4. Amino Acid and Protein Metabolism (Part I) Flashcards
1
Q
Define the Estimated Average Requirement (EAR).
A
Defines the amount of a nutrient that supports a specific function in the body for HALF of the population
2
Q
Define the Recommended Dietary Allowance (RDA).
A
The EAR + 2 standard deviations, which meets 97.5% of the population’s needs
3
Q
What is the risk of deficiency at the RDA?
A
2.5%
4
Q
What is the daily EAR for protein? How many grams is that for a 70 kg person?
A
- 0.66 g/kg/d
- 46 g/d
5
Q
What is the daily RDA for protein? How many grams is that for a 70 kg person?
A
- 0.8 g/kg/d
- 56 g/d
6
Q
What is the typical intake of protein in North America?
A
- 80 to 100 g/d
- Substantially higher than the RDA
7
Q
What should the EAR and RDA be, according to Dr. Wykes?
A
- EAR: 0.9 g/kg/d
- RDA: 1.2-1.4 g/kg/d
8
Q
What is the risk of deficiency at the EAR?
A
50%
9
Q
How many amino acids are incorporated into proteins?
A
20 amino acids possess tRNAs, and are incorporated into proteins
10
Q
Define indispensable amino acids.
A
Amino acids that are necessary to intake through the diet, as humans are incapable of producing these amino
acids de novo
11
Q
Which amino acid was recently discovered to be indispensable in all ages?
A
- Histidine
- Histidine requirement is difficult to measure, and was initially thought to be necessary solely in children
12
Q
Define conditionally indispensable amino acids.
A
Amino acids that are required under certain metabolic conditions or developmental states, but are generally not required for healthy human
13
Q
Which amino acid is responsible for carrying amino groups through the urea cycle? What is another one of its functions?
A
- Arginine
- Precursor for powerful metabolic regulators (e.g. polyamines, creatine in muscle)
14
Q
What occurs if an individual is deficient in arginine?
A
- Build-up of amino groups, which are toxic to the brain
- Arginine is necessary to carry amino groups through the urea cycle
15
Q
Why is arginine conditionally indispensable? Under what conditions is it required?
A
- Babies require arginine
- Individuals with a high turnover of protein (e.g. burns) require arginine
- Normal healthy adults do NOT require arginine
16
Q
Which amino acid requirements are considered together?
A
- Tyrosine is synthesized from phenylalanine
- Cysteine is synthesized from methionine
17
Q
What is tyrosine synthesized from?
A
Phenylalanine
18
Q
What is cysteine synthesized from?
A
Methionine
19
Q
What are the indispensable amino acids?
A
- Valine
- Phenylalanine
- Histidine
- Leucine
- Methionine
- Tryptophan
- Isoleucine
- Threonine
- Lysine
20
Q
What are the conditionally indispensable amino acids?
A
- Tyrosine
- Cysteine
- Arginine
- Glutamine
- Glycine
- Proline
21
Q
What are the dispensable amino acids?
A
- Alanine
- Aspartate
- Asparagine
- Glutamate
- Serine
22
Q
How did the amino acid EARs between the FAO/WHO in 1985 differ from the DRI report in 2005? What was the exception?
A
- The requirements for the amino acids doubled, and largely increased
- Except for methionine and cysteine, which did not change
23
Q
Why did methionine and cysteine requirements not change from the FAO report in 1985?
A
- Because there was a transcription error in the FAO report
- The proposed RDA, and not the proposed EAR, was written
24
Q
What specific principle does tracer dilution rely on?
A
- We do NOT store amino acids or proteins
- They are either used as building blocks, or catabolized
- We maintain a constant pool of amino acids, unlike lipid or carbohydrate pool
25
What are the four assumptions of tracer dilutions?
1) System is at steady-state
2) Homogeneity of the pool
3) Massless tracer, which behaves like tracee
4) No tracer recycling
26
Why is the system at steady-state in a tracer dilution?
Constant flow of amino acids into the pool = the constant flow out
27
How is the amino acid pool assumed to be homogeneous in a tracer dilution? Is this a correct statement?
- The concentration of an amino acid is assumed to be the same in plasma as in brain cells as in liver cells
- Assumption that there is rapid mixing between sub-compartments
- This is NOT true, but this approximation is necessary for the model
- The concentration of a tracer is measured in a blood sample, and assumed that it represents the concentration in the rest of the body
28
Why is the tracer assumed to be massless in a tracer dilution?
- The tracer administered is assumed to NOT be nutritionally significant
- The tracer amino acid behaves metabolically similar to regular amino acids
29
Why is it assumed that there is no tracer recycling in a tracer dilution?
If there was recirculation of the tracer, there will be extra tracer coming in that is not accounted for, which influences the calculated flow number
30
What is the calculation for flow in a tracer dilution?
Flow = (Tracer infusion rate)/(Tracer concentration in pool)
31
Where are proteins stored?
- Proteins are NOT stored, even not in muscle
| - Muscle proteins are made to accomplish a function
32
What are the two routes of amino acid intake?
- Diet intake
| - Proteolysis
33
What are the two routes of amino acid expenditure?
- Protein synthesis
| - Amino acid oxidation
34
If the rate of tracer infusion is known, what may be determined?
The quantity of dilution may be measured with a plasma sample
35
What are isotopes?
Different forms of the same element, with nuclei that have the same number of protons but different numbers of neutrons
36
What element incorporated into water forms heavy water?
Deuterium (2H)
37
What isotopes are preferred to be used as tracers?
Stable isotopes that are NON-radioactive
38
How do you measure stable isotope tracers, and differentiate them?
- Tracers isotopes are heavier, thus they may be uncovered by weight
- Gas Chromatograph Mass Spectrometer
39
How does a GCMS function?
1) Temperature is increased to 300oC
2) Sample is injected, and heated to a gas
3) The molecules are placed in the middle of a magnet, and electrons are shot at them, producing ions in a specific way
4) Ions are sent through focusing lenses and a quadrupole mass analyzer, causing them to spiral down the rods at hit the detector at the end
40
How was essentiality determined historically?
- They cannot be synthesized by the organism out of materials ordinarily available to the cells at a speed commensurate with the demands for normal growth
- Determined with growth as a measure for accuracy
41
In a strictly metabolic sense (i.e. with the appropriate precursors), which amino acids CANNOT be synthesized by providing precursors?
- Lysine
- Threonine
- Tryptophan
42
Why must amino groups be escorted in the body through transamination reactions?
Because free amino groups are toxic to the brain (ammonia)
43
What is a keto acid?
An amino acid without the amino group
44
Would the body be able to synthesize the branched-chain amino acids if we provided their keto acids? What conclusion does this bring about?
- Yes
- Thus, metabolically, they are not essential
- But, nutritionally, they are essential, as we cannot normally supply the keto acid versions of these amino acids
45
What approach may be used in individuals with liver or kidney disease, who are sensitive to supplementary amino groups?
- Providing branched-chain amino acids in the nutrition support regimen as KETO acids
- Synthesis to amino acids gets rid of the body's amino groups
46
What was the premise behind the Golden Egg Experiment?
- Algae was grown in an atmosphere with 13CO2
- Protein from the algae contained 13CO2, and was fed to the chicken
- The chickens incorporated these amino acids into their own protein, including their egg protein
- The eggs were then analyzed to measure which amino acids they contained
47
What results occurred in the Golden Egg Experiment for indispensable amino acids, such as phenylalanine?
- Two fractions: M (protein breakdown) and M+9 (diet)
- There were NO phenylalanine molecules that contained other types of labels
- Thus, there was no resynthesis of phenylalanine in the body, which demonstrates that it is an essential amino acid
- M decreased over days of feeding, while M+9 increased
48
How did the levels of phenylalanine differ between different tissues, 30 days after the Golden Egg Experiment? What does that suggest?
- The liver had the highest, while muscle had the lowest
- Indicates that protein synthesis is much faster in the liver than in muscle (slow protein-turning pool)
- Other visceral organs have rapid metabolism and protein turnover (high incorporation of label)
49
What is the metabolic fuel for the gut?
Glutamate
50
What is glutamine? How does it compare to glutamate?
Glutamine is a glutamate with an extra amino group
51
What common function do glutamine and glutamate share?
Used in transporting amino groups from protein catabolism in peripheral tissues to the liver for urea synthesis
52
What results occurred in the Golden Egg Experiment for dispensable amino acids, such as glutamine and glutamate?
- Multiple fractions: M (protein breakdown) and M+1-9 (diet)
- 70% of glutamate originated from protein breakdown (endogenous)
- Multiple fractions demonstrated that the body assembled new glutamate in the body from labeled precursors that came from dietary amino acids
- Carbon is exchanged between these amino acids and the precursors and intermediates of the Krebs cycle
53
Why does the body use a substantial quantity of glutamate and glutamine?
Since they are amino acids that are used in the Cori Cycle to transport nitrogen from peripheral tissues to the liver
54
What explains that limited quantities of dietary glutamate was synthesized into egg protein?
The gut uses oxidized glutamate as a metabolic fuel
55
Define requirement.
The minimal intake level, representing a single point on a dose-response curve, which is sufficient to maintain a specific criterion of nutrition adequacy
56
What are 7 ways to measure amino acid requirement?
- Growth
- Nitrogen balance
- Plasma AA response
- Direct AA oxidation
- Indicator AA oxidation
- 24h AA balance
- Or measure of organ or system function
57
What are general aspects to experimental approaches to measuring amino acid requirement?
- All methods should give the same answer
- Measures of individuals' requirements
- Subjects should be studied at above or equal to 6 test amino acid intake levels above and below requirement
- Endpoint should show a clear
58
If an individual consuming a low-amino acid switches to a high-amino acid diet, does the body need time to adapt to the change to increase catabolism of amino acids?
Certain individuals say yes, and others no
59
If you are tracking the nitrogen part of an amino acid, how many hours/days does it take for the urea pool to be adapted for a person on a diet?
- The amino acid pools are extremely small, while the urea cycle is extremely large and possesses slow protein turnover
- It takes 7 to 10 days before the urea pool is adapted
60
If you are tracking the oxidation of the amino acid be labelling the carbon, how many hours/days does it take to analyze the CO2 coming out in the breath?
Couple of hours to days
61
In terms of nitrogen balance, how much dietary nitrogen is assumed to come from protein?
All the dietary nitrogen is assumed to come from protein
62
What percentage nitrogen is protein by weight?
Protein is 16% nitrogen by weight
63
What percentage of protein is absorbed, and what percentage is excreted in feces?
- 95% of protein is absorbed and excreted in urine
| - 5% is excreted in feces
64
What is the formula for nitrogen balance?
Nitrogen Balance = Nitrogen Intake - Fecal Nitrogen - Urinary Nitrogen (- miscellaneous losses)
65
How much nitrogen per kilogram is excreted in miscellaneous losses? What percentage does that represent in terms of protein requirement?
- 5 mg of nitrogen per kilogram
| - Represents between 2 and 5% of protein requirement
66
What factors account for miscellaneous losses?
- Sweat
- Hair
- Fingernails
67
How does nitrogen balance differ for individuals in the tropics?
They excrete 10 mg in miscellaneous losses due to an increased amount of sweating
68
What does "Apparent Nitrogen Balance" mean?
That miscellaneous losses are not considered
69
What are the special products of amino acids? What are they generally assumed to be?
- Ex: neurotransmitters
| - Generally assumed to be 0, which is a bold assumption
70
What does positive nitrogen balance indicate?
- Protein synthesis > breakdown
| - Results from a metabolic scenario of growth, recovery, increasing lean body mass, starting a protein-building regimen
71
What is hypermetabolic positive nitrogen balance?
- Synthesis > breakdown
| - Child growing and catching up from malnutrition
72
What is hypometabolic positive nitrogen balance?
- Synthesis > breakdown
| - Elderly person that is trying to recover from an illness
73
Does increasing the intake above requirement from a well-nourished state drive positive nitrogen balance?
- No, as it does not drive more protein synthesis, or less protein breakdown
- Nitrogen balance should be maintained
74
What is nitrogen balance in a state of malnutrition? How does this affect the quantity of amino acids excreted in urine?
- Nitrogen balance is negative (synthesis < breakdown)
- Imbalance of AA in the free pool, which increases the breakdown of proteins to provide more AA, and causes more AA excreted in urine
75
Define a limiting amino acid.
The amino acid that is present in the lowest amount, relative to the body's need for it, which limits protein synthesis
76
What occurs to other amino acids if the quantity of the limiting amino acid is significantly lower?
Causes the catabolism of all the other amino acids present in "excess", which slows growth and protein synthesis
77
What occurs to nitrogen balance when a limiting amino acid is present?
Nitrogen balance is quite negative, as all the non-limiting amino acids are degraded
78
Which amino acid is one of the most common limiting amino acids? Which types of individuals are more vulnerable?
- Lysine
| - Individuals consuming plant-protein and children
79
Which amino acid reacts in cooking through the Maillard reaction? What does that cause?
- Lysine
| - Causes a lack of availability of lysine for protein synthesis
80
What is the impact on increasing the availability of a limiting amino acid on amino acid catabolism, urinary nitrogen, and nitrogen balance?
- Amino acid catabolism: decrease
- Urinary nitrogen: decrease (still higher than the healthy nitrogen)
- Nitrogen balance: less negative (getting closer to 0)
81
What is the clinical and metabolic method to analyze nitrogen balance?
- Diet must be adapted for a week minimum
- Feces/urine must be collected for a minimum of 3 days
- Involves a huge amount of subject commitment because crystalline amino acids taste bad
82
How do miscellaneous losses influence protein requirement based on nitrogen balance?
Miscellaneous losses increase protein requirement
83
What was the issue with nitrogen balance data in the 1950s?
- They did not account for miscellaneous losses, which undershot the protein requirement
- They did not possess modern statistical methods
84
How did the re-analysis of the nitrogen balance data from the 1950s change in modern years?
By changing the modelling by using modern statistical methods
85
How does the dose response curve of nitrogen retention change when an essential amino acid is limiting, at requirement, and in excess?
- Limiting: nitrogen retention is negative when a amino acid is deficient
- As the intake of the limiting amino acid is increased, the other AA are present in less of an excess, causing less urinary nitrogen, increasing nitrogen balance
- At requirement: nitrogen balance is 0
- Excess: nitrogen balance is 0
86
What is nitrogen retention?
The percentage of dietary amino acids that are retained (0 in healthy adults)
87
How does the dose response curve of direct oxidation change when an essential amino acid is limiting, at requirement, and in excess?
- Limiting: low oxidation since the AA is used to make protein
- As the intake increases, more protein can be synthesized, but the oxidation rate maintains a low plateau
- At requirement: the limiting AA is now present in excess, which increases oxidation
- Excess: every amount that we increase above requirement increases oxidation, and the labeled CO2 in the breath
88
What are the three pros of measuring amino acid requirement through direct oxidation?
1) Following the carbon skeleton does not require long periods of adaptation to the urea pool
2) Does not require a huge participation commitment, and measurements are done rather easily (tracer infusion, breathe into a bag)
3) Receive test amino acids in a sterile solution
89
What are the three cons of measuring amino acid requirement through direct oxidation?
1) Solely the branched-chain amino acids, lysine, and phenylalanine produce a labeled CO2 that may be utilized
2) Changing intake from a low amount to an excess of AA changes the AA pool, which may interfere with results
3) Analytical techniques are much more expensive than nitrogen balance
90
Which amino acids produce a labeled carbon that may be utilized in direct oxidation?
- Branched-chain amino acids (leucine, isoleucine, valine)
- Lysine
- Phenylalanine
91
Why can't you study very low intake of amino acids with direct oxidation?
Because of the non-negligible tracer
92
What are the analytical techniques used for direct oxidation and indicator amino acid oxidation ?
- Breath collection requires an isotope ratio mass spectrometer (IRMS) for CO2 enrichment
- Calorimeter for CO2 production
- Blood samples require a GC mass spectrometer (GCMS) for amino acid enrichment
93
What do the direct oxidation and indicator amino acid oxidation methods require in terms of subject feeding?
- Subjects must be in steady-state
| - Constantly nibbling over the course of the day
94
What is the concept behind the indicator amino acid oxidation (IAAO)?
When an indispensable amino acid is limiting, then all other indispensable amino acids will be oxidized, as amino acids CANNOT be stored
95
Which indicator amino acid is usually used in indicator amino acid oxidation?
Phenylalanine
96
How does the dose response curve of indicator amino acid oxidation change when a test amino acid is limiting, at requirement, and in excess?
- Limiting: when the test amino acid is low, it severely limits protein synthesis, which means that the oxidation of all other amino acids are high (including the indicator)
- As the intake increases, protein synthesis increases, but oxidation of the indicator still occurs
- At requirement: the test amino acid is no longer limiting protein synthesis, so oxidation no longer occurs
- Excess: a plateau is maintained, and past requirement, there is no change in protein synthesis or on the oxidation of other amino acids
97
What is the minimally invasive IAAO model used for?
Model used to determine the amino acid requirements for sick children in a non-invasive way
98
What is the tracer administration method for the minimally invasive IAAO model?
- Repeated oral "nibbling" of tracer solution after 4 hour feeding equilibration
- The child sips the tracer very slowly
99
What are the sampling methods for the minimally invasive IAAO model?
- Breath collection for CO2 enrichment
| - Urine in place of blood for plasma amino acid enrichment (every half hour)
100
How has lysine requirement changed?
- From the 1950s to 2000s, the EAR was based on nitrogen balance studies (12 mg/kg/d)
- The DRI committee re-measured the data through oxidation studies and determined that it was significantly higher (27 to 37 mg/kg/day)
- The nitrogen balance studies were also re-analyzed using modern statistical approaches, and the requirement was also much higher (30 mg/kg/day)
101
Which indispensable amino acid is not very present in dietary protein and has a low requirement as well?
Tryptophan
102
How is the quality of protein determined?
By the amount of indispensable amino acids in a gram of protein
103
What quantity of indispensable amino acids was thought to be necessary in protein according to WHO in 1985? What did the DRI report say?
- WHO: 11mg/g of protein (10%)
| - DRI Report: 285 mg/g of protein (30%)
104
The quantity of indispensable amino acids consumed are particularly important for which group of people?
Individuals consuming a plant-based diet, or individuals that consume a low-variety diet
105
Which amino acids are important and every day scenarios, and are likely to be limiting in diets?
- Lysine
- Tryptophan
- Threonine
- Cysteine
- Methionine
106
Which indispensable amino acid requirement was added in recent years?
Histidine
107
How did a histidine-free diet for 48 days, followed by repletion, affect nitrogen balance, protein turnover, and phenylalanine indicator oxidation (IAAO)?
- No effect on nitrogen balance
- Decreased protein turnover
- Decreased phenylalanine oxidation (IAAO)
108
How did a histidine-free diet for 48 days, followed by repletion, affect the histidine free pool? What does that indicate?
- Decreased the histidine free pool by half
| - Indicates that there is an adaptation or a consequence produced by the diet
109
What is hematocrit? What is it normally?
- The fraction of blood volume composed of RBCs
| - Normally, hematocrit is 49%
110
How did a histidine-free diet for 48 days, followed by repletion, affect the hematocrit? What does that indicate?
- Decreased hematocrit (43%)
| - Indicates that there was a decrease in RBCs
111
How did a histidine-free diet for 48 days, followed by repletion, affect the hemoglobin? What does that indicate?
- Decreased hemoglobin
| - Indicates that the individual is developing anemia
112
How did a histidine-free diet for 48 days, followed by repletion, affect ferritin? What does that indicate?
- Ferritin concentration increased
| - Indicates that iron stores are increasing
113
What is ferritin?
- Iron is stored bound to ferritin in the liver
| - The amount in plasma is an indicator of the quantity in the liver
114
How did a histidine-free diet for 48 days, followed by repletion, affect transferrin? What does that indicate?
- Transferrin concentration decreased
| - Suggests that iron is more abundant
115
How are transferrin stores altered in an individual deficient in iron?
Increased level of transferrin to increase absorption efficiency and transportation from stores
116
How did a histidine-free diet for 48 days, followed by repletion, affect albumin? What does that indicate?
- Albumin concentration decreased
| - Indicates that there is a slowing down of protein synthesis, corroborated by the decrease in protein turnover overall
117
What is the overall effect of histidine deficiency?
- Decreased hemoglobin synthesis, which is high in histidine
- Compromised ability to make RBCs and transport oxygen
- This is NOT related to iron, but to a deficiency in histidine
118
What are accommodations?
Serious adaptations to a deficiency, which indicate that there is a functional problem
119
Which populations does histidine particularly affect?
- Children
| - Critical in long-term health
120
What does histidine deficiency ressemble? How may they be differentiated?
- Ressembles iron deficiency anemia, in which case the individual would be told to consume iron supplements
- However, when they get re-tested, they would see that an iron supplement does not solve the functional issues of histidine deficiencies
