Protein Metabolism II
ANS 520
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
Fate of Absorbed Ammonia
1. Transported to liver by portal vein
2. Converted to urea via urea cycle in liver
NH3
Urea
Urea
cycle
3. Urea released into blood
4. If capacity of urea cycle in liver is exceeded
Ammonia toxicity
Over consumption of urea
Fate of Blood Urea
1. Excreted into urine
2. Recycled to digestive tract, g N/d
• Saliva – Related to concentration of
urea in blood
Sheep: 0.5 to 1.0
Cattle: 1.0 to 7.6
• Diffusion into GIT
Sheep: 2 to 5
Cattle: 25 to 40
Urea Diffusion into Rumen
Rumen wall
Blood
urea
Urea
NH3
1. Total N transferred is
greater when high N
diets are fed.
2. Percentage of diet N
transferred is greater
when low N diet are fed
Bacterial population
Urea Diffusion into Rumen
Rumen wall
Urea transporter
Blood
urea
Urea
High [NH3]
inhibits
NH3
Bacterial population
Adjustments to Low Protein Intake
Kidney
Blood urea
Urea
Urine urea
Urea is predominant form of N in urine
Reabsorption of urea by kidney increased
when 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
N, g/d
Nitrogen Recycling - Cattle
45
40
35
30
25
20
15
10
5
0
GIT
Saliva
Wall
87.6
110.4
147.5
178.7
N intake, g/d
Marini et al. JAS 2003
203.5
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
Significance of Recycled Nitrogen
Source of N for microbes when protein consumption
is limited
• Wild species
Protein intake during winter is very low
Rumen deficient of nitrogen for microbial activity
• Slowly degraded feed proteins
Recycling provides nitrogen for microbial growth
• Infrequent feeding of supplemental protein
• Programs to reduce supplemental nitrogen
Difficult to make ruminants severely protein deficient
Urea Nitrogen - Cattle
Saliva urea
14
12
10
8
6
4
2
0
Urine N
N, g/d
mM
Plasm urea
1.45
2.5
3.4
N, % Diet DM
Marini et al. JAS 2003
Urine, Urea N
140
120
100
80
60
40
20
0
1.45
2.5
N, % Diet DM
3.4
Amino Acid Synthesis
Ammonia 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 in
cells – Uses ATP
Because of N recycling this reaction may not
be that important
Amino Acid Synthesis
Ammonia Fixation
2. Glutamic dehydrogenase
• -ketoglutarate + NH3 + NADH
Glu
Low affinity for NH3 – High concentration of
enzyme 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)
Sources of Amino Acids for Host Animal
1. Microbial proteins
Quantity determined by:
a) Fermentability of the feed
b) Quantity of feed consumed
c) Nitrogen available to microorganisms
2. Undegraded feed proteins (UIP)
Quantity will vary in relation to:
a) Degradability of feed proteins
b) Quantity of feed proteins consumed
Nutritional Value of Microbial Proteins
1996 NRC for Beef
Microbial protein 80% digestible in the intestine
UIP 80% digestible in the intestine
2001 NRC for Dairy and Level 1 CNCPS
Microbial protein 80% digestible in the intestine
Digestibility of RUP (UIP) is variable in Dairy NRC
UIP 80% digestible in Level 1 CNCPS
Metabolizable Protein Model
Tissue proteins
NH3
Blood urea
Urine
Amino acid
pools
Energy
A
B
NH3
Microbial
protein
Protein
Metabolizable
protein
C
Protein
from diet
Rumen
Intestine
Feces
Protein Metabolism of Ruminants
Concept 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
Protein Metabolism in the Rumen
Less Extensively Degraded Protein
Feed
Rumen
Intestine
Microbes
Digestion
Metabolizable protein
Undegraded feed
Protein Metabolism in the Rumen
Extensively Degraded Protein
Feed
Rumen
Intestine
Microbes
NH3
Digestion
Metabolizable protein
Undegraded feed
Metabolizable Protein
Supply to Host Animal
Metabolizable protein (MP):
Microorganisms – Digestible proteins
Undegraded feed proteins – Digestible proteins
Microorganisms
g/d = 0.13 (TDN intake, g/d) (0.8) (0.8)
Microbes 80% true protein that is 80% digested
Feed
g/d = (Feed protein) (Portion undegraded) (0.8)
Feed proteins 80% digested
Absorption of Amino Acids
Amino acids and small peptides absorbed
by active transport (specific for groups of AA)
From intestines
Portal blood
Transport of amino acids into cells is
similar process
From blood
Cells
Active transport, requires energy
Utilization of Absorbed Amino Acids
Via 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
Requirements for Absorbed Amino Acids
Metabolizable Protein (MP)
Protein (amino acid) requirements
1. Maintenance
2. Growth
3. Lactation
4. Pregnancy
5. Wool
Protein Metabolism
Concept of Net Protein
Net protein = protein gained in tissues,
milk, or fetal growth = NP
Metabolizable protein is used with less
than 100% efficiency
Net protein = (MP - Metabolic loss)
As a quantity, net protein is less than
metabolizable protein
Net Protein Required for Production
Amino Acids
Proteins
Milk
kg/d = (Milk yield, kg/d) (% protein in milk)
Growth
g/d = SWG (268 - (29.4 (RE/SWG)))
SWG = Shrunk weight gain, kg/d
RE = Retained energy, Mcal/d
RE obtained from net energy equations.
Protein Metabolism
Metabolic Loss
Protein synthesis and metabolism of
amino acids draw from the same pool
Proteins
Amino
acids
Metabolism
• Metabolic loss results from continuous
catabolism from amino acid pools
• Continuous turnover of tissue proteins adds
to amino acid pools in tissues
Amino Acid (MP) Requirements
Maintenance (three fractions)
Protein required to support zero gain or production
1. Metabolism
Metabolized
Urine
Milk
Amino acids Feces
Wool
(Synthesis)
GIT
Scurf
(Degradation)
Pregnancy
Tissue proteins
= Endogenous urinary N
2. Proteins lost from body surface (hair, skin,
secretions) = Scurf proteins
3. Proteins lost from undigested digestive
secretions and fecal bacteria = Metabolic fecal N