Is d-methionine bioavailable to the dairy cow?

Rumen-protected forms of Met contain an equimolar mixture of the d- and l-isomers. Only l-Met can be directly used for protein synthesis, but it is unclear how much of the d-isomer can be transformed into l-Met in ruminants. Four lactating dairy cows, with an average milk yield of 32. kg/d, received a basal diet (12.5% crude protein, supplying 1,718 g/d of metabolizable protein) in 12 equal meals per day plus an abomasal infusion of amino acids (59 g/d, casein profile without Met). They were used in 3 consecutive studies to determine utilization of d-Met. First, the cows each received portal vein infusions for d of 5, 10, or 1 g/d of dl-Met in a Youden square. On the last day of each period, 6 arterial samples were collected at 45-min intervals. Concentrations of l- and d-Met were determined on a chiral column by gas chromatography-mass spectrometry. Portal infusion of 5, 10, and 1 g/d of dl-Met increased plasma total Met concentrations (19.7, 25.0, and 34.4 ± 0.6 μM) and the proportion of Met as d (19.4, 30.5, and 37.3 ± 0.7%). The fractional removal of d-Met was 6 to 7 times lower than the fractional removal of L-Met, with mean half-lives of 52 versus 8 min, respectively. Second, the same cows were infused for 8 h with l[methyl-2H3]Met at 1.3 mmol/h; at 2 h, cows received a bolus injection i.v. of d-[1-13C]Met (6.8 mmol), and arterial samples were collected after 10, 20, 30, 40, 60, 90, 120, 150, 180, 240, 300, 360, 420, and 480 min. Expressed relative to l-[12C]Met; that is, as tracer:tracee ratios, enrichments of plasma d-[1-13C]Met and l-[1-13C]Met averaged 1.77 ± 0.14 and 0.144 ± 0.026, respectively, 10 min after the bolus injection and declined exponentially thereafter. A minimum of 75 ± 3% of the d-[1-13C]Met was transformed into l-[1-13C]Met. Third, the cows received, in a crossover design, an abomasal infusion for  d of either dl-Met or l-Met (1 g/d) and, on the last day of each experimental period, blood samples were collected simultaneously from arterial, portal, hepatic, and mammary vessels. Arterial total Met concentrations were higher with dl- versus l-Met infusions (37.4 vs. 25.4 ± 0.5 μM), with 37.1 ± 5.0% as d-Met. The mammary gland did not extract any d-Met. Hepatic removal of d-Met was observed, but was numerically lower than the fractional extraction of l-Met. In conclusion, much of the d-Met is transformed into l-Met by the dairy cow but at a slow rate. No uptake of d-Met occurs across the mammary gland but l-Met synthesized from the d-isomer elsewhere in the body can be utilized for milk protein synthesis.