The role of free amino groups of peptides and proteins in the Folin-Lowry and biuret methods.
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Date
1980-06
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Abstract
On an equal weight basis polymyxin Β and EM 49 which do not contain tyrosine or
tryptophan yielded the same colour intensity as proteins in the Folin-Lowry and biuret methods.
But, in the absence of reagent C (alkaline copper reagent) polymyxin Β and EM 49 yielded no
colour in the Folin-Lowry method. Mono-, di- and tri-formyl polymyxins Β formed identical
amounts of coloured complexes as polymyxin Β in the two methods. However, the tetra- and
penta-formyl polymyxins Β yielded only one-fifth and one-sixth, respectively, of the expected
colour in the Folin-Lowry method. Similarly, 40% and 30%, respectively, of the anticipated
amount of colour is formed in the biuret method. Formylated and methylated lysozyme and
bovine serum albumins form only 70-75% of the expected colour in the Folin-Lowry method.
Since formation of colour by reduction of Folin reagent, in the Folin-Lowry method, is at least
partly due to complexes of copper, it was inferred that polymyxin Β as well as its mono-, di- and
tri-formyl derivatives on the one hand and the tetra- and penta-formyl derivatives on the other
differ in their ability to complex Cu(II) The former group of compounds was indeed found to
complex as many as three Cu(II) ions whereas the tetra- and penta-formyl polymyxins Β complexed
only one equivalent, under conditions of excess Cu(II). Under conditions of low Cu(II), polymyxin
Β and all its derivatives complexed only one Cu(II). In proteins, sites other than amino groups
which complex Cu(II) probably play a major role in the reduction of the Folin reagent, since
methylated lysozyme and bovine serum albumin yield 70-75% of the colour formed by the
unmodified proteins in the Folin-Lowry reaction.
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Keywords
Polymyxin Β, acylation of polypeptides, reductive methylation of proteins, Folin Lowry and Biuret methods, copper chelate of polymyxin Β, formyl polymyxins B-Cu(II) chelates, Gramicidin S
Citation
Srinivasa B R, Ramachandran L K. The role of free amino groups of peptides and proteins in the Folin-Lowry and biuret methods. Journal of Biosciences. 1980 Jun; 2(2): 99-106.