I think it's because choline converts to betaine, which is involved in the BHMT pathway of remethylation conversion of homocysteine back to methionine, in one-carbon metabolism. There are various papers discussing deficiencies in choline and how they cause NAFLD (Non-alcoholic fatty liver disease).
The issue with methionine is that it contains a sulfur group, which soaks up ROS at a faster rate than other amino acids that do not contain a sulfur group.
Switching of methionine metabolism between the disposal (a) and conservation (b) modes is regulated by AdoMet. The thick arrows qualitatively depict increased metabolic flux. The dashed lines indicate activation (red ) and inhibition (blue) by AdoMet to control flux in response to low and high methionine supply. Abbreviations: AHC, AdoHcy hydrolase; GNMT, glycine N-methyltransferase; MAT-I/III, liver-specific methionine adenosyltransferase isoforms I and III; MT, methyltransferase. (Kabil 2014)
I think the issue in low methionine diets is inappropriate recycling of AdoMet which often shows up as higher levels of homocysteine, such that choline becomes quite important. The other issue with choline is that certain gut bacteria metabolize it to TMAO at different rates, and everyone seems to have a different choline requirement based on their own intestinal microbiome. (Romano 2017)
Darryl or someone with more knowledge of one-carbon metabolism of choline can probably add greater depth to this issue. Looking at this matter is on my long long to-do list.
Kabil O, Vitvitsky V, Banerjee R. Sulfur as a Signaling Nutrient Through Hydrogen Sulfide. Annual review of nutrition. 2014;34:171-205. doi:10.1146/annurev-nutr-071813-105654.
Romano, K., Martinez-del Campo, A., Kasahara, K., Chittim, C., Vivas, E., & Amador-Noguez, D. et al. (2017). Metabolic, Epigenetic, and Transgenerational Effects of Gut Bacterial Choline Consumption. Cell Host & Microbe, 22(3), 279-290.e7. doi:10.1016/j.chom.2017.07.021
Edited by prophets, 11 November 2017 - 01:53 PM.