Structure Elucidation - Biochemical Interpretation

After the verification of choline, the relative concentrations of choline in the urine samples were quantified. It turned out that for samples of group 2, choline concentrations were up to 50-fold increased, and for samples of group 3, choline concentrations were up to 8-fold increased. Choline is an important endogenous metabolites. It is a constituent for phospholipids (lecithin), which are important component of membrans, and for acetylcholine, which is an important neurotransmitter. In additon, choline and its degradation product betaine are important methyl donors, for example for the synthesis of methionine. Choline defficiency, resulting in a decrease of performance, has been described several time in literature.

To interpret the finding of extreme choline excretion, choline has to be seen in the context of biochemical pathways. Choline is a degradation product of lecithin via phosphocholine. As choline is an important metabolite, it is preserved in the body very well. Only 3 % of the choline filtered by glomerulus are excreted. The vast majority of choline is oxidized by choline oxidase via betaine aldehyd to betaine, which is reabsorbed in the proximal tubules. For an extreme excretion of choline, two hypotheses can be formulated. The first explanation is an extreme degradation of lecithin, for example due to an inflammation. This results in an increased serum concentration of choline. In literature it is reported that similar to a step function, the excretion of choline into urine singificantly increases, if choline concentrations in serum exceed a certain abnormal high level, as the choline oxidase is saturated. The second explanation would be a disturbance of the oxidation of choline to betaine in the kidney and consequently an excretion of choline. To validate any of these hypothese further investigations are needed with determinations of choline concentrations in serum and more specific investigations of the kidney including determinations of concentrations of choline, betaine and choline oxidase.

Normal physiological state of lecithin metabolism (first row) two possible explanations for the extreme excretion of choline into urine in terms of biochemical pathways.