Kinetics of methionine and choline and their incorporation into plasma lipids and milk components in lactating goats.

Rates of entry of L-methionine and choline and their incorporations into lipids of blood plasma and milk components were studied in lactating goats fed a concentrate-roughage ration with primed constant infusion of L-[methyl-carbon-14] methionine and [methyl-carbon-14] choline. Entry rates of methionine and choline were 30 and 47 mumol/h/kg body weight. Of the methionine pool, 10 and 28% were recovered in carbon dioxide and choline of blood plasma, but the contribution of methionine to the total carbon dioxide production was negligible. Two percent of serine was derived from methionine. Six percent of the choline pool was derived from methionine, whereas labeled choline was not recovered in methionine. Methionine and choline contributed to at least 4 and 19% of the phospholipid pool. Of the methionine dose, .2 and 3.7% appeared in milk lactose and protein. Little (.3%) labeled choline was recovered in milk; however, its contribution to milk fat was 10 times as high as that of methionine. The lipogenic effect of methionine in the liver is proposed as one mechanism among others stimulating production of milk and milk fat.