Fluorometric determination of free and total isocitrate in bovine milk

Isocitrate is an intermediate metabolite in the citric acid cycle found both inside the mitochondria as well as outside in the cytosolic shunt. Oxidation of isocitrate is believed to deliver large fractions of energy [i.e., reducing equivalents (NADPH) in the bovine udder] used for fatty acid and cholesterol synthesis. This study describes a new analytical method for determination of free and total isocitrate in bovine milk where time-consuming pretreatment of the sample is not necessary. Methods for estimation of both total isocitrate and free isocitrate are described, the difference being the esterified or even lactonized isocitrate. On average, 20% (6-27%) of cow milk isocitrate was esterified and free isocitrate correlated significantly with total isocitrate (r = 0.98). The present fluorometric determination correlated well with the traditional spectrophotometric determination of isocitrate. Milk samples from Danish Holstein cows (984) contained significantly less isocitrate than milk from Danish Jersey cows (760; i.e., 0.134 vs. 0.211 mmol/L). Isocitrate in milk is correlated to milk protein, fat, and citrate, and it is speculated, based on biochemistry, former studies, and the present, that isocitrate may reflect the energy situation in the mammary gland. The use of isocitrate as a biomarker of the energy status in the dairy cow is warranted.