CAN CHOLINE SPARE METHIOININE FROM CATABOLISM IN LACTATING MICE AND DAIRY COWS

Title of Thesis: CAN CHOLINE SPARE METHIOININE FROM CATABOL ISM IN LACTATING MICE AND DAIRY COWS? Sarah Lee Ann Benoit, Master of Science, 2009 Thesis directed by: Professor Richard A. Erdman Department of Animal and Avian Science Several studies have demonstrated that supplementation of rumen-protected choline (RPC) improves milk production in the lactating dairy cow; however, there a re an equal number of studies failing to observe production responses. To date, there are only three studies that provide quantitative information in ruminants on the metabolic fates of methyl groups derived from choline and Methionine (Met). This has limited the abil ity to predict when, and under what conditions, RPC supplementation will be beneficial. The objectives of this thesis were to determine the interaction of choline and Met m ethyl group metabolism and the extent of methyl group transfer during lactation, a nd define what role, if any, is there for RPC in remethylation of homocysteine and in the s paring of Met in lactating animals. A preliminary study with lactating mice consuming a low protein basal di et (10%) was conducted. From 11 to 15 d postpartum, stable isotopes of [methylH3] choline and [methyl,H3]Met replaced the unlabeled choline and Met in the basal diet to measure the metabolic fates of choline and Met including Met remethylation and sources of Met methyl in the mammary gland. Isotopic analysis revealed that the liver i s a major site of Met remethylation from dietary choline with a minimum choline methyl group contribution to Met remethylation of 35%. Mammary tissue was not a major sit e of Met remethylation from dietary choline (< 10% of Met methyl) as measure by Met methyl in mammary tissue and milk casein. However, there was a significant unlabel ed source of methyl groups contributing at a minimum of 45% Met remethylation in the mam mary tissue, presumably by de novo synthesis. This suggested that in addition to the liver, the mammary gland is an active site of methyl group transactions. In a subsequent study, lactating dairy cows were fed a total mixed ration formulated to meet the nutrient requirements with exception of metabolizable Met that was restricted to 1.49 % of metabolizable protein. Treatments included a Control ( basal diet) and RPC supplemented diet where the basal diet was top dressed with 15g/d R PC, diets were fed in a single reversal design with 2 week experimental periods. Stable isotopes of Met, [1C] Met, [CH3] Met, and [methylH3] choline were continuously infused on d 14 of each period to determine the metabolic fate and methyl transactions of Met methyl as measured in blood and milk casein. Treatment had no effect on milk production or composition. However, plasma free Met from choline transmethylation was shown to act as a significant c ontributor to casein synthesis. Fractional Met remethylation rates in the control a d RPC treatments were 26 and 23%, respectively. Methionine Met methyl loss within the mammary tissue appears to be minimal. Based on casein Met enrichment, upwards of 40% of Met present in casein had undergone transmethylation with choline serving as the ultimate methyl donor. Furthermore, plasma versus casein Met methyl enrichment data suggest ed that a significant amount of de novo methyl group synthesis occurs in the dairy cow’s mammary gland with choline serving as a major methyl donor. CAN CHOLINE SPARE METHIOININE FROM CATABOLISM IN LACTATING MICE AND DAIRY COWS?