Measuring concentration of metabolites from MS - (Feb/28/2018 )

Hi All,

I have observed that the concentration of metabolites present in a living cell is commonly reported in mole/ gram protein.

Could someone explain what gram protein represents?

gram protein is exactly what it says - a gram of protein. In the example you gave, the mol per gram protein means how many mole of substance X in each gram of protein. 

Note that cells are not entirely made up of protein - there is a lot of water and various lipids in there too, so the experiments determine how much protein there is and use this as a standard for comparison across substances.