L22: Protein Systems Flashcards
Crude protein:
most of N requirement of animals is used for protein synthesis, and most N in plants present as protein.
CP = N x 6.25
This assumes:?
- All food N part of protein
- All food contains 16% N
How is digestible crude protein determined?
Not all faecal protein is of dietary origin
-where does the rest come from?
- Determined by digestibility trials
- Endogenous N sources other protein origin:
- saliva
- bile
- gastric secretion
- bacterial
- gut MM
- MICROBIAL PROTEIN
How do you determine endogenous N?
- Most common is using labelled 15N in feed
- non-radioactive stable isotope
- *-known conctration of 15N in diet
- proportion of 15N in faeces**
- difference = endogenous N
- Amount is also affected by fibre in diet and protein status of animal
- high fibre = high endogenous N losses in faeces
Is protein digestibility a good indicator of protein value?
No. Efficiency (k value) of absorbed protein varies from source to source
List the protein quality measures applicable to monogastrics
- Protein efficiency ratio
- gain in weight/protein eaten
- Net protein retention
- weight gain eating protein - weight lost eating no protein) / weight of protein eaten
- Gross protein value
- weight gain per g protein eaten / weight gain g casein (protected protein)
- N Balance (more commonly used)
- N in diet consumed - N excreted (faece, milk, urine etc)
What is biological value? (simple explanation)
- proportion of absorbed N retained by body
Describe factors affecting biological value
- Depends on the number and composition of AA in the molecule
- Closer the composition of the food protein to the body protein = Higher BV
- Unable to store AA in free state
- AA either converted to non-essential AA or used as an energy source
- Low BV value
- Diets with high variety of AA = High BV
- Diets with low variety of AA = Low BV
- Theoretically could compare AA profile in feed to that required for different animals and functions – Amino Acid Assays
- Practically difficult due to analysis losses of some AA’s
Describe some food protein measures for pigs and poultry
- Digestible crude protein
- Content of essential aa most likely to be limiting (eg lysine)
Describe some protein quality measures for horses
- Dietary CP commonly used
-inappropriate as horses mostly digest protein in SI
-little MCP produced in hindgut is absorbed = high endogenous N excreted - More accurate method is calculating available protein (AP)
-CP less NPN (x6.25) - acid detergent insuluble n (ADIN) (x6.25)
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Microbes provide most of energy requirement of ruminant. To do this effectively they must synthesise protein, what do they use to synthesise?
Dietary N
Soluble CHO digesting microbes derive about ……….% of their N from aa and peptides
65%
Current protein quality systems for ruminants require determination of:
• Protein degradability
- Microbial protein yield
- Small intestinal digestibility of microbial and dietary protein • Efficiency of use of absorbed amino acids (biological value)
Protein degradability depends on:
• Depends on
- Surface area available for microbial attack
- Presence of protective layers
- Physical and chemical nature of protein
• Extent of degradation in rumen depends on this and on rate of passage
What are the 3 ways of measuring protein degradability?
- *• In vivo** – Gold Standard
- N intake, endogenous N, non-ammonia N, microbial N of dietary origin passing the duodenum
- Laborious, error prone, but benchmark
- *• In sacco**
- Also error prone
- *• Lab methods**
- solubility in buffers, enzymes, chemicals • NIR
Efficiency of N capture is dependant on?
Not only dependent on level of protein degradation but also the provision of energy for MCP synthesis
• Insufficient ME = Absorption of excess ammonia by host
- Excretion of urea
• Urea recycling via rumen and saliva high (70%) on low protein diets, low (10%) on high protein diets
- Efficiency of using degraded protein about 0.8
