Quantitation of peptides and proteins by matrix-assisted laser desorption/ionization mass spectrometry using O-18-labeled internal standards

A method for quantitating proteins and peptides in the low picomole and sub -picomole range has been developed using matrix-assisted laser desorption/i onization mass spectrometry (MALDI-MS) with internal O-18-labeled standards . A simple procedure is proposed to produce such internal standards for the tested sample by enzymatic hydrolysis of the same sample (with known conce ntration) in O-18-water. A mathematical algorithm was developed which uses the isotopic patterns of the substance, the internal standard, and the subs tance/internal standard mixture for accurate quantitation of the substance. A great advantages of the proposed method is the absence of molecular weig ht limitation for the protein quantitation and the possibility of quantitat ion without previous fractionation of proteins and peptides, Using this str ategy, the peptide angiotensinogen and two proteins, RNase and its protein inhibitor, were quantified by MALDI-time-of-flight (TOF) mass spectrometry, Copyright (C) 2000 John Wiley & Sons, Ltd.