are small cell vesicles that contain enzymes for degrading proteins,
nucleic acids, and polysaccharides. The lysosomes segregate these
dangerous enzymes from the body of the cell, thereby permitting
them to play a digestive role without damaging their host. A white
blood cell, scavenging for foreign bacteria, will absorb an intruder
and destroy it with the hydrolytic enzymes in its lysosomes. Upon
the death of a cell in a multicelled organism, lysosomes rupture
and digest the cell contents. They have been called "suicide vesicles"
and compared with the cyanide capsules familiar from spy novels,
but this may unfairly neglect the digestive and degradative functions
that they carry out during the life of the cell.
Peroxisomes are more of a mystery. They contain the enzyme catalase,
which is possibly one of the earliest heme proteins and a precursor,
or at least a predecessor, of cytochromes and globins. Catalase
is one of the largest single-chain enzymes, containing more than
500 amino acids in one polypeptide chain, and a heme group. Its
only known function is to destroy hydrogen peroxide, either with
or without the release of oxygen:
In the non-oxygen-releasing reaction above, HR
represents any oxidizable organic compound.