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VOLATILE FATTY ACIDS
Volatile Fatty Acids
Major VFA: acetic acid; propionic acid; butyric acid.
Major VFAs are absorbed and used as primary energy source by ruminants.
The tissue use of VFA is lower than tissue use of the sugars (e.g., glucose).
~10 % of energy consumed is used for fermentation (methane).
Cont.
Regulation of microbial growth/function Bacteria vs. protozoa Competition Environmental pH Alteration with diet Alteration with intake
Cont.
Acetate: mostly from cellulose Important for milk fat synthesis
Propionate: mostly from starch Important to produce glucose
Butyrate: mostly derived from acetate Important in ketones usage as an energy
source
Volatile Fatty Acids (VFA)
Produced from the fermentation of pyruvate Rumen and hind gut Types/ratios depends on diet
3 major VFAs Acetic acid CH3COOH Propionic acid CH3CH2COOH Butyric acid CH3CH2CH2COOH
Rumen Fermentation
Cellulose Glucose
Pyruvate
Hemicellulose
Starch
Sugars
Pectins
Lactic Acetic Formic H2+CO2
ButyricPropionic Methane(CH4)
Acetate
Pyruvate + Pi + ADP Acetate + ATP +
H2 + CO2
Cellulolytic bacteria Energy source for rumen epithelium and
muscle Not utilized by liver
Acetate utilization
Important as a precursor to de novo fatty acid synthesis Adipose Lactating mammary gland
Oxidized via TCA Activated to acetyl CoA Used by skeletal muscle, kidneys, and heart for
energy Net gain of 10 ATP per mole of acetate
Acetate utilization
Dependent upon Energy balance
Generates CO2 and H2O (i.e., ATP) when in low energy balance
Used for fatty acid synthesis when animal is in high energy balance
Arterial concentration Tissue uptake is directly related to rate of
rumen fermentation [blood concentration]
Propionate
Pyruvate + CoA + 4H+ Propionate + H2O
Amylolytic bacteria Utilized by rumen epithelium
Converted to lactate and pyruvate Important as a precursor for
gluconeogenesis
Hepatic propionate metabolism
Glucose
Propionate
Propionyl CoA
Methylmalonyl CoA
Succinyl CoA
OAA TCACycle
Coenzyme B12
Biotin, Mg ++
ATP
AMP + 2 Pi
ATP
ADP + Pi
CoA
Butyrate
Pyruvate + CoA Acetyl-CoA + H2 + CO2
2 Acetyl-CoA + 4H+ Butyrate + H2O + CoA
Metabolized by rumen epithelium to ketone bodies (acetoacetate, -hydroxybutyrate) Later metabolized in liver
Net ATP production is 25 per mole
Ruminal VFA absorption
Rumenlumen
Rumenwall
Portalvein
Acetate
Propionate
Butyrate
50
10
1
70
20
10
Values are relative flux rates
20
10
9
Hepatic metabolism of VFA
Rumen Portalvein
Acetate
Propionate
Butyrate
50
Glucose
3-hydroxyButyrate (BHB)
70
20
10
10
1
Liver
3-OHbutyrate
Acetate
Peripheralblood
Glucose
CO2
4
Absorption to portal blood
Passively absorbed by rumen epithelium Rate:
Concentration pH Chain length
Tissue uptake related to rate of fermentation Absorbed in undissociated acid form
CH3COOH (acetic acid) vs CH3COO- (acetate) pK ~4.75
Cont.
In converting acetate to pyruvate also CO2; CH4
Ionophore feed additives Increases propionate Decreases acetate
Normal process
Propionate to lactate (normal process) Causes lowering pH
Lactate to pyruvate Requires lactate fermenters (altering pH) this pyruvate is mainly used to
synthesize glucose (hepatic tissues)
Sudden dietary changes
Propionate to lactate; reduced pH Lactate needs to be converted to
puruvate Microbes converting lactate grow
slow !!!!! pH continues to drop Too acidic environment Lactic acidosis; can be lethal
why
Sudden changes in diet; too much concentrate
Stress + reduced feed intake Empty feed bunks Reduced feed intake; how palatable ?
IMBALANCE BETWEEN MICROBES PRODUCING LACTATE AND MICROBES CONVERTING LACTATE TO PYRUVATE
End products
VFAs CO2
CH4
NH3
Microbes
How pH is altered
Diet Intake Feeding frequency Chewing/rumination