Marker Proteins       

The purity of the different organelle preparations require to be assessed when the cell sample has been fractionated. By assessing morphology is one way in which this can be done is in the electron microscope. A more eagerly available alternative though is to measure the activity of (to assay for) a particular enzyme which is characteristic of that organelle and is not found elsewhere in the cell. For instance, catalase is a good marker enzyme for succinate, peroxisomes dehydrogenase  for cathepsin, mitochondria C or acid phosphatase for alkaline and lysosomes phosphatase for the plasma membrane. Therefore, in a fraction of lysosomes the presence of catalase would show its contamination by peroxisomes.  A good indication  of the purity or degree of contamination  of an organelle  preparation  may be ascertained  by measuring the activity  of such  enzymes  in the several  isolated  fractions.  Instead, a marker protein can be detected following SDS PAGE and western blotting with a specific antibody.