Lysosome

Lysosomes are spherical, membrane-enclosed organelles, roughly fifty to seventy nanometers (nm) in diameter, that are found in all animal cells. Knowledge of their construction and function has undergone a radical reinvestigation in recent years. Medical researchers, armed with sophisticated software and optical technology, have studied the lysosome and have found that it is not as basic or as clearly understood as was assumed when it was first discovered and isolated in the mid-1950s.

Background

Just after World War II, Christian de Duve (1917–2003), a cytologist and biochemist at the Université catholique de Louvain in Belgium, began to investigate how insulin, which is produced by the pancreas, works within liver cells. Despite suspecting that powerful enzymes were responsible for maintaining the body’s critical blood-sugar levels, de Duve’s team could not locate the enzyme within the cells they examined.

In 1949, after more than eighteen months of frustration, de Duve suggested that they subject the cells to centrifugation in the hope of isolating the elusive enzyme. Five days later, the team noticed that refrigerated samples showed a measurable increase in enzyme activity, equal to that of a fresh sample. Something had allowed the enzyme action to occur, acting as a kind of delivery system to introduce the enzyme into the cell without interference. De Duve deduced that there must be a molecular structure within the cell that was capable of containing the enzyme and limiting its access to the rest of the cell.

This was the first recognition of the cellular component de Duve would later call “lysosome,” from the Greek words lysis and soma, meaning “to loosen” and “body,” respectively. Other research teams from the United States quickly confirmed de Duve’s hypotheses by flooding cells with light and using an electron microscope to isolate the lysosomes. For his pioneering work, de Duve was awarded the 1974 Nobel Prize in Physiology or Medicine.

Overview

Every cell can hold hundreds of lysosomes, each of which contains more than fifty different varieties of powerful hydrolytic enzymes. For the better part of four decades, lysosomes were defined largely through two dominant metaphors. Because they attach themselves to complex molecular food elements that are absorbed into the cell—everything from proteins and sugars to carbohydrates and lipids—and quickly act to break them down, lysosomes were described as the cell’s “stomach,” able to act as a waste-management system to keep the cell clean and operationally sound. In addition, because lysosomes attack invasive and unwanted elements from the bloodstream, such as viruses and bacteria, that enter to the cell’s cytoplasm, they have also been described as “suicide bags”—a term coined by de Duve himself—in reference to the fact that they sacrifice their healthy internal structure to absorb the invasive elements. This causes the cell to die in order to protect other cells, a process known as apoptosis.

In the event that nutrients become scarce, whether from food deprivation or diseases interfering with the body’s nutritional system, lysosomes release their enzymes to break down other, noncritical organelles within the cell to digest. Far from damaging the cell, this process, known as autophagy, actually saves it, maintaining the cell’s health until the nutrient flow is restored.

The powerful enzymes of the lysosome are first produced in the rough endoplasmic reticulum, one of two types of endoplasmic reticula found in the cell. These enzymes are then packaged and sent to the Golgi apparatus, which in turn fuses the enzyme packages with endosomes to form the lysosomes.

Lysosomes are critical to cell maintenance, security, repair, and metabolism. They are also necessary for maintaining internal cellular communications, which serve to alert the cell when it has absorbed biomolecules for food or sound the alarm when it has been invaded by unwanted entities. Scientists are still not entirely certain how an organelle with a simple membrane wall can contain such powerful acidic enzymes without ultimately breaking down itself, although theories include the protective layer of sugars coating the lysosomal membrane and the difference in pH between the lysosome and the cytoplasm.

In the first decades of the twenty-first century, research laboratories have significantly challenged and corrected several assumptions made about lysosomes. While they were long thought to be found only in animal cells, researchers have isolated evidence of lysosomes in a variety of plants. Scientists also discovered that, rather than “suicide machines” responsible for regulating invasive materials and initiating cell death, individual lysosomes are in fact more like storage facilities for the enzymes, remaining inactive until they bond with other organelles. This creates a hybrid organelle structure that works to maintain a clean cell.

Researchers have begun to investigate ties between lysosomes and chronic diseases. Although many of the disorders that have been traced to genetic defects that produce dysfunctional lysosomes, such as Tay-Sachs disease and Gaucher’s disease, are relatively rare, they serve as evidence that lysosomes may play a far more important role in cell health than merely maintaining sanitation.

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