The use of maleic anhydride for the reversible blocking of amino groups in polypeptide chains.

1. Maleic anhydride was shown to react rapidly and specifically with amino groups of proteins and peptides. Complete substitution of chymotrypsinogen was achieved under mild conditions and the extent of reaction could be readily determined from the spectrum of the maleyl-protein. 2. Maleyl-proteins are generally soluble and disaggregated at neutral pH. Trypsin splits the blocked proteins only at arginine residues and there is frequently selectivity in this cleavage, e.g. in yeast alcohol dehydrogenase and pig glyceraldehyde 3-phosphate dehydrogenase. 3. The group is removed by intramolecular catalysis at acid pH. The half-time was 11-12hr. at 37 degrees at pH3.5 in in-maleyl-lysine or in maleyl-chymotrypsinogen. 4. The unblocking reaction can be used as the basis for a ;diagonal'-electrophoretic separation of lysine peptides and N-terminal peptides, as shown by studies with beta-melanocyte-stimulating hormone.