Metabolic Labeling Using 32P

Protocol:

1.    Prepare the work area for use with 32P by placing all necessary items (e.g., pipets, Geiger counter) within easy reach. Place the flask with the cells behind a beta shield.
CAUTION: The initial 32P labeling of cells may involve large amounts of 32P (see safety precautions in UNIT 10.9 and in critical parameters and troubleshooting). Rigorous precautions should be taken to avoid exposure of individuals and contamination of laboratory equipment. Be sure to think ahead to carry out the following steps in an efficient but unrushed manner.

2.    Place the source vial containing 10 mCi of 32P in a vial beta shield. Open source vial using forceps to remove protective cover. Resuspend 32P in 0.5 ml phosphate-free RPMI using a transfer pipet. Add 32P to the cells, mix, and incubate 90 min.
CAUTION: To protect yourself and others from the 10 mCi of 32P during transport, place the container with the cells over absorbent paper inside a ⅜-in.-thick Plexiglass box. Place the Plexiglass box behind a lucite screen covered with absorbent paper during transport to the incubator.

During this incubation, the 32P will be taken into the cells and incorporated into ATP. Continue the incubation until a steady state is reached. Empirically, the time to steady state is cell-type dependent and is determined as that length of time where radiolabeling of a sample phosphoprotein is not increased by lengthening the incubation time.

3.    Carry out any manipulation of the cells required in the experiment you have designed. To do so, pipet the 32P-labeled cells up-and-down to suspend and then aliquot into 15-ml screw-cap tubes. Cap tightly and incubate for the desired length of time.
Frequently, in a phosphorylation experiment, unstimulated cells are compared to cells stimulated through a cell-surface receptor or stimulated pharmacologically by reagents that activate protein kinases (see UNIT 3.12; however, doses may need to be adjusted). For example, phorbol esters may be used to activate protein kinase C, and agents that alter cellular cAMP levels (dibutyryl cAMP or forskolin) may be used to activate protein kinase A.

To avoid excessive exposure to radioactivity, it is helpful to add ligands for stimulation to tubes first. Stimulation can be carried out in a shaking water bath to prevent pelleting of cells.

4.    Stop the incubation by diluting five- to ten-fold with ice-cold PBS/PI (to dilute radioactivity).

Centrifuge the cells 5 min at 500 × g, 4°C. Pour off the radioactive supernatant directly into a well-shielded liquid radioactive waste container (to remove unincorporated label). Keep the cells on ice for the remainder of the protocol.
To avoid contamination, use a waste container with a wide mouth. Hold a Kimwipe below the opening of the tube to catch any drips.