7.1 Physiologic Effects of Triterpenes and Sterols

7.1 Physiologic Effects of 7.1 Physiologic Effects of Triterpenes and SterolsTriterpenes and Sterols

RA Macahig FM Dayrit

CH3

CH3

HO

H CH3

HH

H

-sitosterol

071 Physiologic Effects of Triterpenes & Sterols (Dayrit) 2

Topics

1. The Cholesterol controversy2. Vitamin D3. Phytosterols and health4. Biotransformation of triterpenes5. Steroids as human hormones


1. The Cholesterol controversyAncel Keys was an American physiologist who studied the influence of diet on health. Keys postulated a correlation between cholesterol levels and cardiovascular disease (CVD). His most famous study was known as the Seven Countries Study, which claimed a strong association between CVD rate in a population, average serum cholesterol and per capita intake of saturated fatty acids (he did not distinguish among chain lengths). Critics have rightfully pointed out that Keys selected only the data to support his hypothesis.


1. The Cholesterol controversy

Keys was able to convince the US government to promote his idea that reducing the intake of saturated fat would reduce the incidence of CVD. However, the resulting health condition of the Americans who took this advice indicates that Keys' basic premise was wrong: while consumption of dietary fat decreased from the 1960s to the 1990s, the rate of CVD did not change substantially and the incidence of obesity and Type II diabetes soared.

USDA dietary pyramid


(from: Homo sapiens cholesterol biosynthesis)

• Cholesterol is an important structural component of eukaryotic cell membranes. It is also a precursor of steroid hormones and bile acids. • Cholesterol plays an important role in the regulation of human health. Nobel prizes in Physiology were awarded to Konrad Bloch in 1963 and to M. S. Brown and J. L. Goldstein in 1985 for their work on the metabolism and regulation of cholesterol. • Cholesterol is a very common compound that is found in many biological processes; high levels of cholesterol have been linked to various disorders. However in most cases, causal mechanisms have not been established.

Cholesterol biosynthesis in humans


• The enzymology of the multistep conversion of lanosterol to cholesterol was largely determined in rat liver, and the human pathway is therefore inferred from this work. The order of some of the reactions in this pathway vary. In rat liver, the cholesterol biosynthetic enzymes are localized in the endoplasmic reticulum. • Human genes have been identified for all the enzymes in this pathway and human disorders of cholesterol metabolism have been associated with genetic defects in most of these enzymes. • The mevalonate pathway is highly regulated. HMG-CoA reductase from the mevalonate pathway is the rate-determining enzyme for the entire pathway from acetate to cholesterol.


7

(from: Homo sapiens cholesterol biosynthesis)

HO

CH3

CH3

H

H3C CH3

H

CH3

H3CCH3

CH3

CH3

CH3

HO

CH3H3CH

CH2OH

H CH3

CH3

CH3

HO

CH3H3CH

CHO

H CH3

CH3

CH3

HO

CH3H3CH

H CH3

8

HO

H

H

CH3

CH3

HO

CH3H3CH

H CH3

CH3

CH3

HO

CH3HOCH2

H

H CH3

CH3

CH3

HO

CH3HO2CH

H CH3

9

HO

CH3

CH3

HH3CCH3

CH3



• Dolichyl phosphate acts as a “carrier lipid” in the assembly of diphosphate-linked oligosaccharides.

H3C SCoA

O

HO2C COSCoA

HO CH3

3

3-Hydroxy-3-methylglutaryl CoAHO2C

HO CH3

OH

OPP

MVAIPP

Farnesyl diphosphate

Squalene

Cholesterol

other steroids, bile acid

Dilochols

OP2 15

Ubiquinones

H3CO

O

H3CO

O

CH3

n

Coenzyme Q (n = 4 to 6)

Dolichyl phosphate

1.1.1.34(HMG-CoA reductase)

• Ubiquinones, such as coenzyme Q, are synthesized in the body and probably function as antioxidants.



• Because of the interest in lowering cholesterol levels in susceptible humans, this has attracted much research interest. Cholesterol biosynthesis inhibitors have been developed which are based on the inhibition of (3S)-3-hydroxy-3-methyl glutaryl-CoA (HMG-CoA) reductase. • One such drug - lovastatin, a fermentation product of Aspergillus terreus – is a prodrug which is hydrolyzed in vivo to mevinolinic acid. It is believed that mevinolinic acid binds tightly to HMG-CoA reductase which blocks endogenous cholesterol biosynthesis. However, the synthesis of other important endogenous compounds is also blocked.

1212

http://www.ucl.ac.uk/~sjjgsca/CellMembraneComplex.gif

Cholesterol is the most abundant sterol in animals:– Membrane component– Precursors of bile acids and

hormones

Sterol functions

Regulation of bile acids. To prevent build-up of free cholesterol in the liver, cholesterol stimulates its own conversion to bile acids by activating the nuclear receptor LXR, which increases transcription of the Cyp7 gene. This requires conversion of cholesterol to oxysterols, which are the ligands for LXR. Bile acids are regulated and recycled. BAR: bile acid receptor

I-BABP:intestinal bile acid binding protein

1313

Sterol functionsLong-chain saturated fats taken in the diet are incorporated into cholesterol esters (and triglycerides) which form part of the LDL complexes in the blood. LDL contains 80% fat (half is cholesterol) and 20% protein. HDL contains 55% fat (1/4 is cholesterol), and 45% protein. High levels of LDL have been linked to an increased risk of coronary heart disease and led to its description as "bad cholesterol". Conversely increased HDL levels reduce the risk of coronary heart disease and is referred to as "good cholesterol". HDL lipoproteins transport excess cholesterol from peripheral tissues back to the liver where it is broken down. Since cholesterol is an essential component of cells and has numerous other roles throughout the body, we must keep in mind the need for an appropriate balance. Biosynthesis of HDL


Lovastatin is the first of the statins which is one of the most widely prescribed type of drugs in the US today. Used for the treatment of hypercholesterolemia, statins control high cholesterol by inhibiting its endogenous biosynthesis in the liver, which accounts for over two-thirds of total body cholesterol. Statins inhibit HMG-CoA reductase, an enzyme required in terpene biosynthesis.

H3C

O

O

CH3 CH3

O

O OH

Lovastatin

Statin drugsFrom fermentation:Lovastatin (Mevacor, Altocor)Simvastatin (Zocor, Lipex)Synthetic:Fluvastatin (Lescol)Atorvastatin (Lipitor)

Endo and Kuroda (1971) isolated mevastatin from Penicillium citrinum, a rice fungus. They hypothesized that the fungus uses mevastatin to interfere with the target organism’s steroid synthesis. Following this lead, Merck isolated lovastatin from the fungus Aspergillus terreus.


Lovastatin (Pfizer), a cholesterol biosynthesis inhibitor, is a prodrug. Hydrolysis opens the lactone which is able to bind

tightly to HMG-CoA reductase.

H3C

O

O

CH3 CH3

O

O OH

LovastatinH3C

OH

CH3

HO OH

OOH

resembles 3-hydroxy-3-methylglutaryl portion of HMG-CoA

interacts with HMG-binding domain

hydrolysis

HO2C COSCoA

HO CH3

3-Hydroxy-3-methylglutaryl CoA

Some reported side-effects of statins:• muscle aches• loss of memory and cognitive function• liver damage


2. Vitamin DVitamin D, calciferol, is a group of fat-soluble vitamins, which is

found in food, but can also be made in the body after exposure to ultraviolet rays from the sun. The liver and kidney convert vitamin D to its active forms.

The major biological function of vitamin D is to absorb and maintain normal blood levels of calcium and phosphorus, helping to form and maintain strong bones. It prevents rickets in children and osteomalacia in adults, which are skeletal diseases that result in defects that weaken bones.

Vitamin D is also believed to play an important role in the endocrine system as it controls several of the adrenal hormones, growth of cells, production of enzymes and has other direct genomic functions.


Vitamin D is derived from cholesterol via 7-hydroxycholesterol. The chemistry of Vitamin D is a very interesting example of a photochemical retro-Diels Alder [4 + 2] to yield a triene (precholecalciferol). A second photochemical 1,7-H shift rearrangement yields Vitamin D3. These transformations take place on the skin and the intervention of sunlight is a central feature of this natural process.

Vitamin D

HO

HO

R

H

HO

R

1,7-H shift7

1

753

7-dehydrocholesterol 7-hydroxycholesterol

3 5 7

HO

R

OHHO

cholesterol

cholesterol-7 -monooxygenase

h (UVB: 260~285 nm)

H

HO

R

PrecholecalciferolCholecalciferol (Vitamin D3)


Chemically, the various forms of vitamin D are secosteroids; i.e., broken-open steroids. Several forms of vitamin D are known. The two major forms are:•Vitamin D2 (ergocalciferol): produced by fungi and plants. •Vitamin D3 (cholecalciferol): from animal sources. Vitamin D3 can made in the skin after only ten to fifteen minutes of sun exposure from 7-dehydrocholesterol reacts under UVB sunlight (270–300 nm). With longer exposure to UVB rays, an equilibrium is achieved in the skin, and the vitamin simply degrades as fast as it is generated.

HO

Cholecalciferol(Vitamin D3)

R =

R =

22

25-Hydroxycholecalciferol .

Calciferol

o

o OH

o OH

OH

24(R),25-Hydroxycholecalciferol .R =

R =o OH

O


3. Phytosterols and health• Plants contain phytosterols which act as a structural component in the plant cell membrane, a role which is played by cholesterol in mammalian cells. • Phytosterols are found in small quantities in vegetable oils, such as corn oil (968mg/100g) and soybean oil (327mg/100g).

CH3

CH3

HO

H CH3

HH

H

-sitosterol ergosterol

CH3

CH3

HO

H CH3

HH

CH3

CH3

HO

H CH3

HH

H

stigmasterol


• Diet with phytosterol-enriched foods decreased Total Cholesterol and LDL levels but not HDL.

• Phytosterols compete with cholesterol for micellar solubilization and absorption.

• Common bile salts include cholate (C) and deoxycholate (DC)

Phytosterols for sterols

Source: http://www.nature.com/nm/journal/v7/n12/fig_tab/nm1201-1282_F1.html


• As a food ingredient or additive, phytosterols lower cholesterol by reducing cholesterol absorption in the intestines. They can reduce cholesterol in human subjects by up to 15%. Phytosterols may also have cancer preventive properties (maybe by modulating hormone uptake?).

Phytosterols for sterols

• Phytosterol patents as food supplement to lower cholesterol:– Wyeth: WO/2007/038596 – WIPO website: the keyword “phytosterol supplements”

yielded more than 450 patents


Reduction of cholesterol to cholestanol or coprostanol in rodents. Left: catalytic reduction of cholesterol yields cholestanol. Right: hypothetical direct pathway for microbiological reduction of cholesterol into coprostanol. Middle: indirect pathway. (Eyssen and Parmentier, “Biohydrogenation of sterols and fatty acids by the intestinal microflora”, Am. J. Clin. Nutr., 1974, 1329. )

4. Biotransformation of triterpenes and steroids


Putative metabolic pathways of ginsenosides in the human body Ginsenosides are deglycosylated to M1 or M4 by intestinal bacteria followed by absorption into the blood or the mesenteric lymphatics. Some M1 may be esterified in the liver with fatty acids at C-3 or C'-6 of the glucose moiety. Most M4 is esterified with fatty acids and accumulated in the tissues including the liver and lung followed by excretion of esterified M4 (EM4) as bile. (Hasegawa, “Proof of the Mysterious Efficacy of Ginseng”, J Pharmacol Sci 95, 153 (2004). )


5. Steroids as human hormones

Steroids of the controversial “hormone replacement therapy” (HRT). HRT was given to menopausal women and was supposed to extend desirable youthful characteristics, as well as prevent bone brittleness and heart attack. However, HRT is also suspected to increase the susceptibility to breast cancer and cardiovascular disease.

20

O

CH3

CH3H

H3C

H

O

progesterone (Progestin)

estrogenHO

OCH3

H

H

H


Steroids as human pheromones

The Chemistry of Scents: Natural Products in Human Behavior (Ref: National Geographic)

• What does body odor have to do with human attraction and love? Maybe a lot. In one scientific study, researchers Manfred Milinski and Claus Wedekind have found that a group of immune system genes—the major histocompatibility complex (MHC)—influences women's attraction to men's body odors. As author Lauren Slater explains, each woman in the study seemed most attracted to men whose MHC genes were very different from her own. • Milinski and Wedekind discovered that people who share the same MHC genotype have similar preferences for perfumes. But they also found that those preferences are really about the scents people select for themselves, not the scents they want their partners to have. Perfumes amplify natural body odors.


Steroids as human pheromones

What might these have to do with steroids in human physiology?

Why steroid hormones?• Human steroid hormones are unique secondary metabolites in humans and are among the most powerful and subtle of human pheromones.

Sender: Recepient:DNA

Protein

Scent compound

(Steroid hormone?)

DNA

Protein

Specific receptor

7.1 Physiologic Effects of Triterpenes and Sterols

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