Micromolar Protein Concentrations and Metalloprotein Stoichiometries Obtained by Inductively Coupled Plasma Atomic Emission Spectrometric Determination of Sulfur

The concentrations of several micromolar solutions of proteins with known sulfur contents were determined by inductively coupled plasma atomic emission spectrometry (1CP-AES) of sulfur in the vacuum ultraviolet. These values are compared to concentrations obtained by using spectrophotometric measurements and published specific and molar absorptivities based on various conventional methods of protein determination. The two sets of values are in close agreement, Indicating that ICP-AES of sulfur is an accurate means of determining microgram quantities of proteins. Standard deviations are within 2% of the mean values obtained for data sets of six replicate measurements. Dilute buffered protein solutions may be directly pumped into the nebulizer; sample digestion and other special sample preparations are not necessary. It is also demonstrated for several metalloproteins that multielement ICP-AES is an excellent means of determining stoichlometries of bound metal ions as both protein and metal assays may be rapidly performed on a single sample.