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

A chromatographic method has been developed for enrichment of methionine-bo und sulfur in casein, for stable-isotope analysis. Casein is precipitated f rom milk samples and cleaved by acid hydrolysis in 6 M hydrochloric acid at 95 degreesC. The amino acids released are converted into their N-acetyl de rivatives by addition of acetic acid anhydride. After lyophilization, N-ace tylmethionine is separated from the accompanying components on octadecylsil ica by use of a gradient prepared from 0.02 nn formic acid and methanol. Th e fractions containing N-acetylmethionine are pooled and freeze dried. Proc edures for hydrolysis and derivatization were optimized to furnish the high est yields. The influence of the abundance ratio on the chromatographic sep aration is shown and discussed. From 1.0 g casein 16.5 mg N-acetylmethionin e were isolated.