably
SYNTHESIS OF BILE SALTS
• Bile salts are just bile acids with the carboxylate
function modified by adding an amine to it via an
amide linkage.
• Bile Salts are even more polar than their
corresponding acids.
• Glycocholate: Cholate with glycine added as an
amide function.
• Taurocholate: Cholate with Taurine added as an
amide function. Taurine is a modified cysteine.
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Primary and secondary bile acids
• Chenodeoxycholic acid (45%) and cholic acid (31%) are
referred to as the primary bile acids.
• Within the intestines the primary bile acids are acted upon by
bacteria and converted to the secondary bile acids, identified
as deoxycholate (from cholate) and lithocholate (from
chenodeoxycholate).
• Both primary and secondary bile acids are reabsorbed by the
intestines and delivered back to the liver via the portal
circulation.
Types of Bile Acids/Salts
• Primary bile acids
– Good emulsifying agents
• All OH groups on same side
• pKa = 6 (partially ionized)
• Conjugated bile salts
– Amide bonds with glycine
or taurine
– Very good emulsifier
• pKa lower than bile acids
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Primary Bile Acids and Salts
Cholic acid
BILE ACIDS
Glycocholic
Taurocholic
BILE SALTS
Chenodeoxycholic
acid
Glycochenodeoxycholic
Taurochenodeoxycholic
Bile salts (Conjugated bile acids):
amide-linked with glycine or taurine
The ratio of glycine to taurine forms in the bile is
3:1
Bile acids are excreted in conjugated form, as taurocholic acid
and glycocholic acid
Bile acids are conjugated with glycine or taurine before being
secreted into bile, where the ratio of glycine to taurine-conjugated
acid is about 3:1
Fate of the bile salts and the synthesis of secondary bile acids
Intestinal bacteria deconjugate and dehydroxylate the bile salts removing the
glycine and taurine residues and the hydroxyl group at position 7. The secondary bile
salts are less soluble and less readily resorbed from the intestinal lumen.
Lithocholic acid has a hydroxyl group only at position 3, is the least soluble bile
acid, its major fate is excretion.
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Secondary Bile Acids
Bile salts
Glyco- or Tauro-cholate
Glycine
Taurine
Bile acids
-Chenodeoxycholate
Intestinal bacteria
Cholic acid
Chenodeoxycholic
Intestinal bacteria
OH
2° Bile acids
Deoxycholic acid
Lithocholic
Secondary bile acids
Primary bile acids
•Within the intestines the primary bile acids are converted by bacteria into the
secondary bile acids, identified as deoxycholate (from cholate) and lithocholate
(from chenodeoxycholate).
Steroids
Grups –OH localized at:
C-3
C-7
C-12
cholic acid
OH
OH
OH
chenodeoxycholic acid
OH
OH
H
deoxycholic acid
OH
H
OH
lithocholic acid
OH
H
H
Primary bile acids:
Secondary bile acids:
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The pK of the bile acids is about 6.
Therefore, in the contents of the intestinal lumen, which normally have a pH of 6,
about half of the molecules are present in the protonated form and half are ionized,
forming bile salts.
The carbonyl group at the end of the side chain is activated by a reaction that
requires ATP and coenzyme A.
The acyl CoA derivatives can react with either glycine or taurine, forming
amides that are known as the conjugated bile salts.
The bile acids conjugated with glycine have a pK of about 4, so a higher
percentage of the molecules is present in the ionized form at the pH of the intestine.
The taurine conjugates have a pK of about 2, and even greater percentage of
the molecules of these conjugates are ionized in the lumen of the gut.
Taurine is synthesized from the amino acid cysteine.
Clinical Significance of Bile Acid Synthesis
• Bile acids perform four physiologically significant functions:
• 1. Their synthesis and subsequent excretion in the feces
represent the only significant mechanism for the elimination of
excess cholesterol.
• 2. Bile acids and phospholipids solubilize cholesterol in the
bile, thereby preventing the precipitation of cholesterol in the
gallbladder.
• 3. They facilitate the digestion of dietary triacylglycerols by
acting as emulsifying agents that render fats accessible to
pancreatic lipases.
• 4. They facilitate the intestinal absorption of fat-soluble
vitamins.
•the primary and secondary bile acids are reabsorbed almost exclusively in
the ileum returning to the liver by way of the portal circulation (98 to 99%)
•the entherohepatic circulation
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Recycling of bile salts
Bile salts are synthesized in the
liver, stored in the gallbladder,
secreted into the small intestine,
resorbed in the ileum, and
returned to the liver.
Only 5% are excreted.
Cholesterol
An amphipathic lipid, an essential structural component of membranes and the
outher layer of plasma lipoproteins,
Present in tissues and lipoproteins either as a free cholesterol or cholesteryl ester,
Lipoproteins transport free cholesterol in the circulation, where it readily equilibrates with cholesterol in other lipoproteins and in membranes,
Cholesteryl ester is a storage form of cholesterol in most tissues,
Low density lipoprotein (LDL) is the mediator of cholesterol and cholesteryl ester
uptake into many tissues,
Free cholesterol is removed from tissues by high density lipoprotein (HDL) and
transported to the liver for conversion to bile acids,
Excess cholesterol is excreted from the liver in the bile as cholesterol or bile salts;
a large proportion of bile salts is absorbed into the portal vein and returned to
the liver as part of the entherohepatic circulation,
Elevated levels of cholesterol are associated with atherosclerosis
Cholesterol in the cell membrane
Cholesterol is also present in the membrane.
It maintains the fluidity and increases the
stability of the membrane. Without cholesterol
the membrane would easily split apart
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The Utilization of Cholesterol
•
•
•
Cholesterol is transported in the plasma predominantly as cholesteryl esters
associated with lipoproteins.
Dietary cholesterol is transported from the small intestine to the liver within
chylomicrons.
Cholesterol synthesized by the liver, as well as any dietary cholesterol in
the liver that exceeds hepatic needs, is transported in the serum within
LDLs (Low density lipoproteins).
The Acyl-Co: cholesterol acyltransferase (ACAT)
reaction (Synthesis of Cholesterol Esters by ACAT)
Located mainly in the endoplasmic reticulum ACAT catalyzes the covalent
joining of cholesterol and long-chain fatty acyl-CoA to form cholesterol esters.
Cholesterol ester products of the ACAT reaction are either stored in cytosolic
droplets or secreted from cells as components of apo-B lipoproteins.
HDL
High-density lipoprotein (HDL) begins in the liver and small
intestine as small protein rich particles containing relatively little
cholesterol and cholesterol ester
HDLs contain apoA-I and apoA-II, as well as the enzyme
lecithin-cholesterol acyltransferase (LCAT) which catalyzes the
formation of cholesterol ester
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Functions of HDL
•
•
•
•
Transfers proteins to other lipoproteins
Picks up lipids from other lipoproteins
Picks up cholesterol from cell membranes
Converts cholesterol to cholesterol esters via the LCAT
reaction
• Transfers cholesterol esters to other lipoproteins, which
transport them to the liver (referred to as “reverse cholesterol
transport)
Reverse Cholesterol Transport
• An enzyme present in our circulation, Lecithin Cholesterol
Acyl Transferase (LCAT), removes one fatty acid from
Phosphatidylcholine (PC) and attaches it to cholesterol,
converting cholesterol into a cholesterol ester, which is then
taken up by a HDL lipoprotein and transported back to the
liver, from where the cholesterol is excreted from the body in
the form of a bile salt
Acyltransferase lecithin : cholesterol (LCAT)
The reaction catalyzed by LCAT. This enzyme is present on the surface of
HDL, and stimulated by the HDL component of apoA-1.
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Lipoproteins and Lipid Transport
Steroid hormones
Steroid hormones are grouped into five categories:
- progestins,
- glucocorticoids,
- mineralocorticoids,
- androgens,
- estrogens.
All contain the four-ring structure of the steroid nucleus and are
remarkably similar in structure, considering the enormous differences
in their physiological effects.
They mediate a wide variety of vital physiological functions.
FUNCTIONS: Steroids Hormones
•
Cholesterol: precursor of the five major
classes of steroid hormones
•
Glucocorticoid : carbohydrate metabolism
•
Mineralocorticoid: Sodium retention in
renal tubules
•
Androgen, Estrogen, Progestagens:
reproductive system
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Steroid hormones
The primary organs involved in their biosynthesis are the gonads
and the adrenal cortex, and the placenta in pregnant females.
In steroid hormones, the side chain of cholesterol is either
shortened or nonexistent.
Steroid hormones (SH) are not stored for release after synthesis.
The level of circulating SH is controlled by its rate of synthesis,
which is controlled by signals from the brain.
Activation of SH synthesis involves stimulation of both
hydrolysis of cholesterol esters and uptake cholesterol into
mitochondria of cells in the target organ.
SYNTHESIS OF STEROID HORMONES
CHOLESTEROL ------> PREGNENOLONE
• Removal of 6 of the side-chain
carbons.
This step is common to all steroid
hormone synthesis.
•
•
CHOLESTEROL ------>
PREGNENOLONE
This step occurs on the mitochondrial
membrane.
This leaves a C21 compound,
pregnenolone, which may accurately be
described as the mother of all steroids
(the C21 steroid carbon skeleton is given
the name pregnane).
Steroids
There is an enzyme complex called cholesterol desmolase that hydroxylates
C-20 and C-22 and cleaves it to yield pregnolone
Desmolase (in mitochondria) forms
pregnenolone, precursor to all others
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activated to turn
on pathways
Cholesterol
Progesterone
Pregnenolone
11-Deoxycortisone
Progesterone
Aldosterone
21-hydroxylase
11-Deoxycortisol
Progesterone
21-hydroxylase
Cortisol
General pathways for the synthesis of aldosterone
and cortisol in the adrenal cortex
Pregnenolone
Cholesterol
Progesterone
Progesterone
Testosterone
(pathway ends
here in testes)
Androstenedione
Estrone
(produced in both male and
female adipose cells)
Estradiol
(pathway continues to
here in ovaries)
In obese men, overproduction of estrogen in fat cells can cause
gynecomastia = excessive male breast development
Pathways for the synthesis of testosterone (testes) and the estrogens
estradiol (ovaries) and estrone (adipose cells)
Transformations of Cholesterol: Steroid Hormones
OH
O
CH3
O
HO
OH
CH3
O
HO
O
Cortisol
Cholesterol
Progesterone
OH
OH
OH
O
Testosterone
HO
CH 2
Estradiol
HO
OH
Vitamin D
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Groups of steroid hormones
1. Progestins – regulate events during pregnancy, progesterone is the
major hormone involved in sustaining pregnancy
2. Glucocorticoids –affect basal metabolism, host defense mechanisms,
blood pressure, response to stress, promote gluconeogenesis and supress
inflammation reactions (cortisol, corticosterone)
3. Mineralocorticoids – affect electrolyte balance and ion transport,
regulate ion balance by promoting reabsorption in the kidney of Na+ ,
Cl-, HCO3- (aldosterone)
4. Androgens – promote male sexual differentiation, spermatogenesis, and
maintain male characteristics (androstenedione, testosterone),
5. Estrogens – female sex hormones, stimulate the development of tissues
involved in reproduction, support femal characteristics (estrone,
estradiol)
Table 1. Functions of Hormones
Derived from Cholesterol
Product
Functions
Progesterone
prepares uterus lining for
implantation of ovum
Glucocorticoids (cortisol)
(produced in adrenal cortex)
(catabolic steroid)
promote gluconeogenesis;
favor breakdown of fat and
protein (fuel mobilization);
anti-inflammatory
Mineralocorticoids
(aldosterone) (produced in
adrenal glands)
maintains blood volume and
blood pressure by increasing
sodium reabsorption by kidney
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Table 1. Functions of Hormones
Derived from Cholesterol
Product
Functions
Androgens (strongest = testosterone)
(produced in testes primarily but weak
androgens in adrenal cortex) (anabolic
steroid)
development of male
secondary sex
characteristics; prevents
bone resorption
Estrogen
development of female
(produced in ovaries primarily but also secondary sex
in adipose cells of males and females) characteristics; prevents
bone resorption
Vitamin D (not a steroid hormone)
(produced in the skin in response to
UV light and processed to active form
in kidney)
intestinal calcium
absorption; promotes
bone formation; prevents
phosphate loss by
kidneys
Steroid hormones act in the nucleus to change
gene expression
Steroid hormones are too hydrophobic to dissolve readily in
the blood, are carried on specific carrier proteins from the point
of their release to their target tissues
In the target tissue, these hormones pass through the plasma
membrane by simple diffusion and bind to specific receptor
proteins in the nucleus
The hormone-receptor complexes act by binding to highly
specific DNA sequences and altering gene expression
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Vitamin D
• Vitamin D is a steroid prohormone, by various metabolic
changes in the body it gives rise to a hormone known as
calcitriol, which play a central role in calcium and phosphate
metabolism.
Formation of provitamin
(7-dehydrocholesterol)
D
D Vitamins are generated from the provitamins ergosterol and 7dehydrocholesterol by the action of sunlight.
Ergosterol occurs in plants and 7-dehydrocholesterol in animals.
UV light from sunlight cleaves the B ring of both compounds.
Ergocalciferol (vitamin D2) is formed in plants.
Cholecalciferol (vitamin D3) is formed in exposed skin.
A specific transport protein called the vitamin D-binding protein binds vitamin D3 and
moves its from the skin or intestine to the liver, where it undergoes 25-hydroxylation
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Liver
Diet
OH
25hydroxylase
HO
HO
25(OH) D3
Vitamin D3
UV from
sunlight
Kidney
1-hydroxylase
Skin
OH
HO
Specific receptors in
intestine, bone, kidney
Ca:
Intestinal absorption
Renal reabsorption
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Provitamin D3
(7-dehydrocholesterol:
Intermediate in cholesterol
synthesis)
HO
OH
1,25(OH)2 D3
PO4:
Intestinal absorption
Renal reabsorption
(active hormone form)
Figure 7. Photobiosynthesis of vitamin D3 and its metabolism
1,25-dihydroxycholecalcyferol (active form of D3)
•
Active VITAMIN D3
– B-Ring opens
– 1-Hydroxylation
– 25-Hydroxylation
The product (1α,25 dihydroxyD3)
is the most potent vitamin D
metabolite.
Its production is regulated by its
own concentration, parathyroid
hormone, and serum phosphate.
vitamin D-deficiency diseases
Deficiency of vitamin D
causes
rickets
and
osteomalacia.
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