Liquid chromatographic enrichment ofN-acetylmethionine from casein hydrolysates for stable-isotope-ratio analysis of sulfur

Summary A chromatographic method has been developed for enrichment of methionine-bound sulfur in casein, for stable-isotope analysis. Casein is precipitated from milk samples and cleaved by acid hydrolysis in 6m hydrochloric acid at 95 °C. The amino acids released are converted into theirN-acetyl derivatives by addition of acetic acid anhydride. After lyophilization,N-acetylmethionine is separated from the accompanying components on octadecylsilica by use of a gradient prepared from 0.02m formic acid and methanol. The fractions containingN-acetylmethionine are pooled and freeze dried. Procedures for hydrolysis and derivatization were optimized to furnish the highest yields. The influence of the abundance ratio on the chromatographic separation is shown and discussed. From 1.0 g casein 16.5 mgN-acetylmethionine were isolated.