Quantitation of some amino-terminal residues in proteins using 3H-labelled dansyl chloride and 14C-labelled amino acids.

A method for quantitation of amino-terminal residues in proteins is presented. The method is a modification of a double isotope-labelling technique, using 3H-labelled dansyl chloride and 14C-labelled amino acids as internal standards. The method is demonstrated on human fibrinogen, horse myoglobin and on mouse myoloma IgA. A linear relationship between the ratio 3H/14C in the separated amino-terminal amino acid of the protein and the amount of protein added in the labelling mixture was obtained with standard deviations of +/- 7.4% +/- 3.4% and +/- 10.3%, respectively. An application of the method is demonstrated by measuring the increase in amino-terminal glycine in fibrinogen following the proteolytic action of thrombin. The method seems to be useful when 0.1 nmol or more of protein is used.