Plasma L-Methionine and Supplemental L-Methionine Precursor Responses to Rumen Administration of a Rumen Protected DL-Methionine Source or Different Levels of 2-Hydroxy-4-Methylthio-Butanoic Acid

Abstract Text: The L-enantiomer of methionine (Met) is the form that can be used for biological functions. Supplemental precursors of L-Met such as D-Met or 2-hydroxy-4-methylthio-butanoic acid (HMTBa) must be converted to L-Met to be incorporated into protein. The objective of this study was to evaluate the plasma response of L-Met and supplemental L-Met precursors to a rumen pulse dose of protected DL-Met (Smartamine M, Adisseo, France; RPM) or different levels of HMTBa. Six rumen cannulated Holstein steers (initial BW = 250 ± 6 kg SD) were fed a common basal diet and pulse dosed with different treatments according to a partially replicated Latin square design. Treatments administered to the rumen were 80 (H80), 120 (H120), or 160 (H160) mg HMTBa/kg BW (provided as MFP^® feed supplement, Novus International, St. Charles, MO, USA) or RPM at 80 mg DL-Met/kg BW (RPM80); where, based on previous research, H160 and RPM80 were hypothesized to provide similar levels of absorbed Met activity (64 mg/kg BW). Ruminal pulse dose coincided with morning feeding and occurred at t=0 with 11 plasma samples taken from the coccygeal vein over the ensuing 48 h and analyzed for HMTBa, D-Met, and L-Met. Statistical contrasts were linear and quadratic effects of level of HMTBa and H160 vs RPM80 with significance declared at P<0.05. Baseline L-Met was not different between treatments averaging 2.75 mg/L; L-Met precursors were not detected in baseline samples. Plasma profiles of L-Met and supplemental L-Met precursors differed between levels and sources (P<0.01). The change in L-Met from baseline area under the response curve through 48 h (AUC) was linearly increased (P<0.02) as HMTBa increased; L-Met AUC for H160 did not differ from RPM80 (128 vs 147 ± 17 mg•h/L, respectively; P>0.40). Likewise, supplemental L-Met precursor AUC linearly increased as HMTBa level increased; HMTBa AUC for H160 did not differ from RPM80 D-Met AUC (140 vs 117 ± 11 mg•h/L, respectively; P>0.14). When not separated on a chiral column, D- and L-Met are combined during plasma Met analysis; when analyzed values for D-Met or HMTBa were added to analyzed values for L-Met, there was no difference between AUC for H160 and RPM80 (268 mg•h HMTBa + L-Met/L vs 264 mg•h D- + L-Met/L ± 24, respectively; P>0.90). It is concluded that supplying a pulse dose of 64 mg methionine activity/kg BW as RPM or HMTBa resulted in plasma L-Met concentrations that were not different.

Keywords: methionine, dairy, bioavailability