Studies on the amide and C-terminal residues in proteins. 5. Estimation of asparagine and glutamine residues

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Abstract
A method for estimating the total number of asparagine and of glutamine residues in proteins was explored. The method is based on modifications which aspartyl, glutamyl, C-terminal asparaginyl and C-terminal glutaminyl residues undergo when esterified and then reduced with lithium borohydride. After hydrolysis the 2 latter residues are represented by gamma-hydroxy-beta-aminobutyric acid and delta-hydroxy-gamma-aminovaleric acid respectively. Asparaginyl and glutaminyl residues in the peptide chains do not become modified and after hydrolysis can be estimated as aspartic and glutamic acids respectively. Interference due to reductive cleavage of peptide bonds occurs to a very small extent and can be allowed for. Data are presented for the asparaginyl, glutaminyl, aspartyl and glutamyl residues in ox insulin, [beta]- and [beta]1 -lactoglobulin and lysozyme. Delta-hydroxy-gamma-aminovaleric acid and gamma-hydroxy-beta-aminobutyric acid were synthesized and their properties are described.