Question

The cells accumulate neutral red (pKa=7), their lysosomes swell-up to many times their normal size. The swelling cannot be accounted for simply by the mass of accumulated neutral red. Can you suggest what else may contribute to the swelling and why it occurs?

Answer 1

Lysosomes are subcellular organelles that contain hydrolytic enzymes required to degrade macromolecules in cell debris. These enzymes are active in acidic pH that is maintained by the action of the vacuolar H+-ATPase. The pH within the lysosome is much lower than outside the lysosome.

These organelles are encased within a semi-permeable membrane. Neutral red is a cationic dye that is taken up by cells by diffusion as it is lypophillic. It accumulates within the lysosomes for long duration. The retention of neutral red in lysosomes depends on the acidic pH of lysosome and efficiency of the proton pump present in lysosomal membrane. Damage to the lysosomal membrane or its proton pump causes leakage of neutral red into cytosol.

Neutral red is a weak base and is taken up by diffusion in its neutral form by the lysosomal membrane. However, in its protonated form within the lysosome (due to protonation), it becomes impermeable. Hence, it is concentrated within the lysosome. When the concentration of the dye increases towards isotonicity, water will enter the lysosome causing the lysosome to swell. Hence, water absorption within the lysosome will cause its swelling to form large vacuoles. The neutral red dye induces vacuolation, increase intralysosomal pH and cause proteolysis in the lysosome.