the unknown organelle to remain 
intact. Presumably, it released its 
contents later. 
De Duve's biochemical approach, 
for which he shared the Nobel Prize 
with Claude and Palade in 1974, 
was soon supplemented by electron 
microscopy. But it proved difficult to 
identify the new particles, since, 
unlike other organelles, lysosomes 
vary in shape from cell to cell. Fi- 
nally, in 1955, Alex Novikoff of the 
Albert Einstein College of Medicine 
clearly identified some lysosomes in 
rat liver cells, and it is now known 
that lysosomes (whose name refers to 
the fact that their enzymes can lyse, 
or digest, substances) exist in all 
eukaryotic cells. 
At about the same time that de Duve 
and his colleagues were describing 
the b iochemistry of lysosomes, they 
detected another enzyme-containing 
organelle. In 1965', de Duve pro- 
posed that the organelle be called 
a peroxisome because it appeared 
to both generate and break down 
hydrogen peroxide, a corrosive 
molecule composed of two atoms 
each of hydrogen and oxygen. 
Today it is known that peroxisomes 
exist in most eukaryotic cells, and that 
they are especially prominent in mam- 
malian liver cells. The membrane that 
surrounds a peroxisome is unusually 
permeable, permitting many small 
molecules to enter easily. Peroxisomal 
enzymes remove hydrogen atoms from 
these small molecules and join the 
hydrogen to atoms of oxygen to. form 
hydrogen peroxide. One of the 
peroxisomal enzymes, catalase, then 
neutralizes the hydrogen peroxide 
by breaking it down into water and 
oxygen. This two-step process is the 
method that peroxisomes in the liver 
use to break down molecules of 
alcohol into substances that can be 
eliminated from the body. About 
one-quarter of the alcohol that enters 
the liver is processed in peroxisomes. 
In his early descriptions of per- 
oxisomes, de Duve called them 
"fossil organelles" because of their 
primitive nature and seemingly 
expendable actions. (All of the 
enzymes found in peroxisomes are 
also found elsewhere in the cell.) 
However, it is now known that a 
rare, fatal genetic disorder called 
Zellweger's syndrome is the result of 
malformed peroxisomes, indicating 
that peroxisomes do have a vital role 
in the h ealthy cell. 
The ability of peroxisomes to use 
oxygen in chemical reactions has 
led many scientists to conjecture that 
these organelles represent a relic 
of an attempt by the precursors of 
eukaryotic cells to "cope" with 
oxygen as it accumulated in the 
prehistoric atmosphere. Peroxisomes 
cannot, however, couple oxygen use 
with energy production. That ability 
is restricted to chloroplasts and 
mitochondria — the "energy convert- 
ers" of eukaryotic cells. 
33 
