Proteins

Abstract

Proteins are not a rigidly defined category of substances. One extreme definition would restrict the term to substances composed entirely of amino acids held together by peptide links. That might be too narrow because many generally accepted proteins carry prosthetic groups which are not made up from amino acids; there is indeed no evidence that chlorophyll and nucleic acid have ever been found in significant quantity in a plant except in combination with proteins. Furthermore it is not always possible to demonstrate that peptide bonds are responsible for holding the structure together. At the other extreme lies the common practice of looking on protein as the only form of combination in which nitrogen occurs in plant material and getting the alleged protein content by multiplying the nitrogen content by some such factor as 6.25. This practice includes free amino-acids, amides, purines, and even some of the nitrate, in the protein category and must be roundly condemned. When the figure given for protein content depends on a nitrogen determination its trustworthyness clearly depends on the completeness with which other forms of nitrogen have been eliminated. The assumption that they have been eliminated is more commonly made than shown to be valid and it is possible, or even probable, that there are several forms of non-protein-nitrogen that have not yet been recognised because of the attraction of the simplifying assumption.

Determination of total nitrogen see Vol. I. Seed proteins see p. 69.