Studies on the amide and C-terminal residues in proteins. 1. The characterization of the C-terminal residue

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Abstract
The carboxyl groups of C-terminal residues in proteins, after preliminary esterification, can be reduced with lithium borohydride, and on subsequent acid hydrolysis the modified residues are present in the hydrolysate as amino alcohols or hydroxyamino acids. The suitability of the procedure for determining the C-terminal residues in proteins was explored, and in agreement with Crawhall and Elliott (1955), it was found that the issue was complicated by the simultaneous reductive cleavage of peptide bonds which occurs, under the conditions chosen, to the extent of 1-2% of the total peptide bonds. Data are presented snowing that with a protein of low molecular weight such as insulin (5732) the interference is not serious and a satisfactory determination of the C-terminal residues can be made. With a protein of much higher molecular weight such as /3-lactoglobulin (37000), however, the interference is serious. The procedure is not recommended as a reliable one for proteins but it may be of use with peptides of low molecular weight.
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