Methionine (Met) is a precursor for protein synthesis and the primary donor of labile methyl groups. It was hypothesized that milk production responses  to rumen-protected choline (RPC) relate to the choline sparing Met as a methyl donor. The objectives of this study were to determine the bio-availability of RPC and whether Met methyl group flux is reduced when dairy cows are fed RPC.

What

Four multiparous Holstein cows in mid lactation were fed a nutritionally complete basal diet except for Met that was limited to 1.49% of metabolizable protein. Treatments included the basal diet or the basal diet plus 15g/d RPC as choline chloride (ReaShure, Balchem Corp., New Hampton, NY) in single reversal design with 2 wk periods. Metabolic fates of Met were measured by continuous i.v. infusion of [1- 13 C] and [methyl- 13 C] Met, and [trimethyl-C 2 H3] choline for 12 h on d 14 of each period.

Parameters Monitored

Milk and milk components and Methionine methyl group flux.

Key Results

1) Supplementation with RPC did not affect total milk yield or milk fat and protein yields which averaged 39 kg/d, 1634 g/d and 1110 g/d, respectively.

2) Based on plasma [1-13C] Met and [methyl-13C] Met enrichments, total Met flux, irreversible loss, and remethylation were not affected by treatment, averaging 15.2, 11.5, and 4.2 mmol/h, suggesting that 24% of Met was remethylated.

3) In contrast, using plasma [1-13C] homocysteine as the true intracellular precursor, total Met flux, irreversible loss, and remethylation rates (mmol/h) were 80.1, 67.6 (p = 0.04); 38.3, 33.5; and 41.8, 34.1 (p = 0.07), for control and RPC, respectively.

Implications

Differences in plasma vs. casein [methyl-2H3] Met labeling, which arose from [trimethyl-C2H3] choline, suggested that ~40% of Met in the mammary  land underwent transmethylation with choline serving as the methyl donor. Finally, based on treatment differences in Met methyl flux, the bio-availability of RPC approximated to 72%. These results illustrate the central role of Met and choline in methyl metabolism and the importance of methyl group transactions in the high-producing dairy cow.

Abstract

Methionine (Met) is a precursor for protein synthesis and the primary donor of labile methyl groups. We hypothesized that milk production responses to rumen-protected choline (RPC) relate to choline sparing Met as a methyl donor. The objectives of this study were to determine the bioavailability of RPC and whether Met methyl group flux is reduced when dairy cows are fed RPC. Four multiparous Holstein cows in mid lactation were fed a nutritionally complete basal diet except for Met that was limited to 1.49% of metabolizable protein. Treatments included the basal diet or the basal diet plus 15g/d RPC as choline chloride (ReaShure, Balchem Corp., New Hampton, NY) in single reversal design with 2 wk periods. Metabolic fates of Met were measured by continuous i.v. infusion of [1-13C] and [methyl-13C] Met, and [trimethyl-C2H3] choline for 12 h on d 14 of each period. Milk was collected at 3 h intervals and blood taken over the last 6 h. Supplementation with RPC did not affect total milk yield or milk fat and protein yields which averaged 39 kg/d, 1634 g/d and 1110 g/d, respectively. Based on plasma [1-13C] Met and [methyl- 13C] Met enrichments, total Met flux, irreversible loss, and remethylation were not affected by treatment, averaging 15.2, 11.5, and 4.2 mmol/h, suggesting that 24% of Met was remethylated. In contrast, using plasma [1- 13C] homocysteine as the true intracellular precursor, total let flux, irreversible loss, and remethylation rates (mmol/h) were 80.1, 67.6 (p = 0.04); 38.3, 33.5; and 41.8, 34.1 (p = 0.07), for control and RPC, respectively. Differences in plasma vs. casein [methyl-2H3] Met labeling, which arises from [trimethyl-C2H3] choline, suggested that ~40% of Met in the mammary gland underwent transmethylation with choline serving as the methyl donor. Finally, based on treatment differences in Met methyl flux, the bioavailability of RPC approximated to 72%. These results illustrate the central role of Met and choline in methyl metabolism and the importance of methyl group transactions in the high producing dairy cow.