DETERMINATION OF METHIONINE SULFOXIDE IN BIOLOGICAL-MATERIALS USING HPLC AND ITS DEGRADABILITY IN THE RUMEN OF CATTLE

A method was developed for estimating concentrations of methionine sul foxide, methionine and other amino acids in feed protein using reverse phase high performance liquid chromatography. This incorporated pre-c olumn o-phthaldialdehyde (OPA) derivatization and fluorometric detecti on. Samples were hydrolyzed prior to analysis using 4 M methanesulfoni c acid containing 0.2% 3- (2-aminoethyl) indole (tryptamine) under vac uum at 120-degrees-C for 17 h. Hydrolyzates were diluted, adjusted to pH 7 and filtered. A Beckman 507 autosampler with a precolumn derivati zation cassette was used for the derivatization process. Fluorometric detection (excitation 338 nm and emission 425 nm) was used to measure OPA derivatives. Samples were prepared such that on column concentrati on was in the 10-200 pmole range. The mean recovery of the standard so lution added to the feed samples was 96.8 +/- 1.4% for seven tested fe eds. Reproducibility of the method was evaluated by analyzing eight gr ass hay samples (0.39 +/- 0.21 mg g-1 DM) and eight alfalfa hay sample s that had been incubated for 96 h in the rumen (0.14 +/- 0.09 mg g-1 DM). Relative standard deviations were 2.74% and 2.97%, respectively. The use of methanesulfonic acid as a hydrolyzing agent for feed protei n, allowed for higher recovery of total methionine when compared with hydrochloric acid with or without sample oxidation 108.5%). The rumen degradability of protein from grass hay decreased as the amount of met hionine sulphoxide increased.