Question

1. Explain the concept of an organelle marker protein or enzyme.

2. How can a marker enzyme/protein aid in isolating and enriching a preparation for an organelle?

3. A student prepares mitochondria from a cauliflower which has been stored at 4°C for three weeks and finds the SDH specific activity of their PMS is about the same as their mitochondrial fraction. The same protocol used on fresh cauliflowers showed a 10-fold higher specific activity in the mitochondrial fraction compared to the PMS. Can you think of a plausible explanation?

Answer 1

Every cell or organelles specialized for specific functions. Specialized function attributed to the presence of specific protein in the cell or organelles. For example, the lysosome is specialized in digesting foreign particles. It contains an elevated concentration of digesting enzymes. These enzymes may not present in mitochondria or Golgi-complex or in the nucleus. If we are isolating and enriching a lysosome and mitochondrial preparation, we have to detect the presence of digesting enzymes and succinate Dehydrogenase in lysosomal and mitochondrial preparation, respectively. A preparation containing lysosomal fraction should contain digesting enzymes. Whereas, mitochondrial preparation should contain succinate dehydrogenase, but not digesting enzyme. Similarly, nuclear fraction contains the protein specific to the nucleus such as histone. Whereas, cytoplasmic preparation does not contain histone protein. Because histones are not present in the cytoplasm.

Succinate dehydrogenase (SDH) also called as succinate-coenzyme Q reductase (SQR) present on the inner mitochondrial membrane of eukaryotes. The role of SDH is implicated in the citric acid cycle and electron transport chain. In general, an enzyme that is stored at 4°C, losses its activity with time. Student stored the mitochondrial fraction at 4°C for three weeks. which is sufficient time for enzyme to become inactive. Therefore, the student did not observe any difference in SDH specific activity between PMS and mitochondrial fraction.

Freshly prepared mitochondrial fraction contains functionally active succinate dehydrogenase. Therefore, freshly prepared mitochondrial fraction exhibits higher specific activity compared to PMS.