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Protein Metabolism II
ANS 520
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Fate of Rumen Ammonia
1. Bacterial protein synthesis
2. Absorbed from reticulorumen and omasum
NH3 passes from rumen by diffusion into portal blood. (High concentration to low)
Form of ammonia dependent on pH of rumen
NH3 + H+ NH4+
Less absorption at more acid pH
3. At pH of rumen, no NH3 lost as gas
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Fate of Absorbed Ammonia1. Transported to liver by portal vein2. Converted to urea via urea cycle in liver
NH3 Urea
Urea cycle
3. Urea released into blood4. If capacity of urea cycle in liver is exceeded
Ammonia toxicityOver consumption of urea
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Fate of Blood Urea
1. Excreted into urine
2. Recycled to digestive tract, g N/d
• Saliva – Related to concentration ofurea in blood
Sheep: 0.5 to 1.0Cattle: 1.0 to 7.6
• Diffusion into GITSheep: 2 to 5Cattle: 25 to 40
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Urea Diffusion into Rumen
Rumen wall
Bloodurea Urea
NH3
Bacterial population
1. Total N transferred isgreater when high Ndiets are fed.
2. Percentage of diet Ntransferred is greaterwhen low N diet are fed
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Urea Diffusion into Rumen
Rumen wall
Urea transporterBloodurea Urea
High [NH3]
inhibits
NH3
Bacterial population
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Adjustments to Low Protein Intake
KidneyBlood urea Urea
Urine urea
Urea is predominant form of N in urine
Reabsorption of urea by kidney increasedwhen ruminants fed low N diets
• Conserves nitrogen in the body• Greater portion recycled to digestive tract• Sheep fed the same diet tend to reabsorb more urea than cattle
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Nitrogen Recycling - Cattle
05
1015202530354045
N,
g/d
87.6 110.4 147.5 178.7 203.5
N intake, g/d
GIT
Saliva
Wall
Marini et al. JAS 2003
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Sources of Nitrogen Recycled to GIT
1. Urea flowing back into digestive tract Rumen
• Saliva• Diffusion from blood
Lower digestive tract (large intestine, colon,cecum)• Diffusion from blood• Endogenous protein secretions into GIT
Mucins Enzymes Sloughing of tissue
2. Turnover of microbial cells in rumen & reticulum
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Significance of Recycled Nitrogen
Source of N for microbes when protein consumptionis limited
• Wild speciesProtein intake during winter is very lowRumen deficient of nitrogen for microbial activity
• Slowly degraded feed proteinsRecycling provides nitrogen for microbial growth
• Infrequent feeding of supplemental protein• Programs to reduce supplemental nitrogen
Difficult to make ruminants severely protein deficient
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Urea Nitrogen - Cattle
0
2
4
6
8
10
12
14
mM
1.45 2.5 3.4
N, % Diet DM
Plasm urea Saliva urea
0
20
40
60
80
100
120
140
N,
g/d
1.45 2.5 3.4
N, % Diet DM
Urine N Urine, Urea N
Marini et al. JAS 2003
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Amino Acid SynthesisAmmonia Fixation
1. Glutamine synthetase/glutamate synthase• Glutamine synthetase Glu + NH3 + ATP Gln
• Glutmate synthase -ketoglutarate + glutamine + NADPH2
2 Glu
High affinity for NH3 - Concentrates NH3 incells – Uses ATPBecause of N recycling this reaction may notbe that important
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Amino Acid SynthesisAmmonia Fixation
2. Glutamic dehydrogenase• -ketoglutarate + NH3 + NADH Glu
Low affinity for NH3 – High concentration ofenzyme in rumen bacteria – Does not use ATP
Probably predominant pathway
3. Other AA can be synthesized by transamination reactions with glutamic acid
Estimates of NH3 requirements range from 5 (culture)to 20 mg/100 ml (in situ digestion)
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Sources of Amino Acids for Host Animal
1. Microbial proteinsQuantity determined by:
a) Fermentability of the feedb) Quantity of feed consumedc) Nitrogen available to microorganisms
2. Undegraded feed proteins (UIP)Quantity will vary in relation to:
a) Degradability of feed proteinsb) Quantity of feed proteins consumed
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Nutritional Value of Microbial Proteins
1996 NRC for BeefMicrobial protein 80% digestible in the intestine
UIP 80% digestible in the intestine
2001 NRC for Dairy and Level 1 CNCPSMicrobial protein 80% digestible in the intestine
Digestibility of RUP (UIP) is variable in Dairy NRCUIP 80% digestible in Level 1 CNCPS
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NH3 Blood urea Urine Amino acid pools
Energy NH3 Metabolizable Microbial protein
protein Protein
Proteinfrom diet
Rumen Intestine Feces
A
B
C
Metabolizable Protein Model
Tissue proteins
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Protein Metabolism of RuminantsConcept of Metabolizable Protein
Metabolizable protein (MP) = Absorbed amino acids or = Digestible fraction of microbial proteins + digestible fraction of undegraded feed proteins
Digestible protein (amino acids) available for metabolism
Concept is similar to Metabolizable energy
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Feed Rumen Intestine
DigestionMicrobes
Undegraded feedMetabolizable protein
Protein Metabolism in the RumenLess Extensively Degraded Protein
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Feed Rumen Intestine
DigestionMicrobes
Undegraded feedMetabolizable protein
Protein Metabolism in the RumenExtensively Degraded Protein
NH3
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Metabolizable ProteinSupply to Host Animal
Metabolizable protein (MP):Microorganisms – Digestible proteinsUndegraded feed proteins – Digestible proteins
Microorganismsg/d = 0.13 (TDN intake, g/d) (0.8) (0.8) Microbes 80% true protein that is 80% digested
Feedg/d = (Feed protein) (Portion undegraded) (0.8) Feed proteins 80% digested
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Absorption of Amino Acids
Amino acids and small peptides absorbedby active transport (specific for groups of AA)
From intestines Portal blood
Transport of amino acids into cells issimilar process
From blood Cells
Active transport, requires energy
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Utilization of Absorbed Amino AcidsVia portal vein to liver
• Used for synthesis of proteins in liver• Metabolized (deaminated) - Used for energy – Carbon for glucose• Escape the liver
Carried by blood to body tissues• Used for synthesis of tissue proteins, milk, fetal growth, wool• Metabolized - Used for energy
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Requirements for Absorbed Amino AcidsMetabolizable Protein (MP)
Protein (amino acid) requirements1. Maintenance2. Growth3. Lactation4. Pregnancy5. Wool
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Protein MetabolismConcept of Net Protein
Net protein = protein gained in tissues, milk, or fetal growth = NP
Metabolizable protein is used with lessthan 100% efficiency
Net protein = (MP - Metabolic loss)
As a quantity, net protein is less thanmetabolizable protein
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Net Protein Required for Production
Amino Acids Proteins
Milkkg/d = (Milk yield, kg/d) (% protein in milk)
Growthg/d = SWG (268 - (29.4 (RE/SWG)))
SWG = Shrunk weight gain, kg/dRE = Retained energy, Mcal/d
RE obtained from net energy equations.
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Protein MetabolismMetabolic Loss
Protein synthesis and metabolism ofamino acids draw from the same pool
ProteinsAmino acids
Metabolism
• Metabolic loss results from continuous catabolism from amino acid pools• Continuous turnover of tissue proteins adds to amino acid pools in tissues
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Amino Acid (MP) Requirements
Maintenance (three fractions)Protein required to support zero gain or production
1. Metabolism Metabolized Urine
Milk Amino acids FecesWool (Synthesis) GITScurf (Degradation)Pregnancy Tissue proteins= Endogenous urinary N
2. Proteins lost from body surface (hair, skin, secretions) = Scurf proteins3. Proteins lost from undigested digestive secretions and fecal bacteria = Metabolic fecal N