glycine betaine in serum ~35µM: choline in serum ~8µM: choline in seawater 0-45nM
||Wargo MJ. Homeostasis and catabolism of choline and glycine betaine: lessons from Pseudomonas aeruginosa. Appl Environ Microbiol. 2013 Apr79(7):2112-20. doi: 10.1128/AEM.03565-12. p.2113 left column 2nd paragraphPubMed ID23354714
|| Melse-Boonstra A, Holm PI, Ueland PM, Olthof M, Clarke R, Verhoef P . 2005. Betaine concentration as a determinant of fasting total homocysteine concentrations and the effect of folic acid supplementation on betaine concentrations. Am. J. Clin. Nutr. 81: 1378–1382.  Roulier MA, Palenik B, Morel FMM . 1990. A method for the measurement of choline and hydrogen-peroxide in seawater. Mar. Chem. 30: 409–421 doi:10.1016/0304-4203(90)90084-PPubMed ID15941890
||Primary source  abstract: "A double-blind randomized trial of 6 incremental daily doses of folic acid (50-800 microg/d) or placebo was carried out in 308 Dutch men and postmenopausal women (aged 50-75 y). Fasted blood concentrations of tHcy [total homocysteine], betaine, choline, dimethylglycine, and folate were measured at baseline and after 12 wk of vitamin supplementation." Primary source  abstract: "The horseradish peroxidase-mediated dimerization of hydroxyphenylpropionic acid can be used to measure hydrogen peroxide in seawater. The method was optimized and interferences investigated, and the method was then adapted to the measurement of choline in seawater. The enzyme choline oxidase is used specifically to oxidize choline to produce betaine and H2O2, and the latter is measured."
||P.2113 left column 2nd paragraph: "Glycine betaine (GB) is an important osmoprotectant for many species in all domains of life (refs 17, 18, 20). The widespread use of GB has been explained, in part, by the superior osmoprotection offered by GB as a compatible solute for many organisms (ref 21). GB functions as a compatible solute in these organisms and also plays important roles in methyl group metabolism (refs 22, 24 primary source 23). Soluble GB is at low concentrations in most animal fluids (∼35 μM in serum [primary source 23]), and concentrations are also thought to be low in most environments, although no data could be found for soluble GB concentrations in soil or water. However, its metabolic precursor choline is predicted to be more abundant, predominately as a moiety on larger molecules. Evidence from experimental and natural systems points to rapid bacterial uptake of choline (refs 25–28). This rapid uptake suggests that, while choline release rates may be high, free choline concentrations may be low. Soluble choline is measurable in some environments (∼8 μM in serum, 0 to 45 nM in seawater [primary sources 23, 29]), but the most abundant source is thought to be the choline headgroup moiety on the eukaryotic phospholipids phosphatidylcholine and sphingomyelin, which together typically comprise 50 to 90% of the outer leaflet of eukaryotic plasma membranes (BNID 112936)."