protein metabolism ii ans 520. fate of rumen ammonia 1. bacterial protein synthesis 2. absorbed from...

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 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

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

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

Urea Diffusion into Rumen

Rumen wall

Urea transporterBloodurea Urea

High [NH3]

inhibits

NH3

Bacterial population

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

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

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 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

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

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

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)

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

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

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

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

Feed Rumen Intestine

DigestionMicrobes

Undegraded feedMetabolizable protein

Protein Metabolism in the RumenLess Extensively Degraded Protein

Feed Rumen Intestine

DigestionMicrobes

Undegraded feedMetabolizable protein

Protein Metabolism in the RumenExtensively Degraded Protein

NH3

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

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

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

Requirements for Absorbed Amino AcidsMetabolizable Protein (MP)

Protein (amino acid) requirements1. Maintenance2. Growth3. Lactation4. Pregnancy5. Wool

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

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.

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

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