Hi,
By substracting the minimum obligatory methionine requirement from the total sulfur amino acid requirement (met) I should receive an approximate value on how much cysteine spares the total sulfur amino acid requirement on a methionine weight basis.
However, considering the molecular weight of cysteine being approximately 20% less than methionine would I therefore adjust/reduce the approximate sparing value by 20% to have the value on a cysteine weight basis?
Example: Cysteine spared 40% of the TSAA in pigs on a methionine weight basis. Would I then add or substract 20% of the 40% if cysteine 0.8g= methionine 1g?
I would assume that I would have to substract 20% of the 40% resulting in 32% of TSAA being spared by cysteine on a cysteine weight basis.
For reference please see
Dietary Cysteine Reduces the Methionine Requirement by an Equal Proportion in Both Parenterally and Enterally Fed Piglets
Thank you in advance
You always have to compare molecules on their molecular weight basis and never on just a weight basis. That is like comparing apples and pears!
To complicate things further: you should compare amino acids (usually incorporated in proteins) on the availability basis, that allows for a true comparison. However that value is seldom known.
Bioassays - like growth and digestibility trials - have a large variability in their data, so it is not always easy or even possible to obtain correct comparison data for amino acids.
C.A. (Kees) Kan Thank you for your reply!
So I gave this a bit of a thought. The good thing is that I did a digestibility study on these diets and that I do know the ADC for methionine (met) and cysteine (cys). I do realize that the cysteine sparing value is more than an estimate than anything else.
But to be more specific, in order to calculate the approximate/potential cysteine sparing effect:
1. I need a "clean" total sulfur amino acid (TSAA) requirement only expressed in the molecular weight of methionine.
So, if I have a TSAA requirement (met+cys) consistent of methionine and cysteine, I will have to convert the cysteine part of the TSAA requirement into the molecular weight of methionine. Therefore --> TSAA requirement (met)
2. Then I can substract MOM (met) from TSAA (met) requirement, which equals the cysteine sparing potential on a methionine molecular weight basis and potentially digestible intake of methionine.
3. In order to correct for the cysteine molecular weight and digestibility I would then take the final value from step 2 and multiply it with 0.812 (cysteine molecular weight in comparison to methionine)
4. Then i would correct the value for the methionine digestibility and convert it back to 100% ADC and multiply it with cysteine's digestibility.
Sorry, if this is too specific but i am not sure if step 3 and 4 are correct.
Caroline Candebat I can see no faults in your plan, but I must admit that the types of calculations are certainly not my strongest point. Just discuss it (once again may be) with a colleague or supervisor. If you can explain things to another person it means that you know and understand them yourself.
Still be aware that also the requirement data are not absolute values, their determination also carries large variability.
So being very precise in your calculations may cost quite some time and afterwards appear not to be warranted due to the variability in feedstuffs, animals etc etc.
To complicate things even further the interconversion between cystein and methionin does not have a 100 % efficiency and theoretically that should also be taken into account at different cystein/methionin ratios.
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