MEASUREMENT OF PROTEIN IN WHOLE-PLANT SAMPLES WITH NINHYDRIN

The ninhydrin method has been used widely for measuring amino acids in hydrolysed purified proteins. It is also a useful method of measuring the total amino acid or protein content of hydrolysed plant samples. In this study the conditions used in a modified ninhydrin method were found to incompletely hydrolyse protein. Therefore, standard conditions for protein hydrolysis (6 M HCl at 110 degrees C) were tested. The accuracy of protein measurements using the standard ninhydrin method was evaluated using pure proteins, protein-carbohydrate standards and plant samples augmented with known amounts of protein. Standard hydrolysis conditions completely hydrolysed proteins, but protein content was overestimated when these conditions were used on whole plant samples and standards containing carbohydrates. This error may be quantified and corrected easily in plant samples, however, by measuring the recovery of protein added to control samples before hydrolysis.